This section describes the SQL-compliant conditional expressions available in PostgreSQL.
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 WHENconditionTHENresult[WHEN ...] [ELSEresult] 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, “UNION, CASE, and Related Constructs” for more detail.
The following “simple” CASE expression is a
specialized variant of the general form above:
CASEexpressionWHENvalueTHENresult[WHEN ...] [ELSEresult] 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;
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 non-null argument are
not evaluated. This SQL-standard 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, “UNION, CASE, and Related Constructs” 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.