Procedural Flow

At first glance, it appears that T-SQL is weak in procedural-flow options. Although it's less rich than some other languages, it suffices. The data-handling boolean extensions — such as EXISTS, IN, and CASE — offset the limitations of IF and WHILE.

Using If for Conditional T-SQL

This is your grandfather's IF. The T-SQL IF command determines the execution of only the next single statement — one IF, one command. In addition, there's no THEN and no END IF command to terminate the IF block:

IF Condition
Statement;

In the following script, the IF condition should return a false, preventing the next command from executing:

IF 1 = 0

PRINT ‘Line One';
PRINT ‘Line Two';

Result:

Line Two
Note
The IF statement is not followed by a semicolon; in fact, a semicolon causes an error. That's because the IF statement is actually a prefix for the following statement; the two are compiled as a single statement.

Using Begin/End to Conditionally Execute Multiple Statements

An IF command that can control only a single command is less than useful. However, a BEGIN/END block can make multiple commands appear to the IF command as the next single command:

IF Condition
  Begin;
   Multiple lines;
  End;

Using If exists() as an Existence-Based Condition

While the IF command may seem limited, the condition clause can include several powerful SQL features similar to a WHERE clause, such as IF EXISTS() and IF. . .IN().

The IF EXISTS() structure uses the presence of any rows returned from a ...

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