A NULL value is a special value that may be assigned to a column. This value normally is used when information in a column is unknown or not applicable. For example, in the case of an unknown home telephone number for a company’s employee, it is recommended that the NULL value be assigned to the home_telephone column.
Any arithmetic expression results in a NULL if any operand of that expression is itself a NULL value. Therefore, in unary arithmetic expressions (if A is an expression with a NULL value), both +A and –A return NULL. In binary expressions, if one (or both) of the operands A or B has the NULL value, A + B, A – B, A * B, A / B, and A % B also result in a NULL. (The operands A and B have to be numerical expressions.)
If an expression contains a relational operation and one (or both) of the operands has (have) the NULL value, the result of this operation will be NULL. Hence, each of the expressions A = B, A <> B, A < B, and A > B also returns NULL.
In the Boolean AND, OR, and NOT, the behavior of the NULL values is specified by the following truth tables, where T stands for true, U for unknown (NULL), and F for false. In these tables, follow the row and column represented by the values of the Boolean expressions that the operator works on, and the value where they intersect represents the resulting value.
Any NULL value in the argument of aggregate functions AVG, SUM, MAX, MIN, and COUNT is eliminated before the respective function is calculated (except for the function COUNT(*)). If a column contains only NULL values, the function returns NULL. The aggregate function COUNT(*) handles all NULL values the same as non-NULL values. If the column contains only NULL values, the result of the function COUNT(DISTINCT column_name) is 0.
A NULL value has to be different from all other values. For numeric data types, there is a distinction between the value zero and NULL. The same is true for the empty string and NULL for character data types.
A column of a table allows NULL values if its definition explicitly contains NULL. On the other hand, NULL values are not permitted if the definition of a column explicitly contains NOT NULL. If the user does not specify NULL or NOT NULL for a column with a data type (except TIMESTAMP), the following values are assigned:
- NULL If the ANSI_NULL_DFLT_ON option of the SET statement is set to ON
- NOT NULL If the ANSI_NULL_DFLT_OFF option of the SET statement is set to ON
If the SET statement isn’t activated, a column will contain the value NOT NULL by default. (The columns of TIMESTAMP data type can be declared only as NOT NULL.)