Table of Contents for

MySQL in a Nutshell, 2nd Edition

An ENUM column is one in which all possible choices are enumerated (e.g., ENUM('yes', 'no', 'maybe')). It’s possible for it to contain a blank value (i.e., '') and NULL. If an ENUM column is set up to allow NULL values, NULL will be the default value. If an ENUM column is set up with NOT NULL, NULL isn’t allowed and the default value becomes the first element given.

MySQL stores a numeric index of the enumerated values in the column, 1 being the first value. The values can be retrieved when the column is used in a numeric context (e.g., SELECT col1 + 0 FROM table1;). The reverse may be performed when entering data into a column (e.g., UPDATE table1 SET col1 = 3; to set the value to the third element). The column values are sorted in ascending order based on the numeric index, not on their corresponding enumerated values. If you want to use a character set for the column other than the default for the table, you can give one for the column. Values are sorted based on the collation of the character set for the column.