Comparison operators compare one expression to another. The result is always TRUE, FALSE, or NULL. Typically, comparisons are used in conditional control statements and allow comparisons of arbitrarily complex expressions.

The logical operators can be used to build more complex queries. The logical operators AND, OR, and NOT operate according to the tri-state logic illustrated by the following truth tables. AND and OR are binary operators; NOT is a unary operator.

As shown in the truth tables, AND returns the value TRUE only if both its operands are true. On the other hand, OR returns the value TRUE if either of its operands is true. NOT returns the opposite value (logical negation) of its operand. For example, NOT TRUE returns FALSE.

NOT NULL returns NULL because nulls are indeterminate.

If you do not use parentheses to specify the order of evaluation, operator precedence determines the order.

Note that "true" and "false" are not literals accepted by SQL parser but values. Logical expression value can be interpreted as a numeric variable:

false = 0 or NULL

true = 1 or any other numeric value

Example:

can be simply written

The IS NULL operator returns the Boolean value TRUE if its operand is null, or FALSE if it is not null. Comparisons involving nulls always yield NULL. To test whether a value is NULL, do not use the expression,

because it never evaluates to TRUE.

Instead, use the following statement:

Note that when using multiple logical operators in solidDB stored procedures the individual logical expressions should be enclosed in parentheses like: