9.12.1. CASE

The SQL CASE expression is a generic conditional expression, similar to if/else statements in other languages:

CASE WHEN condition THEN result
     [WHEN ...]
     [ELSE result]
END

CASE clauses can be used wherever an expression is valid. condition is an expression that returns a boolean result. If the result is true then the value of the CASE expression is the result that follows the condition. If the result is false any subsequent WHEN clauses are searched in the same manner. If no WHEN condition is true then the value of the case expression is the result in the ELSE clause. If the ELSE clause is omitted and no condition matches, the result is null.

An example:

SELECT * FROM test;

 a
---
 1
 2
 3


SELECT a,
       CASE WHEN a=1 THEN 'one'
            WHEN a=2 THEN 'two'
            ELSE 'other'
       END
    FROM test;

 a | case
---+-------
 1 | one
 2 | two
 3 | other

The data types of all the result expressions must be convertible to a single output type. See Section 10.5 for more detail.

The following "simple" CASE expression is a specialized variant of the general form above:

CASE expression
    WHEN value THEN result
    [WHEN ...]
    [ELSE result]
END

The expression is computed and compared to all the value specifications in the WHEN clauses until one is found that is equal. If no match is found, the result in the ELSE clause (or a null value) is returned. This is similar to the switch statement in C.

The example above can be written using the simple CASE syntax:

SELECT a,
       CASE a WHEN 1 THEN 'one'
              WHEN 2 THEN 'two'
              ELSE 'other'
       END
    FROM test;

 a | case
---+-------
 1 | one
 2 | two
 3 | other

A CASE expression does not evaluate any subexpressions that are not needed to determine the result. For example, this is a possible way of avoiding a division-by-zero failure:

SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 1.5 ELSE false END;

9.12.2. COALESCE

COALESCE(value [, ...])

The COALESCE function returns the first of its arguments that is not null. Null is returned only if all arguments are null. This is often useful to substitute a default value for null values when data is retrieved for display, for example:

SELECT COALESCE(description, short_description, '(none)') ...

Like a CASE expression, COALESCE will not evaluate arguments that are not needed to determine the result; that is, arguments to the right of the first non-null argument are not evaluated.

9.12.3. NULLIF

NULLIF(value1, value2)

The NULLIF function returns a null value if and only if value1 and value2 are equal. Otherwise it returns value1. This can be used to perform the inverse operation of the COALESCE example given above:

SELECT NULLIF(value, '(none)') ...