CASE expressions

The CASE expression provides conditional SQL expressions. You can use case expressions anywhere you can use an expression.

The syntax of the CASE expression is as follows:

CASE expression 
WHEN expression THEN expression [, …]
[ ELSE expression ] END

You cannot use a subquery as a value expression in a CASE statement.

If the expression following the CASE statement is equal to the expression following the WHEN statement, then the expression following the THEN statement is returned. Otherwise, the expression following the ELSE statement is returned, if it exists.

For example, the following code uses a case expression as the second clause in a SELECT statement.

SELECT ID,
  (CASE name
  WHEN 'Tee Shirt' THEN 'Shirt'
  WHEN 'Sweatshirt' THEN 'Shirt'
  WHEN 'Baseball Cap' THEN 'Hat'
  ELSE 'Unknown'
  END) as Type
FROM "GROUPO".Products

An alternative syntax is:

CASE
WHEN search-condition THEN expression [, …]
[ ELSE expression ] END

If the search condition following the WHEN statement is satisfied, the expression following the THEN statement is returned. Otherwise the expression following the ELSE statement is returned, if it exists.

For example, the following statement uses a case expression as the third clause of a SELECT statement to associate a string with a search condition.

SELECT ID, name,
  (CASE 
  WHEN name='Tee Shirt' THEN 'Sale'
  WHEN quantity >= 50  THEN 'Big Sale'
  ELSE 'Regular price'
  END) as Type
FROM "GROUPO".Products

NULLIF function for abbreviated CASE expressions

The NULLIF function provides a way to write some CASE statements in short form. The syntax for NULLIF is as follows:

NULLIF ( expression-1, expression-2 )

NULLIF compares the values of the two expressions. If the first expression equals the second expression, NULLIF returns NULL. If the first expression does not equal the second expression, NULLIF returns the first expression.