|PostgreSQL 7.4.30 Documentation|
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This section describes the SQL-compliant conditional expressions available in PostgreSQL.
Tip: If your needs go beyond the capabilities of these conditional expressions you might want to consider writing a stored procedure in a more expressive programming language.
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.
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
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;
COALESCE(value [, ...])
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.
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)') ...