MySQL Manual
1 General Information
1.8.5.3 Constraint ENUM and SET
In MySQL 4.x ENUM is not a real constrain but a more efficient
way to store fields that can only contain a given set of values.
This is because of the same reasons NOT NULL is not honoured.
See section 1.8.5.2 Constraint NOT NULL and DEFAULT values.
If you insert an wrong value in an ENUM field, it will be set to
the reserved enum number 0, which will be displayed as an empty
string in string context. See section 6.2.3.3 The ENUM Type.
If you insert an wrong option in a SET field, the wrong value
will be ignored. See section 6.2.3.4 The SET Type.
