Syntax:
OR, ||

Logical OR. When both operands are non-NULL, the result is 1 if any
operand is non-zero, and 0 otherwise. With a NULL operand, the result
is 1 if the other operand is non-zero, and NULL otherwise. If both
operands are NULL, the result is NULL.

URL: http://dev.mysql.com/doc/refman/5.0/en/logical-operators.html

