PostgreSQL Conditional Expressions

References

• PostgreSQL Conditional Expressions

Notes

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 ELSE clause is omitted and no condition is true, the result is null. The data types of all the result expressions must be convertible to a single output type. 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 not match is found, the result of the ELSE clause is returned. 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

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 default values when data is retrieved for display. The arguments must all be convertible to a common data type, which will be the type of the result. 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.

NULLIF

NULLIF(value1,value2)
NULLIF(value,' (none) ') -- IF value is ' (none) ', NULL is returned

The NULLIF function returns a null value if value1 equals value2; otherwise it returns value. This can be used to perform the inverse operation of COALESCE. The two arguments must be comparable types.

GREATEST and LEAST

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

The GREATEST and LEAST functions select the largest r 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.

NULL values in the list are ignored. The result will be NULL only if all the expressions evaluate to NULL.