PostgreSQL 9.6.9 Documentation
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# 9.17. 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.

## 9.17.1. `CASE`

The SQL `CASE` expression is a generic conditional expression, similar to if/else statements in other programming languages:

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

`CASE` clauses can be used wherever an expression is valid. Each `condition` is an expression that returns a `boolean` result. If the condition's result is true, the value of the `CASE` expression is the `result` that follows the condition, and the remainder of the `CASE` expression is not processed. If the condition's result is not true, any subsequent `WHEN` clauses are examined in the same manner. If no `WHEN` `condition` yields true, the value of the `CASE` expression is the `result` of the `ELSE` clause. If the `ELSE` clause is omitted and no condition is true, 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 details.

There is a "simple" form of `CASE` expression that is a variant of the general form above:

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

The first `expression` is computed, then compared to each of the `value` expressions in the `WHEN` clauses until one is found that is equal to it. If no match is found, the `result` of 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;

> Note: As described in Section 4.2.14, there are various situations in which subexpressions of an expression are evaluated at different times, so that the principle that "`CASE` evaluates only necessary subexpressions" is not ironclad. For example a constant `1/0` subexpression will usually result in a division-by-zero failure at planning time, even if it's within a `CASE` arm that would never be entered at run time.

## 9.17.2. `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. 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)') ...

This returns `description` if it is not null, otherwise `short_description` if it is not null, otherwise `(none)`.

Like a `CASE` expression, `COALESCE` only evaluates the arguments that are 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.

## 9.17.3. `NULLIF`

    NULLIF(value1, value2)

The `NULLIF` function returns a null value if `value1` equals `value2`; otherwise it returns `value1`. This can be used to perform the inverse operation of the `COALESCE` example given above:

    SELECT NULLIF(value, '(none)') ...

In this example, if `value` is `(none)`, null is returned, otherwise the value of `value` is returned.

## 9.17.4. `GREATEST` and `LEAST`

    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. Some other databases make them return NULL if any argument is NULL, rather than only when all are NULL.

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