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ENUM and SET
In MySQL 4.x, ENUM is not a real constraint, but is a more efficient
way to define columns that can only contain a given set of values.
This is because of the same reasons NOT NULL is not honored.
See section 1.8.6.2 Constraint NOT NULL and DEFAULT values.
If you insert an incorrect value into an ENUM column, it will be set to
the reserved enumeration value 0, which will be displayed as an empty
string in string context. See section 11.3.3 The ENUM Type.
If you insert an incorrect value into a SET column, the incorrect value
is ignored. For example, if the column can contain the values
'a', 'b', and 'c', an attempt to assign 'a,x,b,y'
results in a value of 'a,b'.
See section 11.3.4 The SET Type.