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11.4.4 The ENUM Type

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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. 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 11.8, “Data Type Storage Requirements” for the storage requirements for ENUM types.

  • Readable queries and output. The numbers are translated back to the corresponding strings in query results.

and these potential issues to consider:

  • If you make enumeration values that look like numbers, it is easy to mix up the literal values with their internal index numbers, as explained in Enumeration Limitations.

  • Using ENUM columns in ORDER BY clauses requires extra care, as explained in Enumeration Sorting.

Creating and Using ENUM Columns

An enumeration value must be a quoted string literal. For example, you can create a table with an ENUM column like this:

  1. CREATE TABLE shirts (
  2.     name VARCHAR(40),
  3.     size ENUM('x-small', 'small', 'medium', 'large', 'x-large')
  4. );
  5. INSERT INTO shirts (name, size) VALUES ('dress shirt','large'), ('t-shirt','medium'),
  6.   ('polo shirt','small');
  7. SELECT name, size FROM shirts WHERE size = 'medium';
  8. +---------+--------+
  9. | name    | size   |
  10. +---------+--------+
  11. | t-shirt | medium |
  12. +---------+--------+
  13. UPDATE shirts SET size = 'small' WHERE size = 'large';
  14. 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 actual string 'medium' in a VARCHAR column.

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Index Values for Enumeration Literals

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 SELECT statement to find rows into which invalid ENUM values were assigned:

    1. mysql> SELECT * FROM tbl_name WHERE enum_col=0;

  • The index of the NULL value is NULL.

  • 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 ENUM('Mercury', 'Venus', 'Earth') can have any of the values shown here. The index of each value is also shown.

Value Index
NULL NULL
'' 0
'Mercury' 1
'Venus' 2
'Earth' 3

An ENUM column can have a maximum of 65,535 distinct elements.

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:

  1. mysql> SELECT enum_col+0 FROM tbl_name;

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

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Handling of Enumeration Literals

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. Note that ENUM columns 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 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:

  1. 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).

  1. mysql> INSERT INTO t (numbers) VALUES(2),('2'),('3');
  2. mysql> SELECT * FROM t;
  3. +---------+
  4. | numbers |
  5. +---------+
  6. | 1       |
  7. | 2       |
  8. | 2       |
  9. +---------+

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.

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 28.7.5, “C API Data Structures”.

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Empty or NULL Enumeration Values

An enumeration value can also be the empty string ('') or NULL under certain circumstances:

  • 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 ENUM values result in an error.

  • If an ENUM column is declared to permit NULL, the NULL value is a valid value for the column, and the default value is NULL. If an ENUM column is declared NOT NULL, its default value is the first element of the list of permitted values.

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Enumeration Sorting

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 'a' for ENUM('b', 'a'). The empty string sorts before nonempty strings, and NULL values sort before all other enumeration values.

To prevent unexpected results when using the ORDER BY clause on an ENUM column, use one of these techniques:

  • Specify the ENUM list in alphabetic order.

  • Make sure that the column is sorted lexically rather than by index number by coding ORDER BY CAST(col AS CHAR) or ORDER BY CONCAT(col).

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Enumeration Limitations

An enumeration value cannot be an expression, even one that evaluates to a string value.

For example, this CREATE TABLE statement does not work because the CONCAT function cannot be used to construct an enumeration value:

  1. CREATE TABLE sizes (
  2.     size ENUM('small', CONCAT('med','ium'), 'large')
  3. );

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

  1. SET @mysize = 'medium';
  2.  
  3. CREATE TABLE sizes (
  4.     size ENUM('small', @mysize, 'large')
  5. );

We strongly recommend that you do not use numbers as enumeration values, because it does not save on storage over the appropriate TINYINT or 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.


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Dokument erstellt 26/06/2006, zuletzt geändert 26/10/2018
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