CREATE [UNIQUE|FULLTEXT|SPATIAL] INDEX index_name
    [index_type]
    ON tbl_name (index_col_name,...)
    [index_option ...]

index_col_name:
    col_name [(length)] [ASC | DESC]

index_type:
    USING {BTREE | HASH}

index_option:
    KEY_BLOCK_SIZE value
  | index_type
  | WITH PARSER parser_name

CREATE INDEX is mapped to an ALTER TABLE statement to create indexes. See Section 13.1.2, “ALTER TABLE Syntax”. CREATE INDEX cannot be used to create a PRIMARY KEY; use ALTER TABLE instead. For more information about indexes, see Section 7.4.5, “How MySQL Uses Indexes”.

Normally, you create all indexes on a table at the time the table itself is created with CREATE TABLE. See Section 13.1.8, “CREATE TABLE Syntax”. CREATE INDEX enables you to add indexes to existing tables.

A column list of the form (col1,col2,...) creates a multiple-column index. Index values are formed by concatenating the values of the given columns.

Indexes can be created that use only the leading part of column values, using col_name(length) syntax to specify an index prefix length:

The statement shown here creates an index using the first 10 characters of the name column:

CREATE INDEX part_of_name ON customer (name(10));

If names in the column usually differ in the first 10 characters, this index should not be much slower than an index created from the entire name column. Also, using column prefixes for indexes can make the index file much smaller, which could save a lot of disk space and might also speed up INSERT operations.

Prefix lengths are storage engine-dependent (for example, a prefix can be up to 1000 bytes long for MyISAM tables, 767 bytes for InnoDB tables). Note that prefix limits are measured in bytes, whereas the prefix length in CREATE INDEX statements is interpreted as number of characters for non-binary data types (CHAR, VARCHAR, TEXT). Take this into account when specifying a prefix length for a column that uses a multi-byte character set. For example, utf8 columns require up to three index bytes per character.

A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. For all engines, a UNIQUE index allows multiple NULL values for columns that can contain NULL. If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within the prefix.

FULLTEXT indexes are supported only for MyISAM tables and can include only CHAR, VARCHAR, and TEXT columns. Indexing always happens over the entire column; column prefix indexing is not supported and any prefix length is ignored if specified. See Section 12.8, “Full-Text Search Functions”, for details of operation.

The MyISAM, InnoDB, NDB, BDB, and ARCHIVE storage engines support spatial columns such as (POINT and GEOMETRY. (Chapter 17, Spatial Extensions, describes the spatial data types.) However, support for spatial column indexing varies among engines. Spatial and non-spatial indexes are available according to the following rules.

Spatial indexes (created using SPATIAL INDEX):

Non-spatial indexes (created with INDEX, UNIQUE, or PRIMARY KEY):

In MySQL 5.1:

An index_col_name specification can end with ASC or DESC. These keywords are allowed for future extensions for specifying ascending or descending index value storage. Currently, they are parsed but ignored; index values are always stored in ascending order.

Following the index column list, index options can be given. An index_option value can be any of the following: