drop table if exists t1,t2,t3;
CREATE TABLE t1 (id int,facility char(20));
CREATE TABLE t2 (facility char(20));
INSERT INTO t1 VALUES (NULL,NULL);
INSERT INTO t1 VALUES (-1,'');
INSERT INTO t1 VALUES (0,'');
INSERT INTO t1 VALUES (1,'/L');
INSERT INTO t1 VALUES (2,'A01');
INSERT INTO t1 VALUES (3,'ANC');
INSERT INTO t1 VALUES (4,'F01');
INSERT INTO t1 VALUES (5,'FBX');
INSERT INTO t1 VALUES (6,'MT');
INSERT INTO t1 VALUES (7,'P');
INSERT INTO t1 VALUES (8,'RV');
INSERT INTO t1 VALUES (9,'SRV');
INSERT INTO t1 VALUES (10,'VMT');
INSERT INTO t2 SELECT DISTINCT FACILITY FROM t1;
select id from t1 group by id;
id
NULL
-1
0
1
2
3
4
5
6
7
8
9
10
select * from t1 order by id;
id	facility
NULL	NULL
-1	
0	
1	/L
2	A01
3	ANC
4	F01
5	FBX
6	MT
7	P
8	RV
9	SRV
10	VMT
select id-5,facility from t1 order by "id-5";
id-5	facility
NULL	NULL
-6	
-5	
-4	/L
-3	A01
-2	ANC
-1	F01
0	FBX
1	MT
2	P
3	RV
4	SRV
5	VMT
select id,concat(facility) from t1 group by id ;
id	concat(facility)
NULL	NULL
-1	
0	
1	/L
2	A01
3	ANC
4	F01
5	FBX
6	MT
7	P
8	RV
9	SRV
10	VMT
select id+0 as a,max(id),concat(facility) as b from t1 group by a order by b desc,a;
a	max(id)	b
10	10	VMT
9	9	SRV
8	8	RV
7	7	P
6	6	MT
5	5	FBX
4	4	F01
3	3	ANC
2	2	A01
1	1	/L
-1	-1	
0	0	
NULL	NULL	NULL
select id >= 0 and id <= 5 as grp,count(*) from t1 group by grp;
grp	count(*)
NULL	1
0	6
1	6
SELECT DISTINCT FACILITY FROM t1;
FACILITY
NULL

/L
A01
ANC
F01
FBX
MT
P
RV
SRV
VMT
SELECT FACILITY FROM t2;
FACILITY
NULL

/L
A01
ANC
F01
FBX
MT
P
RV
SRV
VMT
SELECT count(*) from t1,t2 where t1.facility=t2.facility;
count(*)
12
select count(facility) from t1;
count(facility)
12
select count(*) from t1;
count(*)
13
select count(*) from t1 where facility IS NULL;
count(*)
1
select count(*) from t1 where facility = NULL;
count(*)
0
select count(*) from t1 where facility IS NOT NULL;
count(*)
12
select count(*) from t1 where id IS NULL;
count(*)
1
select count(*) from t1 where id IS NOT NULL;
count(*)
12
drop table t1,t2;
CREATE TABLE t1 (UserId int(11) DEFAULT '0' NOT NULL);
INSERT INTO t1 VALUES (20);
INSERT INTO t1 VALUES (27);
SELECT UserId FROM t1 WHERE Userid=22;
UserId
SELECT UserId FROM t1 WHERE UserId=22 group by Userid;
UserId
SELECT DISTINCT UserId FROM t1 WHERE UserId=22 group by Userid;
UserId
SELECT DISTINCT UserId FROM t1 WHERE UserId=22;
UserId
drop table t1;
CREATE TABLE t1 (a int(10) unsigned not null primary key,b int(10) unsigned);
INSERT INTO t1 VALUES (1,1),(2,1),(3,1),(4,1);
CREATE TABLE t2 (a int(10) unsigned not null, key (A));
INSERT INTO t2 VALUES (1),(2);
CREATE TABLE t3 (a int(10) unsigned, key(A), b text);
INSERT INTO t3 VALUES (1,'1'),(2,'2');
SELECT DISTINCT t3.b FROM t3,t2,t1 WHERE t3.a=t1.b AND t1.a=t2.a;
b
1
INSERT INTO t2 values (1),(2),(3);
INSERT INTO t3 VALUES (1,'1'),(2,'2'),(1,'1'),(2,'2');
explain SELECT distinct t3.a FROM t3,t2,t1 WHERE t3.a=t1.b AND t1.a=t2.a;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	ALL	PRIMARY	NULL	NULL	NULL	4	Using temporary
1	SIMPLE	t3	ref	a	a	5	test.t1.b	2	Using where; Using index
1	SIMPLE	t2	index	a	a	4	NULL	4	Using where; Using index; Distinct
SELECT distinct t3.a FROM t3,t2,t1 WHERE t3.a=t1.b AND t1.a=t2.a;
a
1
create temporary table t4 select * from t3;
insert into t3 select * from t4;
insert into t4 select * from t3;
insert into t3 select * from t4;
insert into t4 select * from t3;
insert into t3 select * from t4;
insert into t4 select * from t3;
insert into t3 select * from t4;
explain select distinct t1.a from t1,t3 where t1.a=t3.a;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	index	PRIMARY	PRIMARY	4	NULL	4	Using index; Using temporary
1	SIMPLE	t3	ref	a	a	5	test.t1.a	11	Using where; Using index; Distinct
select distinct t1.a from t1,t3 where t1.a=t3.a;
a
1
2
select distinct 1 from t1,t3 where t1.a=t3.a;
1
1
explain SELECT distinct t1.a from t1;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	4	Using index
explain SELECT distinct t1.a from t1 order by a desc;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	4	Using index
explain SELECT t1.a from t1 group by a order by a desc;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	4	Using index
explain SELECT distinct t1.a from t1 order by a desc limit 1;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	4	Using index
explain SELECT distinct a from t3 order by a desc limit 2;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t3	index	NULL	a	5	NULL	10	Using index
explain SELECT distinct a,b from t3 order by a+1;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	204	Using temporary; Using filesort
explain SELECT distinct a,b from t3 order by a limit 10;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t3	index	NULL	a	5	NULL	204	Using temporary
explain SELECT a,b from t3 group by a,b order by a+1;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t3	ALL	NULL	NULL	NULL	NULL	204	Using temporary; Using filesort
drop table t1,t2,t3,t4;
CREATE TABLE t1 (name varchar(255));
INSERT INTO t1 VALUES ('aa'),('ab'),('ac'),('ad'),('ae');
SELECT DISTINCT * FROM t1 LIMIT 2;
name
aa
ab
SELECT DISTINCT name FROM t1 LIMIT 2;
name
aa
ab
SELECT DISTINCT 1 FROM t1 LIMIT 2;
1
1
drop table t1;
CREATE TABLE t1 (
ID int(11) NOT NULL auto_increment,
NAME varchar(75) DEFAULT '' NOT NULL,
LINK_ID int(11) DEFAULT '0' NOT NULL,
PRIMARY KEY (ID),
KEY NAME (NAME),
KEY LINK_ID (LINK_ID)
);
INSERT INTO t1 (ID, NAME, LINK_ID) VALUES (1,'Mike',0),(2,'Jack',0),(3,'Bill',0);
CREATE TABLE t2 (
ID int(11) NOT NULL auto_increment,
NAME varchar(150) DEFAULT '' NOT NULL,
PRIMARY KEY (ID),
KEY NAME (NAME)
);
SELECT DISTINCT
t2.id AS key_link_id,
t2.name AS link
FROM t1
LEFT JOIN t2 ON t1.link_id=t2.id
GROUP BY t1.id
ORDER BY link;
key_link_id	link
NULL	NULL
drop table t1,t2;
create table t1 (
id		int not null,
name	tinytext not null,
unique	(id)
);
create table t2 (
id		int not null,
idx		int not null,
unique	(id, idx)
);
create table t3 (
id		int not null,
idx		int not null,
unique	(id, idx)
);
insert into t1 values (1,'yes'), (2,'no');
insert into t2 values (1,1);
insert into t3 values (1,1);
EXPLAIN
SELECT DISTINCT
t1.id
from
t1
straight_join
t2
straight_join
t3
straight_join
t1 as j_lj_t2 left join t2 as t2_lj
on j_lj_t2.id=t2_lj.id
straight_join
t1 as j_lj_t3 left join t3 as t3_lj
on j_lj_t3.id=t3_lj.id
WHERE
((t1.id=j_lj_t2.id AND t2_lj.id IS NULL) OR (t1.id=t2.id AND t2.idx=2))
AND ((t1.id=j_lj_t3.id AND t3_lj.id IS NULL) OR (t1.id=t3.id AND t3.idx=2));
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	index	id	id	4	NULL	2	Using index; Using temporary
1	SIMPLE	t2	index	id	id	8	NULL	1	Using index; Distinct
1	SIMPLE	t3	index	id	id	8	NULL	1	Using index; Distinct
1	SIMPLE	j_lj_t2	index	id	id	4	NULL	2	Using where; Using index; Distinct
1	SIMPLE	t2_lj	ref	id	id	4	test.j_lj_t2.id	1	Using where; Using index; Distinct
1	SIMPLE	j_lj_t3	index	id	id	4	NULL	2	Using where; Using index; Distinct
1	SIMPLE	t3_lj	ref	id	id	4	test.j_lj_t3.id	1	Using where; Using index; Distinct
SELECT DISTINCT
t1.id
from
t1
straight_join
t2
straight_join
t3
straight_join
t1 as j_lj_t2 left join t2 as t2_lj
on j_lj_t2.id=t2_lj.id
straight_join
t1 as j_lj_t3 left join t3 as t3_lj
on j_lj_t3.id=t3_lj.id
WHERE
((t1.id=j_lj_t2.id AND t2_lj.id IS NULL) OR (t1.id=t2.id AND t2.idx=2))
AND ((t1.id=j_lj_t3.id AND t3_lj.id IS NULL) OR (t1.id=t3.id AND t3.idx=2));
id
2
drop table t1,t2,t3;
create table t1 (a int not null, b int not null, t time);
insert into t1 values (1,1,"00:06:15"),(1,2,"00:06:15"),(1,2,"00:30:15"),(1,3,"00:06:15"),(1,3,"00:30:15");
select a,sec_to_time(sum(time_to_sec(t))) from t1 group by a,b;
a	sec_to_time(sum(time_to_sec(t)))
1	00:06:15
1	00:36:30
1	00:36:30
select distinct a,sec_to_time(sum(time_to_sec(t))) from t1 group by a,b;
a	sec_to_time(sum(time_to_sec(t)))
1	00:06:15
1	00:36:30
create table t2 (a int not null primary key, b int);
insert into t2 values (1,1),(2,2),(3,3);
select t1.a,sec_to_time(sum(time_to_sec(t))) from t1 left join t2 on (t1.b=t2.a) group by t1.a,t2.b;
a	sec_to_time(sum(time_to_sec(t)))
1	00:06:15
1	00:36:30
1	00:36:30
select distinct t1.a,sec_to_time(sum(time_to_sec(t))) from t1 left join t2 on (t1.b=t2.a) group by t1.a,t2.b;
a	sec_to_time(sum(time_to_sec(t)))
1	00:06:15
1	00:36:30
drop table t1,t2;
create table t1 (a int not null,b char(5), c text);
insert into t1 (a) values (1),(2),(3),(4),(1),(2),(3),(4);
select distinct a from t1 group by b,a having a > 2 order by a desc;
a
4
3
select distinct a,c from t1 group by b,c,a having a > 2 order by a desc;
a	c
4	NULL
3	NULL
drop table t1;
create table t1 (a char(1), key(a)) engine=myisam;
insert into t1 values('1'),('1');
select * from t1 where a >= '1';
a
1
1
select distinct a from t1 order by a desc;
a
1
select distinct a from t1 where a >= '1' order by a desc;
a
1
drop table t1;
CREATE TABLE t1 (email varchar(50), infoID BIGINT, dateentered DATETIME);
CREATE TABLE t2 (infoID BIGINT, shipcode varchar(10));
INSERT INTO t1 (email, infoID, dateentered) VALUES
('test1@testdomain.com', 1, '2002-07-30 22:56:38'),
('test1@testdomain.com', 1, '2002-07-27 22:58:16'),
('test2@testdomain.com', 1, '2002-06-19 15:22:19'),
('test2@testdomain.com', 2, '2002-06-18 14:23:47'),
('test3@testdomain.com', 1, '2002-05-19 22:17:32');
INSERT INTO t2(infoID, shipcode) VALUES
(1, 'Z001'),
(2, 'R002');
SELECT DISTINCTROW email, shipcode FROM t1, t2 WHERE t1.infoID=t2.infoID;
email	shipcode
test1@testdomain.com	Z001
test2@testdomain.com	Z001
test2@testdomain.com	R002
test3@testdomain.com	Z001
SELECT DISTINCTROW email FROM t1 ORDER BY dateentered DESC;
email
test1@testdomain.com
test2@testdomain.com
test3@testdomain.com
SELECT DISTINCTROW email, shipcode FROM t1, t2 WHERE t1.infoID=t2.infoID ORDER BY dateentered DESC;
email	shipcode
test1@testdomain.com	Z001
test2@testdomain.com	Z001
test2@testdomain.com	R002
test3@testdomain.com	Z001
drop table t1,t2;
CREATE TABLE t1 (privatemessageid int(10) unsigned NOT NULL auto_increment,  folderid smallint(6) NOT NULL default '0',  userid int(10) unsigned NOT NULL default '0',  touserid int(10) unsigned NOT NULL default '0',  fromuserid int(10) unsigned NOT NULL default '0',  title varchar(250) NOT NULL default '',  message mediumtext NOT NULL,  dateline int(10) unsigned NOT NULL default '0',  showsignature smallint(6) NOT NULL default '0',  iconid smallint(5) unsigned NOT NULL default '0',  messageread smallint(6) NOT NULL default '0',  readtime int(10) unsigned NOT NULL default '0',  receipt smallint(6) unsigned NOT NULL default '0',  deleteprompt smallint(6) unsigned NOT NULL default '0',  multiplerecipients smallint(6) unsigned NOT NULL default '0',  PRIMARY KEY  (privatemessageid),  KEY userid (userid)) ENGINE=MyISAM;
INSERT INTO t1 VALUES (128,0,33,33,8,':D','',996121863,1,0,2,996122850,2,0,0);
CREATE TABLE t2 (userid int(10) unsigned NOT NULL auto_increment,  usergroupid smallint(5) unsigned NOT NULL default '0',  username varchar(50) NOT NULL default '',  password varchar(50) NOT NULL default '',  email varchar(50) NOT NULL default '',  styleid smallint(5) unsigned NOT NULL default '0',  parentemail varchar(50) NOT NULL default '',  coppauser smallint(6) NOT NULL default '0',  homepage varchar(100) NOT NULL default '',  icq varchar(20) NOT NULL default '',  aim varchar(20) NOT NULL default '',  yahoo varchar(20) NOT NULL default '',  signature mediumtext NOT NULL,  adminemail smallint(6) NOT NULL default '0',  showemail smallint(6) NOT NULL default '0',  invisible smallint(6) NOT NULL default '0',  usertitle varchar(250) NOT NULL default '',  customtitle smallint(6) NOT NULL default '0',  joindate int(10) unsigned NOT NULL default '0',  cookieuser smallint(6) NOT NULL default '0',  daysprune smallint(6) NOT NULL default '0',  lastvisit int(10) unsigned NOT NULL default '0',  lastactivity int(10) unsigned NOT NULL default '0',  lastpost int(10) unsigned NOT NULL default '0',  posts smallint(5) unsigned NOT NULL default '0',  timezoneoffset varchar(4) NOT NULL default '',  emailnotification smallint(6) NOT NULL default '0',  buddylist mediumtext NOT NULL,  ignorelist mediumtext NOT NULL,  pmfolders mediumtext NOT NULL,  receivepm smallint(6) NOT NULL default '0',  emailonpm smallint(6) NOT NULL default '0',  pmpopup smallint(6) NOT NULL default '0',  avatarid smallint(6) NOT NULL default '0',  avatarrevision int(6) unsigned NOT NULL default '0',  options smallint(6) NOT NULL default '15',  birthday date NOT NULL default '0000-00-00',  maxposts smallint(6) NOT NULL default '-1',  startofweek smallint(6) NOT NULL default '1',  ipaddress varchar(20) NOT NULL default '',  referrerid int(10) unsigned NOT NULL default '0',  nosessionhash smallint(6) NOT NULL default '0',  autorefresh smallint(6) NOT NULL default '-1',  messagepopup tinyint(2) NOT NULL default '0',  inforum smallint(5) unsigned NOT NULL default '0',  ratenum smallint(5) unsigned NOT NULL default '0',  ratetotal smallint(5) unsigned NOT NULL default '0',  allowrate smallint(5) unsigned NOT NULL default '1',  PRIMARY KEY  (userid),  KEY usergroupid (usergroupid),  KEY username (username),  KEY inforum (inforum)) ENGINE=MyISAM;
INSERT INTO t2 VALUES (33,6,'Kevin','0','kevin@stileproject.com',1,'',0,'http://www.stileproject.com','','','','',1,1,0,'Administrator',0,996120694,1,-1,1030996168,1031027028,1030599436,36,'-6',0,'','','',1,0,1,0,0,15,'0000-00-00',-1,1,'64.0.0.0',0,1,-1,0,0,4,19,1);
SELECT DISTINCT t1.*, t2.* FROM t1 LEFT JOIN t2 ON (t2.userid = t1.touserid);
privatemessageid	folderid	userid	touserid	fromuserid	title	message	dateline	showsignature	iconid	messageread	readtime	receipt	deleteprompt	multiplerecipients	userid	usergroupid	username	password	email	styleid	parentemail	coppauser	homepage	icq	aim	yahoo	signature	adminemail	showemail	invisible	usertitle	customtitle	joindate	cookieuser	daysprune	lastvisit	lastactivity	lastpost	posts	timezoneoffset	emailnotification	buddylist	ignorelist	pmfolders	receivepm	emailonpm	pmpopup	avatarid	avatarrevision	options	birthday	maxposts	startofweek	ipaddress	referrerid	nosessionhash	autorefresh	messagepopup	inforum	ratenum	ratetotal	allowrate
128	0	33	33	8	:D		996121863	1	0	2	996122850	2	0	0	33	6	Kevin	0	kevin@stileproject.com	1		0	http://www.stileproject.com					1	1	0	Administrator	0	996120694	1	-1	1030996168	1031027028	1030599436	36	-6	0				1	0	1	0	0	15	0000-00-00	-1	1	64.0.0.0	0	1	-1	0	0	4	19	1
DROP TABLE t1,t2;
CREATE TABLE t1 (a int primary key, b int, c int);
INSERT t1 VALUES (1,2,3);
CREATE TABLE t2 (a int primary key, b int, c int);
INSERT t2 VALUES (3,4,5);
SELECT DISTINCT t1.a, t2.b FROM t1, t2 WHERE t1.a=1 ORDER BY t2.c;
a	b
1	4
DROP TABLE t1,t2;
CREATE table t1 (  `id` int(11) NOT NULL auto_increment,  `name` varchar(50) NOT NULL default '',  PRIMARY KEY  (`id`)) ENGINE=MyISAM AUTO_INCREMENT=3 ;
INSERT INTO t1 VALUES (1, 'aaaaa');
INSERT INTO t1 VALUES (3, 'aaaaa');
INSERT INTO t1 VALUES (2, 'eeeeeee');
select distinct left(name,1) as name from t1;
name
a
e
drop  table t1;
CREATE TABLE t1 (
ID int(11) NOT NULL auto_increment,
NAME varchar(75) DEFAULT '' NOT NULL,
LINK_ID int(11) DEFAULT '0' NOT NULL,
PRIMARY KEY (ID),
KEY NAME (NAME),
KEY LINK_ID (LINK_ID)
);
INSERT INTO t1 (ID, NAME, LINK_ID) VALUES (1,'Mike',0);
INSERT INTO t1 (ID, NAME, LINK_ID) VALUES (2,'Jack',0);
INSERT INTO t1 (ID, NAME, LINK_ID) VALUES (3,'Bill',0);
CREATE TABLE t2 (
ID int(11) NOT NULL auto_increment,
NAME varchar(150) DEFAULT '' NOT NULL,
PRIMARY KEY (ID),
KEY NAME (NAME)
);
SELECT DISTINCT
t2.id AS key_link_id,
t2.name AS link
FROM t1
LEFT JOIN t2 ON t1.link_id=t2.id
GROUP BY t1.id
ORDER BY link;
key_link_id	link
NULL	NULL
drop table t1,t2;
CREATE TABLE t1 (
html varchar(5) default NULL,
rin int(11) default '0',
rout int(11) default '0'
) ENGINE=MyISAM;
INSERT INTO t1 VALUES ('1',1,0);
SELECT DISTINCT html,SUM(rout)/(SUM(rin)+1) as 'prod' FROM t1 GROUP BY rin;
html	prod
1	0.0000
drop table t1;
CREATE TABLE t1 (a int);
INSERT INTO t1 VALUES (1),(2),(3),(4),(5);
SELECT DISTINCT a, 1 FROM t1;
a	1
1	1
2	1
3	1
4	1
5	1
SELECT DISTINCT 1, a FROM t1;
1	a
1	1
1	2
1	3
1	4
1	5
CREATE TABLE t2 (a int, b int);
INSERT INTO t2 VALUES (1,1),(2,2),(2,3),(2,4),(3,5);
SELECT DISTINCT a, b, 2 FROM t2;
a	b	2
1	1	2
2	2	2
2	3	2
2	4	2
3	5	2
SELECT DISTINCT 2, a, b FROM t2;
2	a	b
2	1	1
2	2	2
2	2	3
2	2	4
2	3	5
SELECT DISTINCT a, 2, b FROM t2;
a	2	b
1	2	1
2	2	2
2	2	3
2	2	4
3	2	5
DROP TABLE t1,t2;
CREATE TABLE t1(a INT PRIMARY KEY, b INT);
INSERT INTO t1 VALUES (1,1), (2,1), (3,1);
EXPLAIN SELECT DISTINCT a FROM t1;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	3	Using index
EXPLAIN SELECT DISTINCT a,b FROM t1;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	
EXPLAIN SELECT DISTINCT t1_1.a, t1_1.b FROM t1 t1_1, t1 t1_2;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1_1	ALL	NULL	NULL	NULL	NULL	3	Using temporary
1	SIMPLE	t1_2	index	NULL	PRIMARY	4	NULL	3	Using index; Distinct
EXPLAIN SELECT DISTINCT t1_1.a, t1_1.b FROM t1 t1_1, t1 t1_2
WHERE t1_1.a = t1_2.a;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1_1	ALL	PRIMARY	NULL	NULL	NULL	3	Using temporary
1	SIMPLE	t1_2	eq_ref	PRIMARY	PRIMARY	4	test.t1_1.a	1	Using index; Distinct
EXPLAIN SELECT a FROM t1 GROUP BY a;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	3	Using index
EXPLAIN SELECT a,b FROM t1 GROUP BY a,b;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	
EXPLAIN SELECT DISTINCT a,b FROM t1 GROUP BY a,b;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	
CREATE TABLE t2(a INT, b INT, c INT, d INT, PRIMARY KEY (a,b));
INSERT INTO t2 VALUES (1,1,1,50), (1,2,3,40), (2,1,3,4);
EXPLAIN SELECT DISTINCT a FROM t2;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t2	index	NULL	PRIMARY	8	NULL	3	Using index
EXPLAIN SELECT DISTINCT a,a FROM t2;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t2	index	NULL	PRIMARY	8	NULL	3	Using index
EXPLAIN SELECT DISTINCT b,a FROM t2;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t2	index	NULL	PRIMARY	8	NULL	3	Using index
EXPLAIN SELECT DISTINCT a,c FROM t2;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	3	Using temporary
EXPLAIN SELECT DISTINCT c,a,b FROM t2;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	3	
EXPLAIN SELECT DISTINCT a,b,d FROM t2 GROUP BY c,b,d;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	3	Using temporary; Using filesort
CREATE UNIQUE INDEX c_b_unq ON t2 (c,b);
EXPLAIN SELECT DISTINCT a,b,d FROM t2 GROUP BY c,b,d;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	3	
DROP TABLE t1,t2;
create table t1 (id int, dsc varchar(50));
insert into t1 values (1, "line number one"), (2, "line number two"), (3, "line number three");
select distinct id, IFNULL(dsc, '-') from t1;
id	IFNULL(dsc, '-')
1	line number one
2	line number two
3	line number three
drop table t1;
CREATE TABLE t1 (a int primary key, b int);
INSERT INTO t1 (a,b) values (1,1), (2,3), (3,2);
explain SELECT DISTINCT a, b FROM t1 ORDER BY b;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using filesort
SELECT DISTINCT a, b FROM t1 ORDER BY b;
a	b
1	1
3	2
2	3
DROP TABLE t1;
CREATE TABLE t1 (
ID int(11) NOT NULL auto_increment,
x varchar(20) default NULL,
y decimal(10,0) default NULL,
PRIMARY KEY  (ID),
KEY (y)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
INSERT INTO t1 VALUES
(1,'ba','-1'),
(2,'ba','1150'),
(306,'ba','-1'),
(307,'ba','1150'),
(611,'ba','-1'),
(612,'ba','1150');
select count(distinct x,y) from t1;
count(distinct x,y)
2
select count(distinct concat(x,y)) from t1;
count(distinct concat(x,y))
2
drop table t1;
CREATE TABLE t1 (a INT, b INT, PRIMARY KEY (a,b));
INSERT INTO t1 VALUES (1, 101);
INSERT INTO t1 SELECT a + 1, a + 101 FROM t1;
INSERT INTO t1 SELECT a + 2, a + 102 FROM t1;
INSERT INTO t1 SELECT a + 4, a + 104 FROM t1;
INSERT INTO t1 SELECT a + 8, a + 108 FROM t1;
EXPLAIN SELECT DISTINCT a,a FROM t1 WHERE b < 12 ORDER BY a;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	index	NULL	PRIMARY	8	NULL	16	Using where; Using index
SELECT DISTINCT a,a FROM t1 WHERE b < 12 ORDER BY a;
a	a
DROP TABLE t1;
