# 9.18. Conditional Expressions

9.18.1. CASE

9.18.2. COALESCE

9.18.3. NULLIF

9.18.4. GREATEST and LEAST

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 server-side function in a more expressive programming language.

# Note

Although COALESCE, GREATEST, and LEAST are syntactically similar to functions, they are not ordinary functions, and thus cannot be used with explicit VARIADIC array arguments.

# 9.18.1. 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
### Note

 As described in [Section 4.2.14](sql-expressions.html#SYNTAX-EXPRESS-EVAL), 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.

### 9.18.2. `COALESCE`

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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)`.

 The arguments must all be convertible to a common data type, which will be the type of the result (see [Section 10.5](typeconv-union-case.html) for details).

 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.

### 9.18.3. `NULLIF`

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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.

 The two arguments must be of comparable types. To be specific, they are compared exactly as if you had written `*`value1`* = *`value2`*`, so there must be a suitable `=` operator available.

 The result has the same type as the first argument — but there is a subtlety. What is actually returned is the first argument of the implied `=` operator, and in some cases that will have been promoted to match the second argument's type. For example, `NULLIF(1, 2.2)` yields `numeric`, because there is no `integer` `=` `numeric` operator, only `numeric` `=` `numeric`.

### 9.18.4. `GREATEST` and `LEAST`

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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](typeconv-union-case.html) 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.