Example of Null operator - NiNo Rule
If we wanted to make HIGHER_OF adhere to "NULL in, NULL out"-let's call it the NiNo rule-we would have to write something like what is shown in Example 2.1a.
Example: NiNo version of HIGHER_OF
CREATE FUNCTION HIGHER_OF ( A INTEGER, B INTEGER )
RETURNS INTEGER
CASE
WHEN A IS NULL OR B IS NULL
THEN RETURN CAST (NULL AS INTEGER);
WHEN A > B
THEN RETURN A;
ELSE RETURN B;
END CASE;
Explanation:
- IS NULL is a monadic Boolean operator that evaluates to TRUE when its argument is "the null value", otherwise FALSE. Note, therefore, that it is our first exception to the NiNo rule, which would require it to evaluate to NULL (UNKNOWN) when its argument is "the null value".
- CAST (NULL AS INTEGER) denotes "the null value" of type INTEGER. As in Tutorial D, the operand of RETURN must be an expression that has a declared type and NULL, on its own, does not have a declared type. The CAST operator takes an expression and a type name, separated by the "noise" word AS, and normally expresses a "type conversion"-a function that maps elements of one type to those of another that are considered to be in some defined sense equivalent. In the special case of NULL it is used to confer a type, so to speak, on something that doesn't otherwise have one.
- AS, appearing where you might have expected just a comma, is explained by a matter of policy in SQL whereby invocations of user-defined functions, which always use commas between arguments, can be syntactically distinguished from invocations of system-defined functions.
- OR is SQL's counterpart of the usual logical operator of that name. In A IS NULL OR B IS NULL its operands are clearly restricted to just TRUE and FALSE and that expression therefore results in TRUE if either of those operands does, otherwise FALSE.