当我们多次在同一列上添加UNIQUE约束时，MySQL将多次在该列上创建索引，我们已经添加了UNIQUE约束。
假设我们有一个表“ employee”，其中在“ empid”列上有UNIQUE约束。可以从以下查询中检查-
mysql> Describe employee; +------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +------------+-------------+------+-----+---------+-------+ | empid | int(11) | YES | UNI | NULL | | | first_name | varchar(20) | YES | | NULL | | | last_name | varchar(20) | YES | | NULL | | +------------+-------------+------+-----+---------+-------+ 3 rows in set (0.12 sec)
现在，当我们运行查询SHOW INDEX时，它给出索引的名称，只有一个索引在列'empid'上创建。
mysql> Show index from employee\G; *************************** 1. row *************************** Table: employee Non_unique: 0 Key_name: empid Seq_in_index: 1 Column_name: empid Collation: A Cardinality: 0 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: Index_comment: 1 row in set (0.00 sec)
借助以下查询，我们在同一列“ empid”上又添加了一个UNIQUE约束-
mysql> Alter table employee ADD UNIQUE(empid); Records: 0 Duplicates: 0 Warnings: 0
现在，当我们运行查询SHOW INDEX时，它给出了索引的名称，即在列“ empid”上创建的两个索引“ empid”和“ empid_2”。
mysql> Show index from employee12\G; *************************** 1. row *************************** Table: employee Non_unique: 0 Key_name: empid 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: empid_2 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 rows in set (0.00 sec)
借助以下查询，我们在同一列“ empid”上又添加了一个UNIQUE约束-
mysql> Alter table employee ADD UNIQUE(empid); Records: 0 Duplicates: 0 Warnings: 0
现在，当我们运行查询SHOW INDEX时，它将给出索引的名称，即在“ empid”列上创建的三个索引“ empid”和“ empid_2”，“ empid_3”。
mysql> Alter table employee ADD UNIQUE(empid); Records: 0 Duplicates: 0 Warnings: 0 mysql> Show index from employee\G; *************************** 1. row *************************** Table: employee Non_unique: 0 Key_name: empid 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: empid_2 Seq_in_index: 1 Column_name: empid 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: empid_3 Seq_in_index: 1 Column_name: empid Collation: A Cardinality: 0 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: Index_comment: 3 rows in set (0.00 sec)
从这个意义上讲，我们可以说MySQL将在该列上添加UNIQUE约束的次数上创建该列的唯一索引。