4.10. Conditional Expressions

This section describes the SQL-compliant conditional expressions available in Postgres.

CASE

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

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

=> 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 coercible to a single output type. See Section 5.5 for more detail.

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

This "simple" CASE expression is a specialized variant of the general form above. The expression is computed and compared to all the values in the WHEN clauses until one is found that is equal. If no match is found, the result in the ELSE clause (or NULL) is returned. This is similar to the switch statement in C.

=> 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

COALESCE

COALESCE(value[, ...])

The COALESCE function returns the first of its arguments that is not 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)') ...

NULLIF

NULLIF(value1, value2)

The NULLIF function returns NULL 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)') ...