如果我将多次在同一列上添加UNIQUE约束会怎样？
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。
示例
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SHOWINDEX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SHOWINDEX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SHOWINDEX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。