1.一個列的唯一索引

mysql> create database pay;

Query OK, 1 row affected (0.03 sec)

mysql> use pay;

Database changed

mysql> show tables;

Empty set (0.00 sec)

CREATE TABLE `t1` (

`a1` int,

`a2` int,

`a3` int,

`a4` int,

UNIQUE KEY `t1_log` (`a1`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

mysql> show index from t1\G;

*************************** 1. row ***************************

Table: t1

Non_unique: 0

Key_name: t1_log

Seq_in_index: 1

Column_name: a1

Collation: A

Cardinality: 0

Sub_part: NULL

Packed: NULL

Null: YES

Index_type: BTREE

Comment:

Index_comment:

1 row in set (0.00 sec)

ERROR:

No query specified

mysql> insert into t1 values(1,2,3,4);

Query OK, 1 row affected (0.05 sec)

mysql> commit;

Query OK, 0 rows affected (0.00 sec)

mysql> insert into t1 values(1,2,3,4);

ERROR 1062 (23000): Duplicate entry '1' for key 't1_log'

2.2個列的唯一索引 ，a1和a2列不能相同

CREATE TABLE `t2` (

`a1` int,

`a2` int,

`a3` int,

`a4` int,

UNIQUE KEY `t2_log` (`a1`,`a2`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

mysql> insert into t2 values(1,2,3,4);

Query OK, 1 row affected (0.01 sec)

mysql> insert into t2 values(1,2,7,8);

ERROR 1062 (23000): Duplicate entry '1-2' for key 't2_log'

3.3個列的唯一索引，3個列不能重復

CREATE TABLE `t3` (

`a1` int,

`a2` int,

`a3` int,

`a4` int,

UNIQUE KEY `t3_log` (`a1`,`a2`,`a3`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

mysql> insert into t3 values(1,2,3,4);

Query OK, 1 row affected (0.04 sec)

mysql> insert into t3 values(1,2,7,8);

Query OK, 1 row affected (0.00 sec)

總結

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