Conditional Expressions v11

The following section describes the SQL-compliant conditional expressions available in Advanced Server.

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
(3 rows)

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
(3 rows)

The data types of all the result expressions must be convertible to a single output type.

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.

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
(3 rows)

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

The COALESCE function returns the first of its arguments that is not null. Null is returned only if all arguments are null.

COALESCE(value [, value2 ] ... )

It is often used to substitute a default value for null values when data is retrieved for display or further computation. 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

The NULLIF function returns a null value if value1 and value2 are equal; otherwise it returns value1.

NULLIF(value1, value2)

This can be used to perform the inverse operation of the COALESCE example given above:

SELECT NULLIF(value1, '(none)') ...

If value1 is (none), return a null, otherwise return value1.

NVL

The NVL function returns the first of its arguments that is not null. NVL evaluates the first expression; if that expression evaluates to NULL, NVL returns the second expression.

NVL(expr1, expr2)

The return type is the same as the argument types; all arguments must have the same data type (or be coercible to a common type). NVL returns NULL if all arguments are NULL.'' is considered as unknown and if the arguments data type are not coercible to the common data type then NVL throws an error.

Examples

  1. The following example computes a bonus for non-commissioned employees, If an employee is a commissioned employee,
    this expression returns the employees commission; if the employee is not a commissioned employee (that is, his commission is NULL), this expression returns a bonus that is 10% of his salary.

    bonus = NVL(emp.commission, emp.salary * .10)
  2. In the following example, the type of 1 is numeric and the type of '' is considered as unknown. Therefore PostgreSQL decides that the common type ought to be numeric and tries to interpret the empty string as a numeric value, which produces the indicated error.

    edb=# select nvl('',1);
    ERROR:  invalid input syntax for type numeric: ""

NVL2

NVL2 evaluates an expression, and returns either the second or third expression, depending on the value of the first expression. If the first expression is not NULL, NVL2 returns the value in expr2; if the first expression is NULL, NVL2 returns the value in expr3.

NVL2(expr1, expr2, expr3)

The return type is the same as the argument types; all arguments must have the same data type (or be coercible to a common type).

The following example computes a bonus for commissioned employees - if a given employee is a commissioned employee, this expression returns an amount equal to 110% of his commission; if the employee is not a commissioned employee (that is, his commission is NULL), this expression returns 0.

bonus = NVL2(emp.commission, emp.commission * 1.1, 0)

GREATEST and LEAST

The GREATEST and LEAST functions select the largest or smallest value from a list of any number of expressions.

GREATEST(value [, value2 ] ... )
LEAST(value [, value2 ] ... )

The expressions must all be convertible to a common data type, which will be the type of the result. Null values in the list are ignored. The result will be null only if all the expressions evaluate to null.

Note

The GREATEST and LEAST are not in the SQL standard, but are a common extension.