如果我多次在同一列上添加 UNIQUE 约束,会发生什么情况?
mysqlmysqli database
当我们多次在同一列上添加 UNIQUE 约束时,MySQL 将根据我们添加 UNIQUE 约束的次数在该列上创建索引。
示例
假设我们有表 ‘employee’,其中 ‘empid’ 列上有 UNIQUE 约束。可以通过以下查询进行检查 −
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,它会给出索引的名称,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’上添加了一个 UNIQUE 约束;−
mysql> Alter table employee ADD UNIQUE(empid); Query OK, 0 rows affected (0.21 sec) Records: 0 Duplicates: 0 Warnings: 0
现在,当我们运行查询 SHOW INDEX 时,它会给出索引的名称,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"上添加了另一个 UNIQUE 约束 −
mysql> Alter table employee ADD UNIQUE(empid); Query OK, 0 rows affected (0.21 sec) Records: 0 Duplicates: 0 Warnings: 0
现在,当我们运行查询 SHOW INDEX 时,它会给出索引的名称,在列"empid"上创建的三个索引"empid"和"empid_2","empid_3"。
mysql> Alter table employee ADD UNIQUE(empid); Query OK, 0 rows affected (0.30 sec) 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 将根据我们在该列上添加 UNIQUE 约束的次数在该列上创建唯一索引。