我们如何创建多列UNIQUE索引？
为了创建多列UNIQUE索引，我们需要在多个列上指定索引名称。以下示例将在'employee'表的'empid'，'first_name'，'last_name'列上创建一个名为'id_fname_lname'的多列索引-
Create UNIQUE INDEX id_fname_lname on employee(empid,first_name,last_name); Records: 0 Duplicates: 0 Warnings: 0 describe employee; +------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +------------+-------------+------+-----+---------+-------+ | empid | int(11) | YES | MUL | NULL | | | first_name | varchar(20) | YES | | NULL | | | last_name | varchar(20) | YES | | NULL | | +------------+-------------+------+-----+---------+-------+ 3 rows in set (0.12 sec)
从上面查询的结果集中，我们可以看到在表上定义了多个索引。忘记有关索引的详细信息，我们可以运行以下查询-
Show index from employee\G *************************** 1. row *************************** Table: employee Non_unique: 0 Key_name: id_fname_lname Seq_in_index: 1 Column_name: empid Collation: A Cardinality: 0 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: Index_comment: *************************** 2. row *************************** Table: employee Non_unique: 0 Key_name: id_fname_lname Seq_in_index: 2 Column_name: first_name Collation: A Cardinality: 0 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: Index_comment: *************************** 3. row *************************** Table: employee Non_unique: 0 Key_name: id_fname_lname Seq_in_index: 3 Column_name: last_name Collation: A Cardinality: 0 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: Index_comment: 3 rows in set (0.00 sec)
从上面的结果集中我们可以看到，'key_name'字段中的值是相同的，因为我们已经在表的所有列上创建了多列索引。