Thursday, February 11, 2010

How Do You Begin a SQL Script?

Have you ever noticed that, regardless of where you work, there's always some kind of "standard header" for SQL scripts? Sometimes, it's a formal, documented set of T-SQL commands, with clear goals and a well thought out implementation.

More often, though, it's like that Star Trek episode where everyone's forgotten the meaning of the Preamble to the United States Constitution, but can still kinda grunt the words? In the same way, a few lines of T-SQL, copied from file to file to file, can devolve into, essentially, a religious ritual. (I don't know what all this does, but our other scripts do it, so I better too!)

After getting bitten by a misuse of SET QUOTED_IDENTIFIER, I sat down to research the issue. Before discussing the code, here's what I came up with; this is the code I use, with occasional modifications, to start most of my scripts:

-- MUST be set as shown to support indexes on computed columns, indexed views. 
SET ANSI_NULLS ON                                 -- Deprecated: leave set ON. 
SET ANSI_PADDING ON                               -- Deprecated: leave set ON. 
SET ANSI_WARNINGS ON                              -- No trailing blanks saved. 
SET ARITHABORT ON                                 -- Math failure not ignored. 
SET CONCAT_NULL_YIELDS_NULL ON                    -- NULL plus string is NULL. 
SET NUMERIC_ROUNDABORT OFF                        -- Allows loss of precision. 
SET QUOTED_IDENTIFIER ON                          -- Allows reserved keywords. 

-- These aren't, strictly speaking, required, but are generally good practice. 
SET NOCOUNT ON                                    -- Minimize network traffic. 
SET ROWCOUNT 0                                    -- Reset in case it got set. 
SET XACT_ABORT ON                                 -- Make transactions behave. 

IF DB_NAME() IN ('master', 'tempdb', 'model', 'msdb')                  RAISERROR('   $$$ YOU ARE ATTACHED TO A SYSTEM DB $$$',   20, 1) WITH NOWAIT, LOG
IF @@TRANCOUNT <> 0                                                    RAISERROR('   $$$ OPEN TRANSACTION EXISTS $$$',           20, 1) WITH NOWAIT, LOG

Let's divide this code into three sections and examine them one at a time.

Standard SET commands

This section was easy to develop, because they are all required if you want to have indexed views (or indexed calculated columns) in your database. I came to that conclusion when I researched each command independently; only when I was finished did I notice this in BOL 2008:

ANSI_NULLS is one of seven SET options that must be set to required values when dealing with indexes on computed columns or indexed views. The options ANSI_PADDING, ANSI_WARNINGS, ARITHABORT, QUOTED_IDENTIFIER, and CONCAT_NULL_YIELDS_NULL must also be set to ON, and NUMERIC_ROUNDABORT must be set to OFF.

Optional SET commands

These three commands are "defensive" (read, "paranoid") programming; ideally, you'd never have to do things like this, but better safe than unemployed.

Critical error checks

I like to do these to prevent run-time errors; note that I'm raising error level 20, which severs the session's connection, because if these errors occurs, I want execution to stop now.

If you've ever accidentally run your "create a bunch of tables" script against the master database, you'll appreciate the first line. It just checks the current database against the well-known names of the system databases. (Curiously, I couldn't find an "is_system" column in the system views for databases.)

The second critical error check is a simple SQL Server version check - to me there's nothing worse than downloading a cool script and having it crash because it requires version n + 1.

The final check is just to be sure there's not a transaction left often. This is the line of code I often end up commenting out in production, but during development it's nice to have, as it prevents those annoying "deadlocks" in SSMS when you forget to close a transaction.

So, these are what I use - did I miss a favorite of yours?


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