The IN operator asks a set membership question.
The syntax for the IN operator is:
expression1 IN (expression [, ...] )
It asks a set membership question on the expression list. If the value of expression1 is in the expression lists values, then the result is true. If expression1 is NULL, then the expression is TRUE if and only if there is a NULL in the expression list. If expression1 is not NULL, then a NULL in the expression list does not force the result to be NULL. Rather, the NULL is ignored.