SQL Server Reference Guide

Mar 28, 2003

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In our last tutorial, we spent a little time explaining the various types of development shops that a DBA might be part of. We found that it's important to learn about database development, even if the role that's called for is maintenance work. After all, how can we figure out if a performance issue is platform-related or just incorrect programming, without knowing a little about each?

Recall that a database program (from the client side) can have as few as three steps:

Create a database, recordset or command object
Create a connection object
Create or execute a command object

Client-side code (such as Visual Basic and Java) is covered in lots of guides, articles, and online books at InformIT and Safari. In our SQL tutorials, we're focusing on the T-SQL code used in Server-side programming, such as stored procedures and trigger logic.

The first T-SQL construct we covered involved comments. As I mentioned earlier, I prefer to write T-SQL code by writing the comments first as pseudo-code (English language steps):

	/* Declare a variable for the string */

	/* Find the date the transfer was created, 
	convert it to a string,
	then assign it to the variable */

	/* Return the string to the calling program */

And then fill in those comment blocks with code:

	/* Declare a variable for the string */
	DECLARE @test varchar(12)
	
	/* Find the date the transfer was created, 
	convert it to a string,
	then assign it to the variable */
	SELECT @test = CONVERT(char(12), GETDATE())
	
	/* Return the string to the calling program */
	SELECT @test

In any case, it's important for you to comment the code, since this makes maintenance of the code (which inevitably happens) easier.

We're going to cover more advanced topics in this tutorial, keeping a broad view. In subsequent tutorials we'll put all these concepts together and develop real-world scenarios, writing useful code.

Scope

Scope, as it refers to programming, means grouping code and its results. This means that the variables (we'll cover those later), commands, and other constructs are limited to operate in a block.

While traditional programming languages implement scope at a very controllable level, SQL code is a bit more monolithic. We don't have subroutines, calls, and other advanced constructs to isolate our code, as other languages do. What we get, instead, are two basic types of isolation: transactions and connections.

Let's examine connections first. Whenever we connect to a SQL Server, the code that runs is ignorant of (and isolated from) code running in another connection. In the code shown below, we start up Query Analyzer and set up a variable and then create another query window (which is really another connection) and set up the same variable. SQL Server isolates the two connections and the variables don't clash:

Window 1:

DECLARE @test1 varchar(30)
SELECT @test1 = 'Hello'
PRINT @test1

Window 2:

DECLARE @test1 varchar(30)
SELECT @test1 = 'Hello'
PRINT @test1

The other construct SQL Server provides for scope is a transaction. A transaction is a set of statements that causes data to be changed (called committed) to the database once they are checked to make sure they would run properly (parsed).

You can use the acronym ACID to help understand the use of transactions:

Atomic: Either all the work is performed or none of it is performed.
Consistent: When the work is complete, the system needs to be completely functional. This means none of the indexes can be left inconsistent, the rules have to be applied, and the general structure of the data must conform to standards.
Isolated: This is precisely what we've been talking about. This means that each transaction doesn't know about other transactions.
Durability: When the work is complete, the data should be changed permanently in the database.

SQL Server natively uses implicit transactions. This means we don't have to tell SQL Server when to start a group of statements and when to end them. If we run a set of T-SQL statements that alter 25 rows of data, the 25 operations happen all at once. If there's a failure in the 24th one, the entire set "rolls back," or resets itself.

We can change this behavior in code, setting explicit transactions. Explicit means that we tell SQL Server when to start a transaction and when to commit it. We do this by enclosing the commands we want to run as a block like this:

BEGIN TRANSACTION
(Run some stuff in here)
COMMIT TRANSACTION

It's that simple. Now why would we want to do this? There are two primary reasons.

The first reason to use transactions is when making massive changes or inserting huge amounts of data. Those changes are implemented as one big transaction, unless we specify otherwise. This causes the transaction log to grow very large, and if there is a failure, the entire load has to be rolled back, which can take a lot of time. Especially during inserts, it's often wiser to break them into smaller chunks of anywhere from 1,000 to 10,000 operations. That way, the transaction log never grows out of control; and, if we need to roll a group of data back, it's a smaller operation.

The second reason to use transaction is to perform error-checking. We'll cover the ins and outs of error handling in our next tutorial.

One more thing before we leave transactions. SQL Server has a way to let us interrogate the current transaction number. In future tutorials we'll use this feature to test for a condition, providing some basic branching capabilities. We can query the current transaction number from the command line by using a construct called a "system" variable:

	SELECT @@TRANCOUNT

System variables always start with @@, and we'll examine another one in our next tutorial.

Online Resources

Even though this SQL.org site deals with Postgres databases, it has a great explanation of the concept of transactions.

InformIT Tutorials and Sample Chapters

Kevin Kline and Baya Pavliashvili have a good article on transactions: SQL Server: Transaction and Locking Architecture.

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