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 COLLATE=ucs2_general_ci
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 COLLATE=ucs2_general_ci
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 COLLATE=ucs2_general_ci
check table t1;
Table	Op	Msg_type	Msg_text
test.t1	check	status	OK
drop table t1;