drop table if exists t1,t2,t3,t4,t5; CREATE TABLE t1 ( grp int(11) default NULL, a bigint(20) unsigned default NULL, c char(10) NOT NULL default '' ) TYPE=MyISAM; INSERT INTO t1 VALUES (1,1,'a'),(2,2,'b'),(2,3,'c'),(3,4,'E'),(3,5,'C'),(3,6,'D'),(NULL,NULL,''); create table t2 (id int, a bigint unsigned not null, c char(10), d int, primary key (a)); insert into t2 values (1,1,"a",1),(3,4,"A",4),(3,5,"B",5),(3,6,"C",6),(4,7,"D",7); select t1.*,t2.* from t1 JOIN t2 where t1.a=t2.a; grp a c id a c d 1 1 a 1 1 a 1 3 4 E 3 4 A 4 3 5 C 3 5 B 5 3 6 D 3 6 C 6 select t1.*,t2.* from t1 left join t2 on (t1.a=t2.a) order by t1.grp,t1.a,t2.c; grp a c id a c d NULL NULL NULL NULL NULL NULL 1 1 a 1 1 a 1 2 2 b NULL NULL NULL NULL 2 3 c NULL NULL NULL NULL 3 4 E 3 4 A 4 3 5 C 3 5 B 5 3 6 D 3 6 C 6 select t1.*,t2.* from { oj t2 left outer join t1 on (t1.a=t2.a) }; grp a c id a c d 1 1 a 1 1 a 1 3 4 E 3 4 A 4 3 5 C 3 5 B 5 3 6 D 3 6 C 6 NULL NULL NULL 4 7 D 7 select t1.*,t2.* from t1 as t0,{ oj t2 left outer join t1 on (t1.a=t2.a) } WHERE t0.a=2; grp a c id a c d 1 1 a 1 1 a 1 3 4 E 3 4 A 4 3 5 C 3 5 B 5 3 6 D 3 6 C 6 NULL NULL NULL 4 7 D 7 select t1.*,t2.* from t1 left join t2 using (a); grp a c id a c d 1 1 a 1 1 a 1 2 2 b NULL NULL NULL NULL 2 3 c NULL NULL NULL NULL 3 4 E 3 4 A 4 3 5 C 3 5 B 5 3 6 D 3 6 C 6 NULL NULL NULL NULL NULL NULL select t1.*,t2.* from t1 left join t2 using (a) where t1.a=t2.a; grp a c id a c d 1 1 a 1 1 a 1 3 4 E 3 4 A 4 3 5 C 3 5 B 5 3 6 D 3 6 C 6 select t1.*,t2.* from t1 left join t2 using (a,c); grp a c id a c d 1 1 a 1 1 a 1 2 2 b NULL NULL NULL NULL 2 3 c NULL NULL NULL NULL 3 4 E NULL NULL NULL NULL 3 5 C NULL NULL NULL NULL 3 6 D NULL NULL NULL NULL NULL NULL NULL NULL NULL NULL select t1.*,t2.* from t1 left join t2 using (c); grp a c id a c d 1 1 a 1 1 a 1 1 1 a 3 4 A 4 2 2 b 3 5 B 5 2 3 c 3 6 C 6 3 4 E NULL NULL NULL NULL 3 5 C 3 6 C 6 3 6 D 4 7 D 7 NULL NULL NULL NULL NULL NULL select t1.*,t2.* from t1 natural left outer join t2; grp a c id a c d 1 1 a 1 1 a 1 2 2 b NULL NULL NULL NULL 2 3 c NULL NULL NULL NULL 3 4 E NULL NULL NULL NULL 3 5 C NULL NULL NULL NULL 3 6 D NULL NULL NULL NULL NULL NULL NULL NULL NULL NULL select t1.*,t2.* from t1 left join t2 on (t1.a=t2.a) where t2.id=3; grp a c id a c d 3 4 E 3 4 A 4 3 5 C 3 5 B 5 3 6 D 3 6 C 6 select t1.*,t2.* from t1 left join t2 on (t1.a=t2.a) where t2.id is null; grp a c id a c d 2 2 b NULL NULL NULL NULL 2 3 c NULL NULL NULL NULL NULL NULL NULL NULL NULL NULL explain select t1.*,t2.* from t1,t2 where t1.a=t2.a and isnull(t2.a)=1; id select_type table type possible_keys key key_len ref rows Extra 1 FIRST Impossible WHERE noticed after reading const tables explain select t1.*,t2.* from t1 left join t2 on t1.a=t2.a where isnull(t2.a)=1; id select_type table type possible_keys key key_len ref rows Extra 1 FIRST t1 ALL NULL NULL NULL NULL 7 1 FIRST t2 eq_ref PRIMARY PRIMARY 8 t1.a 1 where used select t1.*,t2.*,t3.a from t1 left join t2 on (t1.a=t2.a) left join t1 as t3 on (t2.a=t3.a); grp a c id a c d a 1 1 a 1 1 a 1 1 2 2 b NULL NULL NULL NULL NULL 2 3 c NULL NULL NULL NULL NULL 3 4 E 3 4 A 4 4 3 5 C 3 5 B 5 5 3 6 D 3 6 C 6 6 NULL NULL NULL NULL NULL NULL NULL explain select t1.*,t2.*,t3.a from t1 left join t2 on (t3.a=t2.a) left join t1 as t3 on (t1.a=t3.a); Cross dependency found in OUTER JOIN. Examine your ON conditions select t1.*,t2.*,t3.a from t1 left join t2 on (t3.a=t2.a) left join t1 as t3 on (t1.a=t3.a); Cross dependency found in OUTER JOIN. Examine your ON conditions select t1.*,t2.*,t3.a from t1 left join t2 on (t3.a=t2.a) left join t1 as t3 on (t2.a=t3.a); Cross dependency found in OUTER JOIN. Examine your ON conditions select t1.*,t2.* from t1 inner join t2 using (a); grp a c id a c d 1 1 a 1 1 a 1 3 4 E 3 4 A 4 3 5 C 3 5 B 5 3 6 D 3 6 C 6 select t1.*,t2.* from t1 inner join t2 on (t1.a=t2.a); grp a c id a c d 1 1 a 1 1 a 1 3 4 E 3 4 A 4 3 5 C 3 5 B 5 3 6 D 3 6 C 6 select t1.*,t2.* from t1 natural join t2; grp a c id a c d 1 1 a 1 1 a 1 drop table t1,t2; CREATE TABLE t1 ( usr_id INT unsigned NOT NULL, uniq_id INT unsigned NOT NULL AUTO_INCREMENT, start_num INT unsigned NOT NULL DEFAULT 1, increment INT unsigned NOT NULL DEFAULT 1, PRIMARY KEY (uniq_id), INDEX usr_uniq_idx (usr_id, uniq_id), INDEX uniq_usr_idx (uniq_id, usr_id) ); CREATE TABLE t2 ( id INT unsigned NOT NULL DEFAULT 0, usr2_id INT unsigned NOT NULL DEFAULT 0, max INT unsigned NOT NULL DEFAULT 0, c_amount INT unsigned NOT NULL DEFAULT 0, d_max INT unsigned NOT NULL DEFAULT 0, d_num INT unsigned NOT NULL DEFAULT 0, orig_time INT unsigned NOT NULL DEFAULT 0, c_time INT unsigned NOT NULL DEFAULT 0, active ENUM ("no","yes") NOT NULL, PRIMARY KEY (id,usr2_id), INDEX id_idx (id), INDEX usr2_idx (usr2_id) ); INSERT INTO t1 VALUES (3,NULL,0,50),(3,NULL,0,200),(3,NULL,0,25),(3,NULL,0,84676),(3,NULL,0,235),(3,NULL,0,10),(3,NULL,0,3098),(3,NULL,0,2947),(3,NULL,0,8987),(3,NULL,0,8347654),(3,NULL,0,20398),(3,NULL,0,8976),(3,NULL,0,500),(3,NULL,0,198); SELECT t1.usr_id,t1.uniq_id,t1.increment, t2.usr2_id,t2.c_amount,t2.max FROM t1 LEFT JOIN t2 ON t2.id = t1.uniq_id WHERE t1.uniq_id = 4 ORDER BY t2.c_amount; usr_id uniq_id increment usr2_id c_amount max 3 4 84676 NULL NULL NULL SELECT t1.usr_id,t1.uniq_id,t1.increment, t2.usr2_id,t2.c_amount,t2.max FROM t2 RIGHT JOIN t1 ON t2.id = t1.uniq_id WHERE t1.uniq_id = 4 ORDER BY t2.c_amount; usr_id uniq_id increment usr2_id c_amount max 3 4 84676 NULL NULL NULL INSERT INTO t2 VALUES (2,3,3000,6000,0,0,746584,837484,'yes'); INSERT INTO t2 VALUES (2,3,3000,6000,0,0,746584,837484,'yes'); Duplicate entry '2-3' for key 1 INSERT INTO t2 VALUES (7,3,1000,2000,0,0,746294,937484,'yes'); SELECT t1.usr_id,t1.uniq_id,t1.increment,t2.usr2_id,t2.c_amount,t2.max FROM t1 LEFT JOIN t2 ON t2.id = t1.uniq_id WHERE t1.uniq_id = 4 ORDER BY t2.c_amount; usr_id uniq_id increment usr2_id c_amount max 3 4 84676 NULL NULL NULL SELECT t1.usr_id,t1.uniq_id,t1.increment,t2.usr2_id,t2.c_amount,t2.max FROM t1 LEFT JOIN t2 ON t2.id = t1.uniq_id WHERE t1.uniq_id = 4 GROUP BY t2.c_amount; usr_id uniq_id increment usr2_id c_amount max 3 4 84676 NULL NULL NULL SELECT t1.usr_id,t1.uniq_id,t1.increment,t2.usr2_id,t2.c_amount,t2.max FROM t1 LEFT JOIN t2 ON t2.id = t1.uniq_id WHERE t1.uniq_id = 4; usr_id uniq_id increment usr2_id c_amount max 3 4 84676 NULL NULL NULL drop table t1,t2; drop table if exists t1,t2,t3,t4; CREATE TABLE t1 ( cod_asig int(11) DEFAULT '0' NOT NULL, desc_larga_cat varchar(80) DEFAULT '' NOT NULL, desc_larga_cas varchar(80) DEFAULT '' NOT NULL, desc_corta_cat varchar(40) DEFAULT '' NOT NULL, desc_corta_cas varchar(40) DEFAULT '' NOT NULL, cred_total double(3,1) DEFAULT '0.0' NOT NULL, pre_requisit int(11), co_requisit int(11), preco_requisit int(11), PRIMARY KEY (cod_asig) ); INSERT INTO t1 VALUES (10360,'asdfggfg','Introduccion a los Ordenadores I','asdfggfg','Introduccio Ordinadors I',6.0,NULL,NULL,NULL); INSERT INTO t1 VALUES (10361,'Components i Circuits Electronics I','Componentes y Circuitos Electronicos I','Components i Circuits Electronics I','Comp. i Circ. Electr. I',6.0,NULL,NULL,NULL); INSERT INTO t1 VALUES (10362,'Laboratori d`Ordinadors','Laboratorio de Ordenadores','Laboratori d`Ordinadors','Laboratori Ordinadors',4.5,NULL,NULL,NULL); INSERT INTO t1 VALUES (10363,'Tecniques de Comunicacio Oral i Escrita','Tecnicas de Comunicacion Oral y Escrita','Tecniques de Comunicacio Oral i Escrita','Tec. Com. Oral i Escrita',4.5,NULL,NULL,NULL); INSERT INTO t1 VALUES (11403,'Projecte Fi de Carrera','Proyecto Fin de Carrera','Projecte Fi de Carrera','PFC',9.0,NULL,NULL,NULL); INSERT INTO t1 VALUES (11404,'+lgebra lineal','Algebra lineal','+lgebra lineal','+lgebra lineal',15.0,NULL,NULL,NULL); INSERT INTO t1 VALUES (11405,'+lgebra lineal','Algebra lineal','+lgebra lineal','+lgebra lineal',18.0,NULL,NULL,NULL); INSERT INTO t1 VALUES (11406,'Calcul Infinitesimal','C�lculo Infinitesimal','Calcul Infinitesimal','Calcul Infinitesimal',15.0,NULL,NULL,NULL); CREATE TABLE t2 ( idAssignatura int(11) DEFAULT '0' NOT NULL, Grup int(11) DEFAULT '0' NOT NULL, Places smallint(6) DEFAULT '0' NOT NULL, PlacesOcupades int(11) DEFAULT '0', PRIMARY KEY (idAssignatura,Grup) ); INSERT INTO t2 VALUES (10360,12,333,0); INSERT INTO t2 VALUES (10361,30,2,0); INSERT INTO t2 VALUES (10361,40,3,0); INSERT INTO t2 VALUES (10360,45,10,0); INSERT INTO t2 VALUES (10362,10,12,0); INSERT INTO t2 VALUES (10360,55,2,0); INSERT INTO t2 VALUES (10360,70,0,0); INSERT INTO t2 VALUES (10360,565656,0,0); INSERT INTO t2 VALUES (10360,32767,7,0); INSERT INTO t2 VALUES (10360,33,8,0); INSERT INTO t2 VALUES (10360,7887,85,0); INSERT INTO t2 VALUES (11405,88,8,0); INSERT INTO t2 VALUES (10360,0,55,0); INSERT INTO t2 VALUES (10360,99,0,0); INSERT INTO t2 VALUES (11411,30,10,0); INSERT INTO t2 VALUES (11404,0,0,0); INSERT INTO t2 VALUES (10362,11,111,0); INSERT INTO t2 VALUES (10363,33,333,0); INSERT INTO t2 VALUES (11412,55,0,0); INSERT INTO t2 VALUES (50003,66,6,0); INSERT INTO t2 VALUES (11403,5,0,0); INSERT INTO t2 VALUES (11406,11,11,0); INSERT INTO t2 VALUES (11410,11410,131,0); INSERT INTO t2 VALUES (11416,11416,32767,0); INSERT INTO t2 VALUES (11409,0,0,0); CREATE TABLE t3 ( id int(11) DEFAULT '0' NOT NULL auto_increment, dni_pasaporte char(16) DEFAULT '' NOT NULL, idPla int(11) DEFAULT '0' NOT NULL, cod_asig int(11) DEFAULT '0' NOT NULL, any smallint(6) DEFAULT '0' NOT NULL, quatrimestre smallint(6) DEFAULT '0' NOT NULL, estat char(1) DEFAULT 'M' NOT NULL, PRIMARY KEY (id), UNIQUE dni_pasaporte (dni_pasaporte,idPla), UNIQUE dni_pasaporte_2 (dni_pasaporte,idPla,cod_asig,any,quatrimestre) ); INSERT INTO t3 VALUES (1,'11111111',1,10362,98,1,'M'); CREATE TABLE t4 ( id int(11) DEFAULT '0' NOT NULL auto_increment, papa int(11) DEFAULT '0' NOT NULL, fill int(11) DEFAULT '0' NOT NULL, idPla int(11) DEFAULT '0' NOT NULL, PRIMARY KEY (id), KEY papa (idPla,papa), UNIQUE papa_2 (idPla,papa,fill) ); INSERT INTO t4 VALUES (1,-1,10360,1); INSERT INTO t4 VALUES (2,-1,10361,1); INSERT INTO t4 VALUES (3,-1,10362,1); SELECT DISTINCT fill,desc_larga_cat,cred_total,Grup,Places,PlacesOcupades FROM t4 LEFT JOIN t3 ON t3.cod_asig=fill AND estat='S' AND dni_pasaporte='11111111' AND t3.idPla=1 , t2,t1 WHERE fill=t1.cod_asig AND Places>PlacesOcupades AND fill=idAssignatura AND t4.idPla=1 AND papa=-1; fill desc_larga_cat cred_total Grup Places PlacesOcupades 10360 asdfggfg 6.0 0 55 0 10360 asdfggfg 6.0 12 333 0 10360 asdfggfg 6.0 33 8 0 10360 asdfggfg 6.0 45 10 0 10360 asdfggfg 6.0 55 2 0 10360 asdfggfg 6.0 7887 85 0 10360 asdfggfg 6.0 32767 7 0 10361 Components i Circuits Electronics I 6.0 30 2 0 10361 Components i Circuits Electronics I 6.0 40 3 0 10362 Laboratori d`Ordinadors 4.5 10 12 0 10362 Laboratori d`Ordinadors 4.5 11 111 0 SELECT DISTINCT fill,t3.idPla FROM t4 LEFT JOIN t3 ON t3.cod_asig=t4.fill AND t3.estat='S' AND t3.dni_pasaporte='1234' AND t3.idPla=1 ; fill idPla 10360 NULL 10361 NULL 10362 NULL INSERT INTO t3 VALUES (3,'1234',1,10360,98,1,'S'); SELECT DISTINCT fill,t3.idPla FROM t4 LEFT JOIN t3 ON t3.cod_asig=t4.fill AND t3.estat='S' AND t3.dni_pasaporte='1234' AND t3.idPla=1 ; fill idPla 10360 1 10361 NULL 10362 NULL drop table t1,t2,t3,test.t4; CREATE TABLE t1 ( id smallint(5) unsigned DEFAULT '0' NOT NULL auto_increment, name char(60) DEFAULT '' NOT NULL, PRIMARY KEY (id) ); INSERT INTO t1 VALUES (1,'Antonio Paz'); INSERT INTO t1 VALUES (2,'Lilliana Angelovska'); INSERT INTO t1 VALUES (3,'Thimble Smith'); CREATE TABLE t2 ( id smallint(5) unsigned DEFAULT '0' NOT NULL auto_increment, owner smallint(5) unsigned DEFAULT '0' NOT NULL, name char(60), PRIMARY KEY (id) ); INSERT INTO t2 VALUES (1,1,'El Gato'); INSERT INTO t2 VALUES (2,1,'Perrito'); INSERT INTO t2 VALUES (3,3,'Happy'); select t1.name, t2.name, t2.id from t1 left join t2 on (t1.id = t2.owner); name name id Antonio Paz El Gato 1 Antonio Paz Perrito 2 Lilliana Angelovska NULL NULL Thimble Smith Happy 3 select t1.name, t2.name, t2.id from t1 left join t2 on (t1.id = t2.owner) where t2.id is null; name name id Lilliana Angelovska NULL NULL explain select t1.name, t2.name, t2.id from t1 left join t2 on (t1.id = t2.owner) where t2.id is null; id select_type table type possible_keys key key_len ref rows Extra 1 FIRST t1 ALL NULL NULL NULL NULL 3 1 FIRST t2 ALL NULL NULL NULL NULL 3 where used; Not exists explain select t1.name, t2.name, t2.id from t1 left join t2 on (t1.id = t2.owner) where t2.name is null; id select_type table type possible_keys key key_len ref rows Extra 1 FIRST t1 ALL NULL NULL NULL NULL 3 1 FIRST t2 ALL NULL NULL NULL NULL 3 where used select count(*) from t1 left join t2 on (t1.id = t2.owner); count(*) 4 select t1.name, t2.name, t2.id from t2 right join t1 on (t1.id = t2.owner); name name id Antonio Paz El Gato 1 Antonio Paz Perrito 2 Lilliana Angelovska NULL NULL Thimble Smith Happy 3 select t1.name, t2.name, t2.id from t2 right join t1 on (t1.id = t2.owner) where t2.id is null; name name id Lilliana Angelovska NULL NULL explain select t1.name, t2.name, t2.id from t2 right join t1 on (t1.id = t2.owner) where t2.id is null; id select_type table type possible_keys key key_len ref rows Extra 1 FIRST t1 ALL NULL NULL NULL NULL 3 1 FIRST t2 ALL NULL NULL NULL NULL 3 where used; Not exists explain select t1.name, t2.name, t2.id from t2 right join t1 on (t1.id = t2.owner) where t2.name is null; id select_type table type possible_keys key key_len ref rows Extra 1 FIRST t1 ALL NULL NULL NULL NULL 3 1 FIRST t2 ALL NULL NULL NULL NULL 3 where used select count(*) from t2 right join t1 on (t1.id = t2.owner); count(*) 4 select t1.name, t2.name, t2.id,t3.id from t2 right join t1 on (t1.id = t2.owner) left join t1 as t3 on t3.id=t2.owner; name name id id Antonio Paz El Gato 1 1 Antonio Paz Perrito 2 1 Lilliana Angelovska NULL NULL NULL Thimble Smith Happy 3 3 select t1.name, t2.name, t2.id,t3.id from t1 right join t2 on (t1.id = t2.owner) right join t1 as t3 on t3.id=t2.owner; name name id id Antonio Paz El Gato 1 1 Antonio Paz Perrito 2 1 NULL Happy 3 1 NULL El Gato 1 2 NULL Perrito 2 2 NULL Happy 3 2 NULL El Gato 1 3 NULL Perrito 2 3 Thimble Smith Happy 3 3 select t1.name, t2.name, t2.id, t2.owner, t3.id from t1 left join t2 on (t1.id = t2.owner) right join t1 as t3 on t3.id=t2.owner; name name id owner id Antonio Paz El Gato 1 1 1 Antonio Paz Perrito 2 1 1 Lilliana Angelovska NULL NULL NULL 1 Thimble Smith NULL NULL NULL 1 Antonio Paz NULL NULL NULL 2 Lilliana Angelovska NULL NULL NULL 2 Thimble Smith NULL NULL NULL 2 Antonio Paz NULL NULL NULL 3 Lilliana Angelovska NULL NULL NULL 3 Thimble Smith Happy 3 3 3 drop table t1,t2; create table t1 (id int not null, str char(10), index(str)); insert into t1 values (1, null), (2, null), (3, "foo"), (4, "bar"); select * from t1 where str is not null; id str 4 bar 3 foo select * from t1 where str is null; id str 1 NULL 2 NULL drop table t1; CREATE TABLE t1 ( t1_id bigint(21) DEFAULT '0' NOT NULL auto_increment, PRIMARY KEY (t1_id) ); CREATE TABLE t2 ( t2_id bigint(21) DEFAULT '0' NOT NULL auto_increment, PRIMARY KEY (t2_id) ); CREATE TABLE t3 ( t3_id bigint(21) DEFAULT '0' NOT NULL auto_increment, PRIMARY KEY (t3_id) ); CREATE TABLE t4 ( seq_0_id bigint(21) DEFAULT '0' NOT NULL, seq_1_id bigint(21) DEFAULT '0' NOT NULL, KEY seq_0_id (seq_0_id), KEY seq_1_id (seq_1_id) ); CREATE TABLE t5 ( seq_0_id bigint(21) DEFAULT '0' NOT NULL, seq_1_id bigint(21) DEFAULT '0' NOT NULL, KEY seq_1_id (seq_1_id), KEY seq_0_id (seq_0_id) ); insert into t1 values (1); insert into t2 values (1); insert into t3 values (1); insert into t4 values (1,1); insert into t5 values (1,1); explain select * from t3 left join t4 on t4.seq_1_id = t2.t2_id left join t1 on t1.t1_id = t4.seq_0_id left join t5 on t5.seq_0_id = t1.t1_id left join t2 on t2.t2_id = t5.seq_1_id where t3.t3_id = 23; Cross dependency found in OUTER JOIN. Examine your ON conditions drop table t1,t2,t3,t4,t5; create table t1 (n int, m int, o int, key(n)); create table t2 (n int not null, m int, o int, primary key(n)); insert into t1 values (1, 2, 11), (1, 2, 7), (2, 2, 8), (1,2,9),(1,3,9); insert into t2 values (1, 2, 3),(2, 2, 8), (4,3,9),(3,2,10); select t1.*, t2.* from t1 left join t2 on t1.n = t2.n and t1.m = t2.m where t1.n = 1; n m o n m o 1 2 11 1 2 3 1 2 7 1 2 3 1 2 9 1 2 3 1 3 9 NULL NULL NULL select t1.*, t2.* from t1 left join t2 on t1.n = t2.n and t1.m = t2.m where t1.n = 1 order by t1.o; n m o n m o 1 2 7 1 2 3 1 2 9 1 2 3 1 3 9 NULL NULL NULL 1 2 11 1 2 3 drop table t1,t2; CREATE TABLE t1 (id1 INT NOT NULL PRIMARY KEY, dat1 CHAR(1), id2 INT); INSERT INTO t1 VALUES (1,'a',1); INSERT INTO t1 VALUES (2,'b',1); INSERT INTO t1 VALUES (3,'c',2); CREATE TABLE t2 (id2 INT NOT NULL PRIMARY KEY, dat2 CHAR(1)); INSERT INTO t2 VALUES (1,'x'); INSERT INTO t2 VALUES (2,'y'); INSERT INTO t2 VALUES (3,'z'); SELECT t2.id2 FROM t2 LEFT OUTER JOIN t1 ON t1.id2 = t2.id2 WHERE id1 IS NULL; id2 3 SELECT t2.id2 FROM t2 NATURAL LEFT OUTER JOIN t1 WHERE id1 IS NULL; id2 3 drop table t1,t2; create table t1 ( color varchar(20), name varchar(20) ); insert into t1 values ( 'red', 'apple' ); insert into t1 values ( 'yellow', 'banana' ); insert into t1 values ( 'green', 'lime' ); insert into t1 values ( 'black', 'grape' ); insert into t1 values ( 'blue', 'blueberry' ); create table t2 ( count int, color varchar(20) ); insert into t2 values (10, 'green'); insert into t2 values (5, 'black'); insert into t2 values (15, 'white'); insert into t2 values (7, 'green'); select * from t1; color name red apple yellow banana green lime black grape blue blueberry select * from t2; count color 10 green 5 black 15 white 7 green select * from t2 natural join t1; count color color name 10 green green lime 7 green green lime 5 black black grape select t2.count, t1.name from t2 natural join t1; count name 10 lime 7 lime 5 grape select t2.count, t1.name from t2 inner join t1 using (color); count name 10 lime 7 lime 5 grape drop table t1; drop table t2; CREATE TABLE t1 ( pcode varchar(8) DEFAULT '' NOT NULL ); INSERT INTO t1 VALUES ('kvw2000'),('kvw2001'),('kvw3000'),('kvw3001'),('kvw3002'),('kvw3500'),('kvw3501'),('kvw3502'),('kvw3800'),('kvw3801'),('kvw3802'),('kvw3900'),('kvw3901'),('kvw3902'),('kvw4000'),('kvw4001'),('kvw4002'),('kvw4200'),('kvw4500'),('kvw5000'),('kvw5001'),('kvw5500'),('kvw5510'),('kvw5600'),('kvw5601'),('kvw6000'),('klw1000'),('klw1020'),('klw1500'),('klw2000'),('klw2001'),('klw2002'),('kld2000'),('klw2500'),('kmw1000'),('kmw1500'),('kmw2000'),('kmw2001'),('kmw2100'),('kmw3000'),('kmw3200'); CREATE TABLE t2 ( pcode varchar(8) DEFAULT '' NOT NULL, KEY pcode (pcode) ); INSERT INTO t2 VALUES ('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw2000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3000'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw3500'),('kvw6000'),('kvw6000'),('kld2000'); SELECT t1.pcode, IF(ISNULL(t2.pcode), 0, COUNT(*)) AS count FROM t1 LEFT JOIN t2 ON t1.pcode = t2.pcode GROUP BY t1.pcode; pcode count kld2000 1 klw1000 0 klw1020 0 klw1500 0 klw2000 0 klw2001 0 klw2002 0 klw2500 0 kmw1000 0 kmw1500 0 kmw2000 0 kmw2001 0 kmw2100 0 kmw3000 0 kmw3200 0 kvw2000 26 kvw2001 0 kvw3000 36 kvw3001 0 kvw3002 0 kvw3500 26 kvw3501 0 kvw3502 0 kvw3800 0 kvw3801 0 kvw3802 0 kvw3900 0 kvw3901 0 kvw3902 0 kvw4000 0 kvw4001 0 kvw4002 0 kvw4200 0 kvw4500 0 kvw5000 0 kvw5001 0 kvw5500 0 kvw5510 0 kvw5600 0 kvw5601 0 kvw6000 2 SELECT SQL_BIG_RESULT t1.pcode, IF(ISNULL(t2.pcode), 0, COUNT(*)) AS count FROM t1 LEFT JOIN t2 ON t1.pcode = t2.pcode GROUP BY t1.pcode; pcode count kld2000 1 klw1000 0 klw1020 0 klw1500 0 klw2000 0 klw2001 0 klw2002 0 klw2500 0 kmw1000 0 kmw1500 0 kmw2000 0 kmw2001 0 kmw2100 0 kmw3000 0 kmw3200 0 kvw2000 26 kvw2001 0 kvw3000 36 kvw3001 0 kvw3002 0 kvw3500 26 kvw3501 0 kvw3502 0 kvw3800 0 kvw3801 0 kvw3802 0 kvw3900 0 kvw3901 0 kvw3902 0 kvw4000 0 kvw4001 0 kvw4002 0 kvw4200 0 kvw4500 0 kvw5000 0 kvw5001 0 kvw5500 0 kvw5510 0 kvw5600 0 kvw5601 0 kvw6000 2 drop table t1,t2; CREATE TABLE t1 ( id int(11), pid int(11), rep_del tinyint(4), KEY id (id), KEY pid (pid) ); INSERT INTO t1 VALUES (1,NULL,NULL); INSERT INTO t1 VALUES (2,1,NULL); select * from t1 LEFT JOIN t1 t2 ON (t1.id=t2.pid) AND t2.rep_del IS NULL; id pid rep_del id pid rep_del 1 NULL NULL 2 1 NULL 2 1 NULL NULL NULL NULL create index rep_del ON t1(rep_del); select * from t1 LEFT JOIN t1 t2 ON (t1.id=t2.pid) AND t2.rep_del IS NULL; id pid rep_del id pid rep_del 1 NULL NULL 2 1 NULL 2 1 NULL NULL NULL NULL drop table t1; CREATE TABLE t1 ( id int(11) DEFAULT '0' NOT NULL, name tinytext DEFAULT '' NOT NULL, UNIQUE id (id) ); INSERT INTO t1 VALUES (1,'yes'),(2,'no'); CREATE TABLE t2 ( id int(11) DEFAULT '0' NOT NULL, idx int(11) DEFAULT '0' NOT NULL, UNIQUE id (id,idx) ); INSERT INTO t2 VALUES (1,1); explain SELECT * from t1 left join t2 on t1.id=t2.id where t2.id IS NULL; id select_type table type possible_keys key key_len ref rows Extra 1 FIRST t1 ALL NULL NULL NULL NULL 2 1 FIRST t2 index id id 8 NULL 1 where used; Using index; Not exists SELECT * from t1 left join t2 on t1.id=t2.id where t2.id IS NULL; id name id idx 2 no NULL NULL drop table t1,t2; create table t1 (bug_id mediumint, reporter mediumint); create table t2 (bug_id mediumint, who mediumint, index(who)); insert into t2 values (1,1),(1,2); insert into t1 values (1,1),(2,1); SELECT * FROM t1 LEFT JOIN t2 ON (t1.bug_id = t2.bug_id AND t2.who = 2) WHERE (t1.reporter = 2 OR t2.who = 2); bug_id reporter bug_id who 1 1 1 2 drop table t1,t2; create table t1 (fooID smallint unsigned auto_increment, primary key (fooID)); create table t2 (fooID smallint unsigned not null, barID smallint unsigned not null, primary key (fooID,barID)); insert into t1 (fooID) values (10),(20),(30); insert into t2 values (10,1),(20,2),(30,3); explain select * from t2 left join t1 on t1.fooID = t2.fooID and t1.fooID = 30; id select_type table type possible_keys key key_len ref rows Extra 1 FIRST t2 index NULL PRIMARY 4 NULL 3 Using index 1 FIRST t1 eq_ref PRIMARY PRIMARY 2 const 1 where used; Using index select * from t2 left join t1 on t1.fooID = t2.fooID and t1.fooID = 30; fooID barID fooID 10 1 NULL 20 2 NULL 30 3 30 select * from t2 left join t1 ignore index(primary) on t1.fooID = t2.fooID and t1.fooID = 30; fooID barID fooID 10 1 NULL 20 2 NULL 30 3 30 drop table t1,t2;