Conditional Logic in SQL Server

SQL Server provides several ways to implement conditional logic within queries and stored procedures. This allows you to execute different code paths based on whether a certain condition is true or false. This tutorial explores the common methods for achieving this, along with best practices for performance and readability.

1. The IF...ELSE Statement

The IF...ELSE statement is a fundamental control flow structure in SQL Server. It allows you to execute a block of code if a condition is true, and an alternative block of code if the condition is false.

IF condition
BEGIN
    -- Code to execute if the condition is true
END
ELSE
BEGIN
    -- Code to execute if the condition is false
END

The BEGIN and END keywords define code blocks. While not always strictly necessary for single-statement blocks, it’s highly recommended to use them for clarity and to prevent errors when adding more statements later.

A common use case is checking for the existence of data in a table. For example:

IF EXISTS (SELECT 1 FROM tblGLUserAccess WHERE GLUserName = 'xxxxxxxx')
BEGIN
    SELECT 1; -- Return 1 if the username exists
END
ELSE
BEGIN
    SELECT 2; -- Return 2 if the username does not exist
END

Note the use of SELECT 1 (or SELECT 2). The EXISTS clause only checks for the presence of any matching rows, so selecting 1 is a common and efficient practice. Avoid SELECT * as it retrieves all columns, which is unnecessary overhead. The TOP 1 keyword can further optimize this, ensuring only one row is checked, although this makes a minimal difference in most cases.

2. The IIF Function

SQL Server provides the IIF function as a shorthand for simple IF...ELSE statements. It’s a ternary operator that returns one value if a condition is true and another value if it’s false.

IIF(condition, value_if_true, value_if_false)

For the username existence check, this can be written as:

SELECT IIF(EXISTS (SELECT 1 FROM tblGLUserAccess WHERE GLUserName = 'xxxxxxxx'), 1, 2);

IIF is concise and often suitable for simple conditions. However, it can become less readable with complex conditions or multiple nested IIF statements.

3. The CASE Statement

The CASE statement offers more flexibility than IIF and is particularly useful when you have multiple conditions to evaluate.

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ...
    ELSE default_result
END

For checking username existence, the CASE statement would look like this:

SELECT
    CASE
        WHEN EXISTS (SELECT 1 FROM tblGLUserAccess WHERE GLUserName = 'xxxxxxxx') THEN 1
        ELSE 2
    END;

CASE statements excel when dealing with complex logic and are generally more readable than deeply nested IIF statements.

4. Using Variables

You can also use variables to store the result of your conditional logic. This can be useful when you need to reuse the result multiple times or when you’re building a more complex stored procedure.

DECLARE @Result INT;

IF EXISTS (SELECT 1 FROM tblGLUserAccess WHERE GLUserName = 'xxxxxxxx')
    SET @Result = 1;
ELSE
    SET @Result = 2;

SELECT @Result;

This approach provides more control and can enhance code organization, especially within larger procedures.

Best Practices

• SELECT 1 with EXISTS: Always use SELECT 1 (or a constant value) with EXISTS to minimize overhead. Avoid SELECT * unless you specifically need the data.
• Clarity and Readability: Choose the method (IF...ELSE, IIF, CASE) that makes your code easiest to understand and maintain.
• Use Variables for Reusability: If you need to reuse a conditional result, store it in a variable.
• Optimize for Performance: For complex queries, consider indexing the columns used in the WHERE clause to speed up the EXISTS check.
• Consider TOP 1: While often negligible, including TOP 1 in your EXISTS subquery can provide a slight performance improvement, particularly in large tables.