PostgreSQL 8.0.0rc4 Documentation
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# 9.13. Conditional Expressions

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.

## 9.13.1. `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;

## 9.13.2. `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.

## 9.13.3. `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)') ...

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