9.1. Logical Operators #

   The usual logical operators are available:
boolean AND boolean → boolean
boolean OR boolean → boolean
NOT boolean → boolean

   SQL uses a three-valued logic system with true, false, and null, which
   represents “unknown”. Observe the following truth tables:
     a     b   a AND b a OR b
   TRUE  TRUE  TRUE    TRUE
   TRUE  FALSE FALSE   TRUE
   TRUE  NULL  NULL    TRUE
   FALSE FALSE FALSE   FALSE
   FALSE NULL  FALSE   NULL
   NULL  NULL  NULL    NULL

     a   NOT a
   TRUE  FALSE
   FALSE TRUE
   NULL  NULL

   The operators AND and OR are commutative, that is, you can switch the
   left and right operands without affecting the result. (However, it is
   not guaranteed that the left operand is evaluated before the right
   operand. See Section 4.2.14 for more information about the order of
   evaluation of subexpressions.)