Search Conditions in SQL Statements

2. Language Elements : Search Conditions in SQL Statements

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Search conditions are used in WHERE and HAVING clauses to qualify the selection of data. Search conditions are composed of predicates of various kinds, optionally combined using parentheses and logical operators (AND, OR, and NOT). The following are examples of legal search conditions:

Description	Example
Simple predicate	salary BETWEEN 10000 AND 20000
Predicate with NOT operator	edept NOT LIKE eng_%
Predicates combined using OR operator	edept LIKE eng_% OR edept LIKE admin_%
Predicates combined using AND operator	salary BETWEEN 10000 AND 20000 AND edept LIKE eng_%
Predicates combined using parentheses to specify evaluation	(salary BETWEEN 10000 AND 20000 AND edept LIKE eng_% ) OR edept LIKE admin_%

Predicates evaluate to true, false, or unknown. They evaluate to unknown if one or both operands are null (the IS NULL predicate is the exception). When predicates are combined using logical operators (that is, AND, OR, NOT) to form a search condition, the search condition evaluates to true, false, or unknown as shown in the following tables:

AND	true	false	unknown
true	true	false	unknown
false	false	false	false
unknown	unknown	false	unknown

OR	true	false	unknown
true	true	true	true
false	true	false	unknown
unknown	true	unknown	unknown

NOT(true) is false, NOT(false) is true, NOT(unknown) is unknown.

After all search conditions are evaluated, the value of the WHERE or HAVING clause is determined. The WHERE or HAVING clause can be true or false, not unknown. Unknown values are considered false. For more information about predicates and logical operators, see Logical Operators (see Logical Operators).