An ENUM is a string object with a value chosen from a list of permitted values that are enumerated explicitly in the column specification at table creation time.

An enumeration value must be a quoted string literal; it may not be an expression, even one that evaluates to a string value. For example, you can create a table with an ENUM column like this:

CREATE TABLE sizes (
    name ENUM('small', 'medium', 'large')
);

However, this version of the previous CREATE TABLE statement does not work:

CREATE TABLE sizes (
    c1 ENUM('small', CONCAT('med','ium'), 'large')
);

You also may not employ a user variable as an enumeration value. This pair of statements do not work:

SET @mysize = 'medium';

CREATE TABLE sizes (
    name ENUM('small', @mysize, 'large')
);

If you wish to use a number as an enumeration value, you must enclose it in quotation marks.

The value may also be the empty string ('') or NULL under certain circumstances:

Each enumeration value has an index:

For example, a column specified as ENUM('one', 'two', 'three') can have any of the values shown here. The index of each value is also shown.

An enumeration can have a maximum of 65,535 elements.

Starting from MySQL 3.23.51, trailing spaces are automatically deleted from ENUM member values in the table definition when a table is created.

When retrieved, values stored into an ENUM column are displayed using the lettercase that was used in the column definition. Before MySQL 4.1.1, lettercase is irrelevant when you assign values to an ENUM column. As of 4.1.1, ENUM columns can be assigned a character set and collation. For binary or case-sensitive collations, lettercase does matter when you assign values to the column.

If you retrieve an ENUM value in a numeric context, the column value's index is returned. For example, you can retrieve numeric values from an ENUM column like this:

mysql> SELECT enum_col+0 FROM tbl_name;

If you store a number into an ENUM column, the number is treated as the index into the possible values, and the value stored is the enumeration member with that index. (However, this does not work with LOAD DATA, which treats all input as strings.) If the numeric value is quoted, it is still interpreted as an index if there is no matching string in the list of enumeration values. For these reasons, it is not advisable to define an ENUM column with enumeration values that look like numbers, because this can easily become confusing. For example, the following column has enumeration members with string values of '0', '1', and '2', but numeric index values of 1, 2, and 3:

numbers ENUM('0','1','2')

If you store 2, it is interpreted as an index value, and becomes '1' (the value with index 2). If you store '2', it matches an enumeration value, so it is stored as '2'. If you store '3', it does not match any enumeration value, so it is treated as an index and becomes '2' (the value with index 3).

mysql> INSERT INTO t (numbers) VALUES(2),('2'),('3');
mysql> SELECT * FROM t;
+---------+
| numbers |
+---------+
| 1       |
| 2       |
| 2       |
+---------+

ENUM values are sorted according to the order in which the enumeration members were listed in the column specification. (In other words, ENUM values are sorted according to their index numbers.) For example, 'a' sorts before 'b' for ENUM('a', 'b'), but 'b' sorts before 'a' for ENUM('b', 'a'). The empty string sorts before nonempty strings, and NULL values sort before all other enumeration values. If you expect sorting to be done alphabetically, you should specify the ENUM list in alphabetic order. You can also use GROUP BY CAST(col AS CHAR) or GROUP BY CONCAT(col) to make sure that the column is sorted lexically rather than by index number.

Functions such as SUM() or AVG() that expect a numeric argument cast the argument to a number if necessary. For ENUM values, the cast operation causes the index number to be used.

If you want to determine all possible values for an ENUM column, use SHOW COLUMNS FROM tbl_name LIKE enum_col and parse the ENUM definition in the Type column of the output.