How do I perform an IF…THEN in an SQL SELECT?

Question

How do I perform an IF...THEN in an SQL SELECT statement?

For example:

SELECT IF(Obsolete = \'N\' OR InStock = \'Y\' ? 1 :          


        

Answer 1

From SQL Server 2012 you can use the IIF function for this.

SELECT IIF(Obsolete = 'N' OR InStock = 'Y', 1, 0) AS Salable, *
FROM   Product

This is effectively just a shorthand (albeit not standard SQL) way of writing CASE.

I prefer the conciseness when compared with the expanded CASE version.

Both IIF() and CASE resolve as expressions within a SQL statement and can only be used in well-defined places.

The CASE expression cannot be used to control the flow of execution of Transact-SQL statements, statement blocks, user-defined functions, and stored procedures.

If your needs can not be satisfied by these limitations (for example, a need to return differently shaped result sets dependent on some condition) then SQL Server does also have a procedural IF keyword.

IF @IncludeExtendedInformation = 1
  BEGIN
      SELECT A,B,C,X,Y,Z
      FROM   T
  END
ELSE
  BEGIN
      SELECT A,B,C
      FROM   T
  END

Care must sometimes be taken to avoid parameter sniffing issues with this approach however.