Boolean value expressions can specify a single search condition or can combine multiple conditions linked by the AND / OR keywords. The order in which Boolean operators are evaluated within an expression is as follows, unless altered by the presence of parentheses: NOT, AND, then OR.

Note that parentheses

(...)

can be placed around individual predicates, groups of predicates, and even the whole Boolean value expression. If a predicate (or collection of predicates) is enclosed by parentheses, it will be evaluated first. If parenthetical expressions are nested, the innermost condition surrounded by parentheses is evaluated before all others. Placing parentheses around logical operations also enhances the readability of the SQL, especially if there are many predicates in the expression.

The following table provides evaluation of different Boolean expressions combined with the AND operator:

Boolean Expression	true	false	unknown
true	true	false	unknown
false	false	false	false
unknown	unknown	false	unknown

The following table provides evaluation of different Boolean expressions combined with the OR operator:

Boolean Expression	true	false	unknown
true	true	true	true
false	true	false	unknown
unknown	true	unknown	unknown