

PostgreSQL 8.1.4 Documentation  

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This section describes the SQLcompliant 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.
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 divisionbyzero failure:
SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 1.5 ELSE false END;
COALESCE
(value [, ...])
The COALESCE
function returns the first of its
arguments that is not null. Null is returned only if all arguments
are null. It is often used 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 nonnull argument are
not evaluated. This SQLstandard function provides capabilities similar
to NVL
and IFNULL
, which are used in some other
database systems.
NULLIF
(value1, value2)
The NULLIF
function returns a null value 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)') ...
If value1 is (none), return a null, otherwise return value1.
GREATEST
(value [, ...])
LEAST
(value [, ...])
The GREATEST
and LEAST
functions select the
largest or smallest value from a list of any number of expressions.
The expressions must all be convertible to a common data type, which
will be the type of the result
(see Section 10.5 for details). NULL values
in the list are ignored. The result will be NULL only if all the
expressions evaluate to NULL.
Note that GREATEST
and LEAST
are not in
the SQL standard, but are a common extension.