9.1. Logical Operators
9.1. Logical Operators
9.1.逻辑运算符
The usual logical operators are available:
常用的逻辑运算符均可用：
AND
OR
NOT
SQL uses a three-valued logic system with true, false, and null, which represents “unknown”. Observe the following truth tables:
SQL使用具有true，false和null（表示“未知”）的三值逻辑系统。 查看以下真值表：
The operators AND and OR are commutative, that is, you can switch the left and right operand without affecting the result. But see Section 4.2.14 for more information about the order of evaluation of subexpressions.
运算符AND和OR是可交换的，也就是说，可以在不影响结果的情况下切换左右操作数。 但是有关子表达式求值顺序的更多信息，请参见第4.2.14节。