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SQL Server TSQL Coding Conventions, Best Practices, and Programming Guidelines

By : Vyas Kondreddi
May 10, 2001

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Always use a column list in your INSERT statements. This helps in avoiding problems when the table structure changes (like adding or dropping a column). Here's an example which shows the problem.

Consider the following table:

CREATE TABLE EuropeanCountries
(
CountryID int PRIMARY KEY,
CountryName varchar(25)
)

Here's an INSERT statement without a column list , that works perfectly:

INSERT INTO EuropeanCountries
VALUES (1, 'Ireland')

Now, let's add a new column to this table:

ALTER TABLE EuropeanCountries
ADD EuroSupport bit

Now run the above INSERT statement. You get the following error from SQL Server:

Server: Msg 213, Level 16, State 4, Line 1
Insert Error: Column name or number of supplied values does not match table definition.

This problem can be avoided by writing an INSERT statement with a column list as shown below:

INSERT INTO EuropeanCountries
(CountryID, CountryName)
VALUES (1, 'England')

Perform all your referential integrity checks and data validations using constraints (foreign key and check constraints) instead of triggers, as they are faster. Limit the use triggers only for auditing, custom tasks and validations that can not be performed using constraints. Constraints save you time as well, as you don't have to write code for these validations, allowing the RDBMS to do all the work for you.

Always access tables in the same order in all your stored procedures and triggers consistently. This helps in avoiding deadlocks. Other things to keep in mind to avoid deadlocks are: Keep your transactions as short as possible. Touch as few data as possible during a transaction. Never, ever wait for user input in the middle of a transaction. Do not use higher level locking hints or restrictive isolation levels unless they are absolutely needed. Make your front-end applications deadlock-intelligent, that is, these applications should be able to resubmit the transaction incase the previous transaction fails with error 1205. In your applications, process all the results returned by SQL Server immediately so that the locks on the processed rows are released, hence no blocking.

Offload tasks, like string manipulations, concatenations, row numbering, case conversions, type conversions etc., to the front-end applications if these operations are going to consume more CPU cycles on the database server. Also try to do basic validations in the front-end itself during data entry. This saves unnecessary network roundtrips.

If back-end portability is your concern, stay away from bit manipulations with T-SQL, as this is very much RDBMS specific. Further, using bitmaps to represent different states of a particular entity conflicts with normalization rules.

Always add a @Debug parameter to your stored procedures. This can be of BIT data type. When a 1 is passed for this parameter, print all the intermediate results, variable contents using SELECT or PRINT statements and when 0 is passed do not print anything. This helps in quick debugging stored procedures, as you don't have to add and remove these PRINT/SELECT statements before and after troubleshooting problems.

Do not call functions repeatedly within your stored procedures, triggers, functions and batches. For example, you might need the length of a string variable in many places of your procedure, but don't call the LEN function whenever it's needed, instead, call the LEN function once, and store the result in a variable, for later use.

Make sure your stored procedures always return a value indicating their status. Standardize on the return values of stored procedures for success and failures. The RETURN statement is meant for returning the execution status only, but not data. If you need to return data, use OUTPUT parameters.

If your stored procedure always returns a single row resultset, consider returning the resultset using OUTPUT parameters instead of a SELECT statement, as ADO handles output parameters faster than resultsets returned by SELECT statements.

Always check the global variable @@ERROR immediately after executing a data manipulation statement (like INSERT/UPDATE/DELETE), so that you can rollback the transaction in case of an error (@@ERROR will be greater than 0 in case of an error). This is important, because, by default, SQL Server will not rollback all the previous changes within a transaction if a particular statement fails. This behavior can be changed by executing SET XACT_ABORT ON. The @@ROWCOUNT variable also plays an important role in determining how many rows were affected by a previous data manipulation (also, retrieval) statement, and based on that you could choose to commit or rollback a particular transaction.

To make SQL Statements more readable, start each clause on a new line and indent when needed. Following is an example:

SELECT title_id, title
FROM titles
WHERE title LIKE '%Computer%' AND
title LIKE '%cook%'

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