select (select 2); (select 2) 2 SELECT (SELECT 1) UNION SELECT (SELECT 2); (SELECT 1) 1 2 drop table if exists t1,t2,t3,t4,t5,attend,clinic,inscrit; create table t1 (a int); create table t2 (a int, b int); create table t3 (a int); create table t4 (a int, b int); insert into t1 values (2); insert into t2 values (1,7),(2,7); insert into t4 values (4,8),(3,8),(5,9); select (select a from t1 where t1.a=t2.a), a from t2; (select a from t1 where t1.a=t2.a) a NULL 1 2 2 select (select a from t1 where t1.a=t2.b), a from t2; (select a from t1 where t1.a=t2.b) a NULL 1 NULL 2 select (select a from t1), a from t2; (select a from t1) a 2 1 2 2 select (select a from t3), a from t2; (select a from t3) a NULL 1 NULL 2 select * from t2 where t2.a=(select a from t1); a b 2 7 insert into t3 values (6),(7),(3); select * from t2 where t2.b=(select a from t3 order by 1 desc limit 1); a b 1 7 2 7 select * from t2 where t2.b=(select a from t3 order by 1 desc limit 1) union (select * from t4 order by a limit 2) limit 3; a b 1 7 2 7 3 8 select * from t2 where t2.b=(select a from t3 order by 1 desc limit 1) union (select * from t4 where t4.b=(select max(t2.a)*4 from t2) order by a); a b 1 7 2 7 3 8 4 8 explain select * from t2 where t2.b=(select a from t3 order by 1 desc limit 1) union (select * from t4 where t4.b=(select max(t2.a)*4 from t2) order by a); id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY t2 ALL NULL NULL NULL NULL 2 where used 2 SUBSELECT t3 ALL NULL NULL NULL NULL 3 Using filesort 3 UNION t4 ALL NULL NULL NULL NULL 3 where used; Using filesort 4 SUBSELECT t2 ALL NULL NULL NULL NULL 2 select (select a from t3 where a1) as tt; (select t3.a from t3 where a<8 order by 1 desc limit 1) a 7 2 explain select (select t3.a from t3 where a<8 order by 1 desc limit 1), a from (select * from t2 where a>1) as tt; id select_type table type possible_keys key key_len ref rows Extra 3 DERIVED t2 ALL NULL NULL NULL NULL 2 where used 1 PRIMARY system NULL NULL NULL NULL 1 2 SUBSELECT t3 ALL NULL NULL NULL NULL 3 where used; Using filesort select * from t1 where t1.a=(select t2.a from t2 where t2.b=(select max(a) from t3) order by 1 desc limit 1); a 2 select * from t1 where t1.a=(select t2.a from t2 where t2.b=(select max(a) from t3 where t3.a > t1.a) order by 1 desc limit 1); a 2 select * from t1 where t1.a=(select t2.a from t2 where t2.b=(select max(a) from t3 where t3.a < t1.a) order by 1 desc limit 1); a select b,(select avg(t2.a+(select min(t3.a) from t3 where t3.a >= t4.a)) from t2) from t4; b (select avg(t2.a+(select min(t3.a) from t3 where t3.a >= t4.a)) from t2) 8 7.5000 8 4.5000 9 7.5000 explain select b,(select avg(t2.a+(select min(t3.a) from t3 where t3.a >= t4.a)) from t2) from t4; id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY t4 ALL NULL NULL NULL NULL 3 2 DEPENDENT SUBSELECT t2 ALL NULL NULL NULL NULL 2 3 DEPENDENT SUBSELECT t3 ALL NULL NULL NULL NULL 3 where used select * from t3 where exists (select * from t2 where t2.b=t3.a); a 7 select * from t3 where not exists (select * from t2 where t2.b=t3.a); a 6 3 insert into t4 values (12,7),(1,7),(10,9),(9,6),(7,6),(3,9); select b,max(a) as ma from t4 group by b having b < (select max(t2.a) from t2 where t2.b=t4.b); b ma select b,max(a) as ma from t4 group by b having b >= (select max(t2.a) from t2 where t2.b=t4.b); b ma 7 12 create table t5 (a int); select (select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a), a from t2; (select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a) a NULL 1 2 2 insert into t5 values (5); select (select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a), a from t2; (select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a) a NULL 1 2 2 insert into t5 values (2); select (select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a), a from t2; (select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a) a NULL 1 2 2 explain select (select a from t1 where t1.a=t2.a union select a from t5 where t5.a=t2.a), a from t2; id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY t2 ALL NULL NULL NULL NULL 2 2 DEPENDENT SUBSELECT t1 system NULL NULL NULL NULL 1 where used 3 DEPENDENT UNION t5 ALL NULL NULL NULL NULL 2 where used select (select a from t1 where t1.a=t2.a union all select a from t5 where t5.a=t2.a), a from t2; Subselect returns more than 1 record create table attend (patient_uq int, clinic_uq int, index i1 (clinic_uq)); create table clinic( uq int primary key, name char(25)); insert into clinic values(1,"Oblastnaia bolnitsa"),(2,"Bolnitsa Krasnogo Kresta"); insert into attend values (1,1),(1,2),(2,2),(1,3); select * from attend where exists (select * from clinic where uq = clinic_uq); patient_uq clinic_uq 1 1 1 2 2 2 select * from t1 where a= (select a from t2,t4 where t2.b=t4.b); Column: 'a' in field list is ambiguous drop table if exists t1,t2,t3; CREATE TABLE t3 (a varchar(20),b char(1) NOT NULL default '0'); INSERT INTO t3 VALUES ('W','a'),('A','c'),('J','b'); CREATE TABLE t2 (a varchar(20),b int NOT NULL default '0'); INSERT INTO t2 VALUES ('W','1'),('A','3'),('J','2'); CREATE TABLE t1 (a varchar(20),b date NOT NULL default '0000-00-00'); INSERT INTO t1 VALUES ('W','1732-02-22'),('A','1735-10-30'),('J','1743-04-13'); SELECT * FROM t1 WHERE b = (SELECT MIN(b) FROM t1); a b W 1732-02-22 SELECT * FROM t2 WHERE b = (SELECT MIN(b) FROM t2); a b W 1 SELECT * FROM t3 WHERE b = (SELECT MIN(b) FROM t3); a b W a drop table if exists inscrit; CREATE TABLE `inscrit` ( `pseudo` varchar(35) character set latin1 NOT NULL default '', `email` varchar(60) character set latin1 NOT NULL default '', PRIMARY KEY (`pseudo`), UNIQUE KEY `email` (`email`) ) TYPE=MyISAM CHARSET=latin1 ROW_FORMAT=DYNAMIC; INSERT INTO inscrit (pseudo,email) VALUES ('joce','test'); INSERT INTO inscrit (pseudo,email) VALUES ('joce1','test1'); INSERT INTO inscrit (pseudo,email) VALUES ('2joce1','2test1'); EXPLAIN SELECT pseudo,(SELECT email FROM inscrit WHERE pseudo=(SELECT pseudo FROM inscrit WHERE pseudo='joce')) FROM inscrit WHERE pseudo=(SELECT pseudo FROM inscrit WHERE pseudo='joce'); id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY inscrit const PRIMARY PRIMARY 35 const 1 4 SUBSELECT inscrit const PRIMARY PRIMARY 35 const 1 2 SUBSELECT inscrit const PRIMARY PRIMARY 35 const 1 3 SUBSELECT inscrit const PRIMARY PRIMARY 35 const 1 SELECT pseudo FROM inscrit WHERE pseudo=(SELECT pseudo,email FROM inscrit WHERE pseudo='joce'); Subselect returns more than 1 field SELECT pseudo FROM inscrit WHERE pseudo=(SELECT * FROM inscrit WHERE pseudo='joce'); Subselect returns more than 1 field SELECT pseudo FROM inscrit WHERE pseudo=(SELECT pseudo FROM inscrit WHERE pseudo='joce'); pseudo joce SELECT pseudo FROM inscrit WHERE pseudo=(SELECT pseudo FROM inscrit WHERE pseudo LIKE '%joce%'); Subselect returns more than 1 record drop table if exists t1,t2,t3,t4,t5,attend,clinic,inscrit; drop table if exists searchconthardwarefr3; CREATE TABLE `searchconthardwarefr3` ( `topic` mediumint(8) unsigned NOT NULL default '0', `date` date NOT NULL default '0000-00-00', `pseudo` varchar(35) character set latin1 NOT NULL default '', PRIMARY KEY (`pseudo`,`date`,`topic`), KEY `topic` (`topic`) ) TYPE=MyISAM ROW_FORMAT=DYNAMIC; INSERT INTO searchconthardwarefr3 (topic,date,pseudo) VALUES ('43506','2002-10-02','joce'),('40143','2002-08-03','joce'); EXPLAIN SELECT DISTINCT date FROM searchconthardwarefr3 WHERE date='2002-08-03'; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE searchconthardwarefr3 index NULL PRIMARY 41 NULL 2 where used; Using index EXPLAIN SELECT (SELECT DISTINCT date FROM searchconthardwarefr3 WHERE date='2002-08-03'); id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY No tables used 2 SUBSELECT searchconthardwarefr3 index NULL PRIMARY 41 NULL 2 where used; Using index drop table searchconthardwarefr3;