Tuesday, November 30, 2010

The Really Cool NTILE() Window Function

If you regularly code queries and have never been introduced to the windowing functions, then you are in for a treat. I've been meaning to write about these for over a year, and now it's time to get down to it.

Support in Major Servers

SQL Server calls these functions Ranking Functions.

PostgreSQL supports a wider range of functions than MS SQL Server, having put them in at 8.4, and PostgreSQL and calls them Window Functions.

Oracle's support is broader (by a reading of the docs) than SQL Server or PostgreSQL, and they call them Analytic Functions.

I try to stay away from MySQL, but I did a quick Google on all three terms and came up with a few forum posts asking when and if they will be supported.

The NTILE() Function

In this post we are going to look at NTILE, a cool function that allows you to segment query results into groups and put numbers onto them. The name is easy to remember because it can create any -tile, a percentile, a decile, or anything else. In short, an n-tile. But it is much easier to understand with an example, so let's go right to it.

Finding percentiles

Consider a table of completed sales, perhaps on an eCommerce site. The Sales Manager would like them divided up into quartiles, four equally divided groups, and she wants the average and maximum sale in each quartile. Let's say the company is not exactly hopping, and there are only twelve sales, which is good because we can list them all for the example. If we already had the quartiles provided then the query would be easy, so if we were lucky enough to be starting with this:

 RETAIL   |   78.00 |   1
 RETAIL   |  234.00 |   1
 DEALER   |  249.00 |   1
 DEALER   |  278.00 |   2
 RETAIL   |  392.00 |   2
 RETAIL   |  498.00 |   2
 DEALER   |  500.00 |   3
 RETAIL   |  738.00 |   3
 DEALER   | 1250.00 |   3
 RETAIL   | 2029.00 |   4
 RETAIL   | 2393.00 |   4
 RETAIL   | 3933.00 |   4

The query would be child's play if we already had the quartile:

Select quartile
     , avg(amount) as avgAmount
     , max(amount) as maxAmount
 GROUP BY quartile
 ORDER BY quartile

The Problem is We Do Not Have Quartile

The problem of course is that we do not usually have handy columns like QUARTILE provided, but we can generate the QUARTILE column during the query by using NTILE.

Select quartile
     , avg(amount) as avgAmount
     , max(amount) as maxAmount
  FROM (
        -- The subquery is necessary
        -- to process all rows and add the quartile column
        SELECT amount
             , ntile(4) over (order by amount) as quartile
          FROM ORDERS
       ) x
 GROUP BY quartile
 ORDER BY quartile

This query will give us what the Sales Manager wants.

Dissecting the Function and The OVER Clause

The NTILE() function takes a single argument, which tells the server how many groups to divide the data into. If there are not an exact number of rows in each group, the server decides which groups will be missing one row. So in an exact case all of your groups have the same count of rows, but when it does not divide evenly, one or more of them will be one row short.

If you pass 100 to NTILE(), you get a percentile. If you pass 10, you get a decile, and so forth.

The magic is in the OVER() function. This supports two clauses, and the example shows one, the ORDER BY. Quite simply, the ORDER BY clause tells the server how to line up the rows when adding the NTILE values. The clause is very flexible, and has nothing to do with your query's overall ORDER BY clause.

The Second Clause: PARTITION

Now we will pretend the Sales Manager is not satisfied, and wants separate numbers for the two Customer Types. We could do this if the NTILE() function would create two sets of quartiles, one for each Customer Type, like so:

 DEALER   |  249.00 |   1
 DEALER   |  278.00 |   2
 DEALER   |  500.00 |   3
 DEALER   | 1250.00 |   4
 RETAIL   |   78.00 |   1
 RETAIL   |  234.00 |   1
 RETAIL   |  392.00 |   2
 RETAIL   |  498.00 |   2
 RETAIL   |  738.00 |   3
 RETAIL   | 2029.00 |   3
 RETAIL   | 2393.00 |   4
 RETAIL   | 3933.00 |   4

We can do this by using the PARTITION BY clause, which tells the server to break the rows into groups and apply the NTILE() numbering separately within each group. The new query would be this:

Select custtype
     , quartile
     , avg(amount) as avgAmount
     , max(amount) as maxAmount
  FROM (
        -- The subquery is necessary
        -- to process all rows and add the quartile column
        SELECT amount
             , ntile(4) over (partition by custtype
                                 order by amount) as quartile
          FROM ORDERS
       ) x
 GROUP BY custtype,quartile
 ORDER BY custtype,quartile

Bonus Points: The Median

Now once again the Sales Manager, who is never satisified, comes down and says that the average is no good, she needs the max and the median sale value within each quartile. To keep it simple, she does not need this broken out by customer type, it can be applied to the entire set.

This is a case where we can use NTILE() twice. The first time we will break all sales up into four groups, to get the quartiles, and then we will break up each quartile into two groups to get the median. The code looks like this:

Select quartile
     , max(case when bitile=1 then amount else 0 end) as medAmount
     , max(amount) as maxAmount
  FROM (
        -- The second pass adds the
        -- 2-tile value we will use to find medians
        SELECT quartile
             , amount
             , ntile(2) over (partition by quartile
                                  order by amount) as bitile
          FROM (
                -- The subquery is necessary
                -- to process all rows and add the quartile column
                SELECT amount
                     , ntile(4) over (order by amount) as quartile
                  FROM ORDERS
               ) x1
       ) x2
 GROUP BY quartile
 ORDER BY quartile

The magic here is that we know we've divided the data evenly into four sets, so the median will be the maximum value half way through each set. In other words, it will be the maximum value when the value of bitile=1 for each quartile.

One More Note About Oracle

Once you get down the basics of the OVER clause, Oracle looks really good, because they support the clause over the largest range of functions, at least going by the respective doc pages for each platform.

Monday, November 29, 2010

Loops Without Cursors

Looping Without Cursors

Sometimes you need to process a table row-by-row, and the established approach is to use cursors, which are verbose, slow, and painful to code and use.

The Cursor Example

Here is the basic minimum syntax required to loop through a table and get something done. The SQL flavor is MS SQL Server, but its not much better in any other flavor.

-- I coded this off the top of my head, there
-- may be a minor syntax error or two

-- Most of this is pseudo-code, but take
-- note that it is ordered on column1
declare someCursorName cursor for
 select column1, column2, column3 
   from anyTable
  ORDER BY column1

-- Have to do this now
open someCursorName

-- Now you need to declare some variables
-- For the example I'm just making everything int
declare @column1 int
      , @column2 int
      , @column3 int

-- Gosh, we're actually about to start the loop!  Finally!
fetch next from someCursorName into @column1,@column2,@column3
while @@fetch_status = 0 begin

   --  If you still remember what you actually wanted
   --  to do inside the loop, code it here:

-- Repeat this line from the top here again:
fetch next from someCursorName into @column1,@column2,@column3

-- Not done yet, these two lines are crucial
close someCursorName
deallocate someCursorName

Call me petty, but what I hate about that code is that I have to refer to specific columns of interest 3 times (not counting the declarations). You refer to them in the cursor declaration and in the two FETCH commands. With a little clever coding, we can vastly simplify this and do it only once.

Using An Ordered Column

We can execute the same loop without the cursor if one of the columns is ordered and unique. Let us say that column1 is the primary key, and is an auto-incremented integer. So it is ordered and unique. The code now collapses down to:

-- I coded this off the top of my head, there
-- may be a minor syntax error or two

-- We can't get around declaring the vars, so do that
declare @column1 int
      , @column2 int
      , @column3 int

-- If you know a safe value for initialization, you
-- can use the code below.  If this is not 100% 
-- safe, you must query for the value or it must
-- be supplied from some other source
set @column1 = -1

-- BONUS POINTS: Can this become an infinite loop?
while 1 = 1 begin

-- Now we code the query and exit condition
 select TOP 1
        @column1 = column1
      , @column2 = column2
      , @column3 = column3 
   from anyTable
  WHERE column1 > @column1  -- this is what advances the loop
  ORDER BY column1

if @@rowcount = 0 begin

        -- Put the actions here        


Final Notes

The only requirement for this approach is that you have a unique ordered column. This usually means a unique key or primary key. If "column1" is not unique, the loop will skip all but the first value in each group.

Also, it is very nice if you know a safe value to use as an initializer. Without that, you must query for the minimum value that matches the condition and then decrement it by one.

Finally, can this loop become infinite? No. Well, if, in the extremely unlikely situation that rows are being added to the base table faster than you are processing them, then yes, it could go on for a very long time. But if that were happening I'd say there was a separate problem to look at.

It should probably go without saying, but if the particular loop is going to happen very often, the table should be indexed on your unique ordered column. If it is a primary key or you already have a unique constraint it is not necessary to create an index explicitly because there will be one as part of the key or constraint.

Saturday, November 27, 2010

Revisiting Normalization and Denormalization

In this blog I have done at many articles on Normalization and Denormalization, but I have never put all of the arguments together in one place, so that is what I would like to do today.

There are links to related essays on normalization and denormalization at the bottom of this post.

This blog has two tables of contents, the Topical Table of Contents and the list of Database Skills.

The What and Why of Normalization

Normalization is the process of designing tables so that each fact is stored in exactly one place. A "fact" in this case is any detail that we have to keep track of, such as a product's description, a product's price, an employee's social security number, and so forth.

The process is all about figuring out what tables you need and what columns each table will have. If we are talking about an employee's social security number, then we can guess right from the start that will have a table of EMPLOYEES, and that one of the columns will be SSN. As we get more details, we add more tables and columns.

The advantage of normalization comes when your application writes data to the database. In the simplest terms, when the application needs to store some fact, it only has to go to one place to do it. Writing this kind of code is very easy. Easy to write, easy to debug, easy to maintain and improve.

When the database is not normalized, you end up spending more time writing more complicated application code that is harder to debug. The chances of bad data in your production database go way up. When a shop first experiences bad data in production, it starts to become tempting to "lock down" access to the database, either by forcing updates to go through stored procedures or by trying to enforce access to certain tables through certain codepaths. Both of these strategies: stored procedures and code paths, are the actually the same strategy implemented in different tiers, they both try to prevent bugs by routing access through some bit of code that "knows what to do." But if the database is normalized, you do not need any magic code that "knows what to do."

So that, in brief, is what normalization is and why we do it. Let's move on now to denormalization.

Denormalization is Harder to Talk About

Normalization is easy to explain because there is a clearly stated end-goal: correct data. Moreover, there are well-defined methods for reaching the goal, which we call the normal forms, First Normal Form, Second Normal Form, and higher forms. By contrast, denormalization is much harder to talk about because there is no agreed-upon end goal. To make matters worse, denormalization violates the original theory of Relational Databases, so you still have plenty of people screaming not to do it all, making things even more confusing. What we have now in our industry is different shops denormalizing in different ways for different reasons.

The arguments that I have heard in my career boil down to two basic groups. The first set of arguments centers around calculated or derived values, and the second set centers around programmer convenience.

Arguments for Derived Values

My own experience comes down heavily in favor of denormalizing by storing derived values directly into the tables, with the extremely signficant caveat that you must have a way to ensure that they are always correct. In this paradigm you maintain strict normalization for facts supplied from the outside, and then layer on additional facts that are calculated during write operations and saved permanently.

Here is a very simple example. A strictly normalized database happens to be missing data that many programmers would automatically assume should be stored. Believe it or not, a simple value in a shopping cart like EXTENDED_PRICE is forbidden by 3rd normal form because it is a non-key dependency, or, in plain English, since it can be derived from other values (QTY * PRICE), then it is redundant, and we no longer have each fact stored in exactly one place. The value of EXTENDED_PRICE is only correct if it always equals QTY * PRICE, and so there is now a "fact" that is spread across three locations. If you store EXTENDED_PRICE, but do not have a way to ensure that it will always 100% of the time equal QTY * PRICE, then you will get bad data.

So, given the risk of bad data, what is to be gained by putting EXTENDED_PRICE into the cart? The answer is that it adds value to the database and actually simplifies application code. To see why, imagine a simple eCommerce shopping cart that does not store any derived values. Every single display of the cart to the user must go all over the place to gather lots of details and recalculate everything. This means re-calculating not just the EXTENDED_PRICE, but adding in item level discounts, taking account of possible tax exemptions for different items, rolling the totals to the cart, adding in tax, shipping, perhaps a customer discount, a coupon, and who knows what else. All of this just to display the cart, every time, no matter what the purpose.

This situation leads to three problems. A pitifully slow application (too many disk reads and lots of cycles calculating the values), maddening bugs when an application update has subtle changes to the calculations so the customer's order no longer displays the same numbers as it did yesterday, and the frustrating requirement that the simplest of reports must route through application code to calculate these values instead of simply reading them off the disk, which leads to reporting systems that are orders of magnitude slower than they could be and horribly more complicated than they need to be because they can't just read straight from the tables.

Now let's look at how that same shopping cart would be used if all of those calculated values were generated and saved when the order is written. Building on your foundation of normalized values (price, qty), you need only one body of code that has to perform calculations. This magic body of code takes the user-supplied values, adds in the calculations, and commits the changes. All other subsequent operations need only to read and display the data, making them faster, simpler, and more robust.

So the obvious question is how to make sure the derived values are correct. If they are correct, we gain the benefits with no down side. If there is the smallest chance of bad data, we will quickly pay back any benefit we gained by chasing down the mistakes.

From a technical standpoint, what we really need is some technology that will make sure the calculations cannot be subverted, it cannot be possible for a stray bit of program code or SQL Statement to put the wrong value in for EXTENDED_PRICE. There are a few generally accepted ways to do this:

  • Require all writes to go through a certain codepath. The only PRO here is that you keep the logic in the application code, and since most shops have more programmers than database people, this makes sense. The only CON is that it never works. One programmer working alone can maintain discipline, but a team cannot. All it takes is one programmer who did not know about the required codepath to screw it all up. Also, it makes your system inflexible, as it is no longer safe to write to the database except through a single application.
  • Require all writes to go through stored procedures. This is nominally better than the codepath solution because it is not subvertible, and you can allow different side apps and utilities to safely write to the database. But it makes a lot of work and tends to be very inflexible.
  • Putting triggers onto tables that perform the calculations and throw errors if a SQL statement attempts to explicitly write to a derived column. This makes the values completely non-subvertible, ensures they will always be correct, and allows access from any application or utility. The downside is that the triggers cannot be coded by hand except at extreme cost, and so must be generated from a data dictionary, which is fairly easy to do but tends to involve extreme psychological barriers. In these days of ORM many programmers mistakenly believe their class files define reality, but this is not true. Reality is defined by the users who one way or another create the paychecks, and by the database, which is the permanent record of facts. But a programmer who thinks his classes define reality simply cannot see this and will reject the trigger solution for any number of invalid reasons.

So denormalizing by putting in derived values can make a database much more valuable, but it does require a clear systematic approach to generating the derived values. There is no technical problem associated with ensuring the values are correct because of course the application has to do that somehow somewhere anyway, the real barriers tend to be the psychological and political.

Arguments For Programmer Convenience

The second set of arguments for denormalization tend to be rather weak, and come down to something like this (you have to picture the programmer whining like a child when he says this), "I don't like my data scattered around so many tables, can't we play some other game instead?"

Many programmers, when they first learn about normalization and build a normalized database, discover that the data they need to build a screen is "scattered" about in many tables, and that it is tedious and troublesome to get it all together for presentation to the user. A simple example might be a contacts list. The main table is CONTACTS, and it contains not much more than first and last name. A second table is a list of PHONES for each contact, and a third table is a list of various mailing addresses. A fourth table of EMAILS stores their email addresses. This makes four tables just to store a simple contact! We programmers look at this and something inside of us says, "That's just way too complicated, can't I do something else instead?"

This is a case of programmer convenience clashing with correctness of data. Nobody argues (at least not that I've heard) that they do not want the data to be correct, they just wonder if it is possible to simplify the tables so that they do not have to go out to so many places to get what they need.

In this case, programmers argue that denormalization will make for simpler code if they deliberately skip one or more steps in the normalizing process. (Technically I like to call the result a "non-normalized" database instead of denormalized, but most people call it denormalized, so we will go with that.)

The argument goes something like this: I know for a fact that nobody in the contacts list will have more than 3 emails, so I'm going to skip the EMAILS table and just put columns EMAIL1, EMAIL2, and EMAIL3 into the main CONTACTS table. In this case, the programmer has decided to skip 1st Normal Form and put a repeating group into the CONTACTS table. This he argues makes for simpler database retrieval and easier coding.

The result is painfully predictable. The simplification the programmer sought at one stage becomes a raft of complications later on. Here is an example that will appear trivial but really gets to the heart of the matter. How do you count how many emails a user has? A simple SELECT COUNT(*)...GROUP BY CONTACT that would have worked before now requires more complicated SQL. But isn't this trivial? Is it really that bad? Well, if all you are coding is a CONTACTS list probably not, but if you are doing a real application with hundreds of tables and this "convenience" has been put out there in dozens of cases, than it becomes a detail that programmers need to know on a table-by-table basis, it is an exception to how things ought to be that has to be accounted for by anybody who touches the table. In any shop with more than 5 programmers, whatever convenience the original programmer gained is lost quickly in the need to document and communicate these exceptions. And this is only a single trivial example.

Other examples come when it turns out you need more than three slots for phone. In the normalized case this never comes up. Any user can have any number of phones, and the code to display the phones is running through a loop, so it does not need to be modified for the case of 1 phone, 2 phones, etc. But in the "convenient" denormalized case you now must modify the table structure and the code that displays the contacts, making it quite inconvenient.

Then you have the case of how to define unused slots. If the user has only one email, do we make EMAIL2 and EMAIL3 empty or NULL? This may also seem like a silly point until you've sat through a flamewar at the whiteboard and discovered just how passionate some people are about NULL values. Avoiding that argument can save your shop a lot of wasted time.

In short, programmer convenience should never lead to a shortcut in skipping normalization steps because it introduces far more complications than it can ever pay for.

Related Essays

This blog has two tables of contents, the Topical Table of Contents and the list of Database Skills.

The normalization essays on this blog are:

Friday, November 19, 2010

Prepare Now For Possible Future Head Transplant

This is the Database Programmer blog, for anybody who wants practical advice on database use.

There are links to other essays at the bottom of this post.

This blog has two tables of contents, the Topical Table of Contents and the list of Database Skills.

Planning For The Unlikely

We programmers love to plan for things that will hardly ever happen, like coding the system's upgrade engine to handle spontaneous human combustion, making sure the SQL scrubbing layer can also launch a rocket into space, and, well, trying to work out ahead of time what to do if we ever need a head transplant.

The boss comes over and says, "can you toss a simple plot onto the sales' staff home page that shows sales by day? Use that 'jquicky' or whatever you call it. Should take a couple of hours, right?" And three days later we're working on the world's greatest plotting system that can report everything except what the boss actually asked for because we haven't gotten around to that part of it yet. (Really, can he expect me to just bang this out without the required Seven Holy Layers of Abstraction and Five Ritual Forms of Parameterization, and the Just and Wholesome Mobile Support, or the features Not Yet Required but visible to the Far Seeing Wise and Learned Men?)

Abstraction Contraptions

So what I am getting at is that programmers of all stripes are addicted to abstraction, it gives us goosebumps and makes us feel warm and tingly, and so we do it even when we do not need to. We build abstraction contraptions.

When it comes to designing a database, this unhealthy proclivity can seriously slow you down, because of what I call:

Ken's Law

Everybody wants to be remembered for something. If I could write my own epitaph, it might be:

Table-based datastores are optimally abstract

This law is not about database access, it is about database design. It can be expressed informally as:

People Understand Tables Just Fine

Or more rigorously as:

Table-based datastores are optimally abstract; they require no additional abstraction when requirements are converted to desgin; they cannot be reduced to a less abstract form.

Structured Atomic Values

I should point out that this essay deals with structured atomic values, who live in the Kingdom of The Relational Database. The concepts discussed here do not apply to free-text documents or images, or sound files, or any other media.

No Additional Abstraction Required

My basic claim here is that you cannot create an abstraction of data schemas that will pay for itself. At best you will create a description of a database where everything has been given a different name, where tables have been designated 'jingdabs' and columns have been designated 'floopies' and in the end all of your jingdab floopies will become columns in tables. Oh, and I suppose I should mention the Kwengars will be foreign keys and the Slerzies will be primary keys.

After that it goes downhill, because if we generate an abstraction that is not a simple one-to-one mapping, we actually obscure the end goal. Consider an example so simple as to border on the trivial or absured. Why would we ever use the terms "One-to-Many" or "Many-to-Many" when the more precise terms "child table" and "cross-reference table" convey the same idea without the noise? I said above that this would sound trivial, and you can accuse me of nit-picking, but this is one of those camel's nose things, or perhaps a slippery slope. When technical folk get together to design a system, we should call things what they are, and not make up new words to make ourselves sound smarter.

No de-Abstraction is Possible

The second half of Ken's law says that you cannot de-Abstract a table schema into some simpler form. This should be very easy to understand, because relational databases deal with atomic values, that is, values which cannot themselves be decomposed. If you cannot decompose something, then it cannot be an abstraction of something more specific.

Going further, if the schema has been normalized, then every fact is stored in exactly one place, so no further simplification is possible. If you cannot simplify it or resolve it into something more specific, then it is not an abstraction of something else.

But Does it Work?

I originally began to suspect the existence of what I call in all humility "Ken's Law" when I was sitting at a large conference table with my boss, her boss, a couple of peers, and 3 or 4 reps from a Fortune 5 company. My job was basically to be C3PO, human-cyborg relations. Some people at the table protested loudly to being non-technical, while others had technical roles. But everybody at the table spent all day discussing table design.

Later on, when at a different position, the programmers received their instructions from Project Managers. The best Project Managers worked with their customers to figure out what they were trying to keep track of, and handed us specs that were basically table layouts. The customers loved these guys because they felt they could "understand what the project manager was talking about", and the project managers, who swore they were not technical, were respected because they handed us requirements we could actually understand and implement.

Since that time I have further learned that it is very easy for anybody who deals with non-technical people to bring them directly to table design without telling them you are doing it. All you have to do is listen to what words they use and adopt your conversation accordingly. If they say things like "I need a screen that shows me orders by customer types" they have told you there will be a table of customer types. Talk to them in terms of screens. If they say, "Our catalog has 3 different price list and four discount schemes" then you know that there will be a PRICELIST table, a DISCOUNTS table, and likely some cross-references and parent-child relationships going on here.

So How Does ORM Come Into This?

One of the greatest abstraction contraptions of this century (so far), is ORM, or Object-Relational Mapping, which I do not use precisely because it is an abstraction contraption.

To be clear, the mistake that ORM makes is not at the design phase, but at the access phase. The ORM meme complex instructs its victims that it is ok to put structured atomic values into a Relational Database, but when it comes time to access and use that data we will pretend we did not put it into tables and we will pretend that the data is in objects. In this sense the term Object- Relational Mapping is a complete misnomer, because the point is not to map data to objects but to create the illusion that the tables do not even exist.

Perhaps ORM should stand for Obscuring Reality Machine.

Getting Back to That Head Transplant

So what about that weird title involving head transplants? Obviously a head transplant is impossible, making it also very unlikely, besides being silly and non-sensical. It came to mind as a kind of aggregrate of all of the bizarre and unrealistic ideas about abstracting data designs that I have heard over the years.

One of these ideas is that it is possible and beneficial to create a design that is abstract so that it can be implemented in any model: relational, hierarchical, or network. I'm not saying such a thing is impossible, it is likely just a small matter of programming, but for heaven's sake, what's the point?


So don't waste time creating abstractions that add steps, possibly obscure the goal, and add no value. Don't plan for things that are not likely to happen, and avoid abstraction contraptions.

Related Essays

This blog has two tables of contents, the Topical Table of Contents and the list of Database Skills.

Other philosophy essays are:

Saturday, November 13, 2010

Database Skills

It seems strange to me that I've been working on this blog for 3 years or so (with one very long break) and somehow never got around to writing a simple list of skills that all database experts need. So here it is!

Various Job Tiles for Database People

There are three common job titles in the database area, which are Database Administrator (DBA), Database Programmer, and Database Architect. These titles tend to be somewhat variable from shop-to-shop, but generally the "Architect" term indicates the highest level of skill combined with considerable management responsibilities. The "Programmer" term is somewhere below that, but the "DBA" is extremely variable. I have seen shops where a person was called a DBA and filled a relatively constrained role closer to IT or operations (routine tasks, no real programming) and other shops where a person with the DBA title was basically the Architect.

Because of this high variability in what titles mean, I am not going to waste time categorizing skills as belonging to one job title or another, I am simply going to list them all out.

The various levels of skills are these:

  • Before Hello World!: The basics of tables, columns, rows
  • The Hello World! Level: SQL Select
  • Just after Hello World!: Writing Data
  • Commands to create, modify and drop tables, or Data Definition Language (DDL)
  • Knowing how to use a Query Analyzer or optimization tool
  • Understanding Normalization
  • Understanding Denormalization
  • Understanding Primary Keys, Foreign Keys and Constraints
  • Understanding Transactions
  • Understanding ACID
  • Understanding Indexes as optimization tool
  • Views
  • Database Security
  • Upgrades and Installs
  • Efficient access of database from application
  • Bulk operations: loading or exporting large amounts of data
  • Understanding of Contexts and how they lead to different sets of Best Practices
  • Preventing performance degradation through various maintenance tasks
  • Deployment strategies: partitioning, tablespaces
  • Deployment strategies, failure protection, from simple backup to hot standbys
  • Server side coding: stored procedures and functions
  • Server side coding: triggers
  • Temporary tables

As long as that list is, it only covers those of us who use database systems. There is an entire set of skills for those who actually create and maintain these systems, but that is not something that will be treated in this blog.

Before Hello World!: Tables and Columns

If you have never so much as typed a single SQL command, or seen a table diagram, or anything like that, then it is worth a few minutes to go through the basics of what a database does, which is to organize atomic values into tables.

I am going to write an essay on this soon, even though it may seem so obvious as to be completely unnecessary. But I will do it because the most popular essay on this blog is about using GROUP BY, which tells me newer programmers are starving for useful tutorials at the beginner level. So it seems to me, why not put something out there at the very beginning of the beginning?

The Hello World! Level: SQL Select

If you are starting completely from scratch and want to know about database programming, you want to start with the SQL SELECT command. This is the (almost) only command used to extract data from a database, and all of the possible ways to combine, filter and extract data are expressed in the many clauses of this command.

Just after Hello World!: Writing Data

When it comes time to change the data in a database there are three commands, listed below. These commands are based on the tables-and-rows nature of databases, and allow to add a row (or rows), change a row (or rows) and delete a row (or rows).

  • The INSERT command
  • The UPDATE command
  • The DELETE command

Commands to create, modify and drop tables, or Data Definition Language (DDL)

The term "DDL" stands for "Data Definition Language" and includes all of the commands use to build the tables that will hold the data for the INSERT, UPDATE, DELETE and SELECT statements. The basic list of commands to be familiar with is:

  • Understanding Data Types (databases are strongly typed)
  • Commands to add and drop primary keys
  • Commands to add and drop foreign keys
  • Commands to add and drop constraints
  • Commands to add and drop indexes

There are also numerous commands that are specific to different products. Those will not be listed here today, but who knows what the future may bring.

Knowing how to use a Query Analyzer or optimization tool

Database programmers, once they get started with the skills listed above, tend to become more and more obsessed with performance. Every major database has some type of tool that lets you examine how the server is going to process a SQL SELECT, and database programmers depend on these tools to discover where they might alter tables or indexes or the SELECT itself to make the queries go faster.

Understanding Normalization

The term "normalization" refers to the process of analyzing the data that your system is required to store, and organizing it so that every fact is stored in exactly one place. Understanding how to normalize data is an absolute requirement for the database programmer who wants to design databases.

We speak of normalization in "forms" as in "first normal form", "second normal form", and so on. It is a good idea to understand The argument for normalization and then to pursue at very least:

Normalization is a a fascinating topic to study, and it extends all they way up to "Domain-key Normal Form" which is considered the most complete normalization for a database.

Understanding Denormalization

Every database programmer, after fully understanding normalization, realizes that there are severe practical problems with a fully normalized database, such a database solves many problems but generates problems of its own. This has led programmer after programmer down the path of denormalization, the deliberate re-intoduction of redundant values to improve the usability of the database.

There is a surprising lack of material available on the web regarding denormalization strategies. Most of what you find is arguments and flame wars about whether or not to do it, with little to nothing on how to actually do it. For this reason, I provide my own essays on this blog on the strategies and methods I have worked out over the years:

After reviewing The Argument For Denormalization it is worthwhile to follow up with:

The arguments for and against denormalization are heavily affected by the Pay me now or pay me later design tradeoff.

Understanding Primary Keys, Foreign Keys and Constraints

One might argue that this list of skills belongs much higher up the list, up there with the CREATE TABLE command. However, I have it useful to distinguish between simply knowing the commands to make a primary key and actually understanding the tremendous power of keys.

In this author's opinion it is not truly possible to understand how powerful and beneficial Primary keys and Foreign Keys are for an entire application stack until you have learned the commands, built some databases, and worked through the concepts of normalization and denormalization. Only then can you revisit these humble tools and realize how powerful they are.

Understanding Transactions

The word "transaction" has two meanings in common day-to-day database talk. One meaning is very loose and refers to some individual command or set of commands. You might hear somebody using the term loosely when they say, "We're seeing about 10 transactions per second this week."

The more rigorous use of the term refers to a statement or set of statements that must be guaranteed to either complete in their entirety or fail in their entirety. This is a profoundly important concept once you get beyond simply making tables with keys and get into real-world heavy multi-user activity. And this leads us to the next topic...

Understanding ACID

Modern relational databases expect multiple simultaneous users to be writing and reading data all of the time. The term "ACID Compliance" refers to both the philosophy of how to handle this and the actual methods that implement that philosophy. The term ACID refers to:

  • The Atomic nature of each transaction
  • The Consistentcy of the database during and after simultaneous overlapping transactions
  • The Isolation of each transaction
  • The Durability of the results

Understanding Indexes as optimization tool

An index is a special tool databases use to provide very rapid access to large amounts of data. Just like keys, it is not enough to know the commands, it is necessary to understand the subtle power of indexes when used with some craftsmanship. The basic uses of indexes are:

  • A simple index on a column to provide rapid search on that column
  • A "covering index" that includes extra columns that can further speed up certain common access patterns
  • Clustered indexes (MS SQL Server) and what they give and what they take away
  • The cost of indexes on write operations


A view looks like a table to the SQL SELECT command. The view itself is a stored SQL SELECT command that encodes some query that is either used very often or is very compex. In all cases, views are used to present the database data to the application in some simplified convenient or secure form. The two major uses of views are:

  • To simplify the application programmer's job
  • To provide a read-only interface for some applications

Upgrades and Installs

If you are a single programmer or hobbyist working with a database, it is all well and good to just add and drop tables as you wish. But as soon as you get into development with quality control stages and multiple programmers, it becomes evident that you need a strategy for handling the schema changes that come with with new versions of the system. There are multiple essays available on this blog, covering:

Database Security

Databases provide incredible security provisions that are just about completely ignored by modern web developers. Sometimes there is good reason for this, but overall anybody who wants to become a truly accomplished Database Programmer or Database Architect must have a thorough understanding of database security and how it can simplify the entire system stack.

Database security comes down to specifying who is allowed to perform the 4 basic operations of INSERT, UPDATE, DELETE and SELECT against which tables:

My basic introduction to security is here.

  • Understanding roles (we used to say users and groups)
  • Simple table-level security
  • Column-level security (not widely supported)
  • Row-level security (not widely supported)

Efficient access of database from application

Imagine you have the perfectly designed database, with every nuance and subtlety excellently crafted in the ares of keys, indexes, normalization, denormalization and security. At this point your job branches out into several new areas, but one of the most important is knowing how to write application code that efficiently accesses the database.

Bulk operations: loading or exporting large amounts of data

Some database applications involve a large number of small transactions, where each trip to the database writes only a single row or reads only a dozen or so rows.

But in many cases you need to bulk load large amounts of data in one shot, thousands or even millions of rows. In these cases the techniques that govern small transactions are useless and counter-productive, and you need to learn some new commands and strategies to handle the bulk loads.

Understanding Contexts and how they lead to different sets of Best Practices

Not all databases are created for the same purpose. If you have a very large operations then it will likely have multiple independent databases that fill the classical roles, while in a smaller shop the roles may be combined in one database. I like to refer to these roles as "contexts" because they determine how the tables will be designed and how acess to the tables will be governed. The major contexts are:

  • OLTP or Online Transaction Processing, characterized by simultaneous reads and writes, generally assumes little or no periods of inactivity, and generally assumes that the individual transactions are very small. The apps we were all writing in the 80s and 90s to do accounting, ERP, MRP, Job control, payroll, airline reservations and many others fall into this context.
  • Data Warehouse context, characterized by periodic bulk loads of new information with most activity being reads. The Data Warehouse context is largely associated with the "Star Schema" table design. Data in a Warehouse is historical, it never changes after it is loaded.
  • CMS or Content Management System, also characterized by very few writes compared to reads, but more likely to have a normalized structure. Unlike a Data Warehouse, the data is subject to change, just not that often.
  • Any other Read Only Context. I include this category because I spent some time working on Direct Marketing databases, which are like a Data Warehouse in that they are updated periodically and the data does not change, but the Star Schema is completely inappropriate for them.

If you consider a huge online shopping system, you can see that within that application there are at least two contexts. The product catalog is likely to see vastly fewer writes than reads, but the shopping cart tables will be in a constant state of reads and writes.

Preventing performance degradation through various maintenance tasks

Once the database and its application stack is up and running, and the reads and writes and coming through, the laws of thermodynamics come into play and system performance can begin to degrade even if the database stays the same size and the load on the system is steady.

Different vendors have different tools for combatting this, but they tend to come down to reclaiming temporary space and occassionally rebuilding indexes. There are also log files that have to be purged, regular backups to be made, and other operations along those lines.

Deployment strategies: partitioning, tablespaces

When systems become sufficiently large, it is no longer possible to just attach some disks to a box and run a database server. The Database Architect must consider breaking different tables out onto different sets of spindles, which is usually done with "tablespaces", and moving older data onto slower cheaper spindles, which is often done with Partitioning.

Deployment strategies, failure protection, from simple backup to hot standbys

Because a database typically experiences simultaneous reads and writes from multiple sources, and may be expected to be up and running 24/7 indefinitely, the concept of making a backup and recovering from a failure becomes more complicated than simply copying a few files to a safe location.

In the most demanding case, you will need to provide a second complete box that can become fully live within seconds of a disastrous failing of the main box. This is called various things, but Postgres calls it a "hot standby" in version 9 and some MS SQL Server shops call it a "failover cluster."

The ability to come up live on a second box when the first one fails is made possible by the way databases handle ACID compliance, and the fact that they produce something called a Write-Ahead-Log (WAL) that can be fed into a second box that "replays" the log so that its copy of the database is getting the same changes as the master copy.

Server side coding: stored procedures and functions

I really could not figure out where to put this entry in the list, so I just punted and put it near the end. It could really go anywhere.

Stored procedures or functions are procedural routines (not object oriented) that are on the database server and can be invoked directly from an application or embedded inside of SQL commands. Generally speaking they provide various flow-control statements and rudimentary variable support so that you can code multi-step processes on the server itself instead of putting them in application code.

Server side coding: Triggers

Triggers are quite possibly the most elegant and beautiful technology that servers support, absolutely the least understood, and definitely the most reviled by the ignorant. You will find virtually no web content today that will explain why and how to use triggers and what they are good for.

Except of course for my own essay on triggers that discusses them in terms of encapsulation.

Temporary tables

Temporary tables are like Stored Procedures inasmuch as I had no idea where to put them in the list, so they just ended up at the end.

As the name implies, a temporary table is a table that you can create on-the-fly, and which usually disappears when your transaction is complete. They are most often found in Stored Procedures. They can impact performance for the worst in many ways, but can be extremely useful when you are doing multi-staged analsysis of data in a Data Warehouse (that's where I use them the most).

Saturday, November 6, 2010

Recursive Queries with Common Table Expressions

This week The Database Programmer returns after almost 18 months with an entry on using Common Table Expressions (CTEs) to do recursive queries. Relational databases were plagued from their inception with a lack of meaningful treatment for recursive operations, and CTEs have finally plugged that hole.

Common Table Expressions appeared in SQL Server 2005, and in PostgreSQL 8.4, and are also available in Oracle. As for mySQL, since I don't use it, I did a quick Google search and looked at the Docs for 5.5, and couldn't really find anything. I generally tend to assume mySQL cannot do the tough stuff.

But First, A Rant

There have always been plenty of people who claimed SQL was a bloated and clumsy language to work with. Most of the time I tend to agree, but I find the advantages of relational/SQL system to be so large that I'm willing to pay that price.

But with Commom Table Expressions (CTEs) I just can't help drifting into conspiracy theories involving the enemies of SQL infiltrating the committees and deliberately suggesting the most twisted, bloated, complicated way they could think of to do what is really a very basic operation. In other words, I am profoundly unimpressed with the syntax of CTEs, but as long as they are here and they work, we'll go along.

The Basic Example

Your basic recursive table contains a foreign key to itself, so that some rows in the table are children of some other row in the table. This recursion can nest to any depth, and the chart below shows a very simple example:

Primary_key   |   Parent_Key  |  Notes  
     A        |     null      |   top level row, no parent
     B        |      A        |   first level child of A
     C        |      B        |   child of B, grandchild
              |               |   of A
     D        |      C        |   child of C, grandchild 
              |               |   of B, great-grandchild 
              |               |   of A
     X        |     null      |   top level row, no parent
     Y        |      X        |   child of X
     Z        |      Y        |   child of Y, grandchild 
              |               |   of X

What we want is a query that can return a given row and all of its children out to any level, including helpful information about the structure of the recursion, something like this:

Primary_key | Level | Top_Key | Immediate_Parent_key 
     A      |   1   |  A      | null
     B      |   2   |  A      | A    
     C      |   3   |  A      | B   
     D      |   4   |  A      | C
     X      |   1   |  X      | null
     Y      |   2   |  X      | X   
     Z      |   3   |  X      | Y   

And Another Rant

At this point the mind boggles at how long this blog entry needs to be to explain this simple operation. But lets get going anyway.

The First Step and Last Step

A Common Table Expression begins with the "WITH" clause and ends with a standard SQL Select:

;WITH myCTEName (primary_key,level,top_key,immediate_parent_key)
as (
  ....we'll get to this below....
select * from myCTEName

The basic idea is that we are going to define a CTE with a name and a list of columns, and then SELECT out of it. Without that final SELECT statement the CTE does not actually do anything. The SELECT can also be arbitrarily complex, doing aggregrations, WHERE clauses and anything else you might need.

The first thing to notice is the leading semi-colon. This is a trick adopted by MS SQL Server users. SQL Server does not require statements to be terminated with a semi-colon, but a SQL Server CTE requires the previous statement to have been terminated with a semi-colon (nice huh?). So SQL Server programmers adopted the strategy of starting the CTE with a semi-colon, which keeps the syntactical requirement with the CTE, where it belongs.

A given CTE sort of has a name. That is, you have to name it something, but think of it as a table alias in a SQL SELECT, such as "Select * from myTable a JOIN otherTable b...", it exists only during the execution of the statement.

The columns listed in the parantheses can have any names (at least in SQL Server). But these column names are what you will refer to in the final SQL SELECT statement.

Coding The Inside of the CTE, Step 1

Now we code the inside of the CTE in two steps. The first step is called the "anchor", and it is a straightforward query to find the top-level rows:

;WITH myCTEName (primary_key,level,top_key,immediate_parent_key)
as (
    select primary_key   as primary_key
         , 1             as level
         , primary_key   as top_key
         , null          as immediate_parent_key
      from myRecursiveTable
     where Parent_key is null
select * from myCTEName

This should be self-explanatory, we are querying only for rows that have no parent (WHERE Parent_key is null) and we are hardcoding the "level" column to 1, and we are also hardcoding the "immediate_parent_key" column to null.

This query alone would return two of the rows from our desired output:

Primary_key | Level | Top_Key | Immediate_Parent_key 
     A      |   1   |  A      | null
     X      |   1   |  X      | null

Coding The Inside of the CTE, Step 2

Now we are going to add the actual recursion. When I first learned CTEs this was the hardest part to figure out, because it turned out my hard-won set-oriented thinking was actually slowing me down, I had to think like a procedural programmer when defining the second half of the query.

;WITH myCTEName (primary_key,level,top_key,immediate_parent_key)
as (
    select primary_key,1,primary_key,null
      from myRecursiveTable
     where Parent_key is null
    select chd.primary_key,par.level+1,par.top_key,chd.parent_key
      FROM myCTEName        par
      JOIN myRecursiveTable chd ON chd.parent_key = par.primary_key
select * from myCTEName

Thinking step-wise, here is what is going on under the hood:

  1. The server executes the "anchor" query, generating a result set called "myCTEName" containing just the top level rows.
  2. The server then executes the second query. At this point the result set "myCTEName" exists and can be referenced, so that you can link children to their parents. (That's why you see the JOIN)
  3. Step 2 is repeated recursively, adding grand-children, great-grand-children, and so on, until no more rows are being added, at which point it stops, and...
  4. The final result set is passed to the trailing SELECT, which pulls results out of "myCTEName" as if it were a table or view.

So when we code the 2nd part of the inside of the CTE, the part after the UNION ALL, act as if the first query has already run and produced a table/view called "myCTEName" that can be referenced. Once you understand that, the query is pretty easy to understand:

  • The "From myCTEName par" clause tells us we are pulling from the previously generated set. I like to use the alias "par" for "parent" to remind myself that the prior result is the parent row.
  • We then join to the original source table and use the alias "chd" to remind ourselves we are pulling child rows from there. The "ON chd.parent_key = par.primary_key" defines how children are joined to parents.
  • Our first column, "chd.primary_key", is the unique key for the results.
  • Our second column, "par.level+1" gives us a nifty automatically incremented "level" column.
  • Our third column, "par.top_key" ensures that all rows contain a reference to their top-most parent.
  • Our final column, "chd.parent_key", makes sure each row contains a reference to its immediate parent.

Finding Various Statistics

Once you have the inside of the CTE coded, the fun part moves to the final SELECT, which is operating on the complete set of results. You do not necessarily have to pull the complete list. For instance, you may want to find out the maximum nesting level for each parent, or the count of children for each parent:

;WITH myCTEName (primary_key,level,top_key,immediate_parent_key)
as (
    select primary_key,1,primary_key,null
      from myRecursiveTable
     where Parent_key is null
    select chd.primary_key,par.level+1,par.top_key,chd.parent_key
      FROM myCTEName        par
      JOIN myRecursiveTable chd ON chd.parent_key = par.primary_key
select top_key
     , max(level) as maxNestingLevel
     , count(*)   as countRows
     , count(*)-1 as countChildren
  from myCTEName


Common Table Expressions give SQL-based databases the (very) long-needed ability to execute recursive queries, albeit with a rather complex syntax. Once you grasp the basics of how to code them, there are many possible uses that go far beyond the simple example shown here.