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create table t1(a int not null, b int, c char(10), d varchar(20), primary key (a)) engine = innodb default charset=ucs2;
insert into t1 values (1,1,'ab','ab'),(2,2,'ac','ac'),(3,2,'ad','ad'),(4,4,'afe','afe');
commit;
alter table t1 add unique index (b);
ERROR 23000: Duplicate entry '2' for key 'b'
insert into t1 values(8,9,'fff','fff');
select * from t1;
a b c d
1 1 ab ab
2 2 ac ac
3 2 ad ad
4 4 afe afe
8 9 fff fff
show create table t1;
Table Create Table
t1 CREATE TABLE `t1` (
`a` int(11) NOT NULL,
`b` int(11) DEFAULT NULL,
`c` char(10) DEFAULT NULL,
`d` varchar(20) DEFAULT NULL,
PRIMARY KEY (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=ucs2
alter table t1 add index (b);
insert into t1 values(10,10,'kkk','iii');
select * from t1;
a b c d
1 1 ab ab
2 2 ac ac
3 2 ad ad
4 4 afe afe
8 9 fff fff
10 10 kkk iii
select * from t1 force index(b) order by b;
a b c d
1 1 ab ab
2 2 ac ac
3 2 ad ad
4 4 afe afe
8 9 fff fff
10 10 kkk iii
explain select * from t1 force index(b) order by b;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL b 5 NULL 6
show create table t1;
Table Create Table
t1 CREATE TABLE `t1` (
`a` int(11) NOT NULL,
`b` int(11) DEFAULT NULL,
`c` char(10) DEFAULT NULL,
`d` varchar(20) DEFAULT NULL,
PRIMARY KEY (`a`),
KEY `b` (`b`)
) ENGINE=InnoDB DEFAULT CHARSET=ucs2
alter table t1 add unique index (c), add index (d);
insert into t1 values(11,11,'aaa','mmm');
select * from t1;
a b c d
1 1 ab ab
2 2 ac ac
3 2 ad ad
4 4 afe afe
8 9 fff fff
10 10 kkk iii
11 11 aaa mmm
select * from t1 force index(b) order by b;
a b c d
1 1 ab ab
2 2 ac ac
3 2 ad ad
4 4 afe afe
8 9 fff fff
10 10 kkk iii
11 11 aaa mmm
select * from t1 force index(c) order by c;
a b c d
11 11 aaa mmm
1 1 ab ab
2 2 ac ac
3 2 ad ad
4 4 afe afe
8 9 fff fff
10 10 kkk iii
select * from t1 force index(d) order by d;
a b c d
1 1 ab ab
2 2 ac ac
3 2 ad ad
4 4 afe afe
8 9 fff fff
10 10 kkk iii
11 11 aaa mmm
explain select * from t1 force index(b) order by b;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL b 5 NULL 7
explain select * from t1 force index(c) order by c;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL c 21 NULL 7
explain select * from t1 force index(d) order by d;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 index NULL d 43 NULL 7
show create table t1;
Table Create Table
t1 CREATE TABLE `t1` (
`a` int(11) NOT NULL,
`b` int(11) DEFAULT NULL,
`c` char(10) DEFAULT NULL,
`d` varchar(20) DEFAULT NULL,
PRIMARY KEY (`a`),
UNIQUE KEY `c` (`c`),
KEY `b` (`b`),
KEY `d` (`d`)
) ENGINE=InnoDB DEFAULT CHARSET=ucs2
check table t1;
Table Op Msg_type Msg_text
test.t1 check status OK
drop table t1;
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