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.
It has these advantages:
Compact data storage in situations where a column has a limited set of possible values. The strings you specify as input values are automatically encoded as numbers. See Section 11.6, “Data Type Storage Requirements” for the storage requirements for
Readable queries and output. The numbers are translated back to the corresponding strings in query results.
and these potential issues to consider:
An enumeration value must be a quoted string literal. For
example, you can create a table with an
ENUM column like this:
CREATE TABLE shirts ( name VARCHAR(40), size ENUM('x-small', 'small', 'medium', 'large', 'x-large') ); INSERT INTO shirts (name, size) VALUES ('dress shirt','large'), ('t-shirt','medium'), ('polo shirt','small'); SELECT name, size FROM shirts WHERE size = 'medium'; +---------+--------+ | name | size | +---------+--------+ | t-shirt | medium | +---------+--------+ UPDATE shirts SET size = 'small' WHERE size = 'large'; COMMIT;
Inserting 1 million rows into this table with a value of
'medium' would require 1 million bytes of
storage, as opposed to 6 million bytes if you stored the
'medium' in a
Each enumeration value has an index:
The elements listed in the column specification are assigned index numbers, beginning with 1.
The index value of the empty string error value is 0. This means that you can use the following
SELECTstatement to find rows into which invalid
ENUMvalues were assigned:
mysql> SELECT * FROM tbl_name WHERE enum_col=0;
The index of the
The term “index” here refers to a position within the list of enumeration values. It has nothing to do with table indexes.
For example, a column specified as
'Venus', 'Earth') can have any of the values shown
here. The index of each value is also shown.
ENUM column can have a
maximum of 65,535 distinct elements. (The practical limit is
less than 3000.) A table can have no more than 255 unique
element list definitions among its
SET columns considered as a
group. For more information on these limits, see
Limits Imposed by .frm File Structure.
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;
Trailing spaces are automatically deleted from
ENUM member values in the table definition
when a table is created.
When retrieved, values stored into an
column are displayed using the lettercase that was used in the
column definition. Note that
can be assigned a character set and collation. For binary or
case-sensitive collations, lettercase is taken into account
when assigning values to the column.
If you store a number into an
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
'2', but numeric index values of
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
it does not match any enumeration value, so it is treated as
an index and becomes
'2' (the value with
mysql> INSERT INTO t (numbers) VALUES(2),('2'),('3'); mysql> SELECT * FROM t; +---------+ | numbers | +---------+ | 1 | | 2 | | 2 | +---------+
To determine all possible values for an
ENUM column, use
FROM and parse the
ENUM definition in the
Type column of the output.
In the C API,
ENUM values are returned as
strings. For information about using result set metadata to
distinguish them from other strings, see
Section 23.7.4, “C API Data Structures”.
An enumeration value can also be the empty string
If you insert an invalid value into an
ENUM(that is, a string not present in the list of permitted values), the empty string is inserted instead as a special error value. This string can be distinguished from a “normal” empty string by the fact that this string has the numeric value 0. See Index Values for Enumeration Literals for details about the numeric indexes for the enumeration values.
If strict SQL mode is enabled, attempts to insert invalid
ENUMvalues result in an error.
ENUMcolumn is declared to permit
NULLvalue is a valid value for the column, and the default value is
NULL. If an
ENUMcolumn is declared
NOT NULL, its default value is the first element of the list of permitted values.
ENUM values are sorted based on their index
numbers, which depend on the order in which the enumeration
members were listed in the column specification. For example,
'b' sorts before
ENUM('b', 'a'). The empty string sorts
before nonempty strings, and
sort before all other enumeration values.
To prevent unexpected results when using the
BY clause on an
ENUM column, use
one of these techniques:
ENUMlist in alphabetic order.
Make sure that the column is sorted lexically rather than by index number by coding
ORDER BY CAST(or
ORDER BY CONCAT(.
An enumeration value cannot be an expression, even one that evaluates to a string value.
For example, this
statement does not work because the
CONCAT function cannot be used to construct
an enumeration value:
CREATE TABLE sizes ( size ENUM('small', CONCAT('med','ium'), 'large') );
You also cannot employ a user variable as an enumeration value. This pair of statements do not work:
SET @mysize = 'medium'; CREATE TABLE sizes ( size ENUM('small', @mysize, 'large') );
We strongly recommend that you do not use
numbers as enumeration values, because it does not save on
storage over the appropriate
SMALLINT type, and it is easy
to mix up the strings and the underlying number values (which
might not be the same) if you quote the
ENUM values incorrectly. If you do use a
number as an enumeration value, always enclose it in quotation
marks. If the quotation marks are omitted, the number is
regarded as an index. See Handling of Enumeration Literals to
see how even a quoted number could be mistakenly used as a
numeric index value.
Duplicate values in the definition cause a warning, or an error if strict SQL mode is enabled.