mysql 唯一索引

1.一个列的唯一索引
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 t1G;
*************************** 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个列的唯一索引 ，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个列的唯一索引，3个列不能重复
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)