SQL Reference

Search Conditions

search-condition |--+------+---+-predicate----------+----------------------------> '-NOT--' '-(search-condition)-' .-----------------------------------------------------. V | >--------+-----------------------------------------------+--+---| '--+-AND-+---+------+---+-predicate----------+--' '-OR--' '-NOT--' '-(search-condition)-'

A search condition specifies a condition that is "true," "false," or "unknown" about a given row.

The result of a search condition is derived by application of the specified logical operators (AND, OR, NOT) to the result of each specified predicate. If logical operators are not specified, the result of the search condition is the result of the specified predicate.

AND and OR are defined in Table 12, in which P and Q are any predicates:

Table 12. Truth Tables for AND and OR

P	Q	P AND Q	P OR Q
True	True	True	True
True	False	False	True
True	Unknown	Unknown	True
False	True	False	True
False	False	False	False
False	Unknown	False	Unknown
Unknown	True	Unknown	True
Unknown	False	False	Unknown
Unknown	Unknown	Unknown	Unknown

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

Search conditions within parentheses are evaluated first. If the order of evaluation is not specified by parentheses, NOT is applied before AND, and AND is applied before OR. The order in which operators at the same precedence level are evaluated is undefined to allow for optimization of search conditions.

Figure 12. Search Conditions Evaluation Order

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