drop table if exists t1,t2,t3,t4,t5,t6; CREATE TABLE t1 (a int not null, b char (10) not null); insert into t1 values(1,'a'),(2,'b'),(3,'c'),(3,'c'); CREATE TABLE t2 (a int not null, b char (10) not null); insert into t2 values (3,'c'),(4,'d'),(5,'f'),(6,'e'); select a,b from t1 union select a,b from t2; a b 1 a 2 b 3 c 4 d 5 f 6 e select a,b from t1 union all select a,b from t2; a b 1 a 2 b 3 c 3 c 3 c 4 d 5 f 6 e select a,b from t1 union all select a,b from t2 order by b; a b 1 a 2 b 3 c 3 c 3 c 4 d 6 e 5 f select a,b from t1 union all select a,b from t2 union select 7,'g'; a b 1 a 2 b 3 c 3 c 3 c 4 d 5 f 6 e 7 g select 0,'#' union select a,b from t1 union all select a,b from t2 union select 7,'gg'; 0 # 0 # 1 a 2 b 3 c 3 c 3 c 4 d 5 f 6 e 7 g select a,b from t1 union select a,b from t1; a b 1 a 2 b 3 c select 't1',b,count(*) from t1 group by b UNION select 't2',b,count(*) from t2 group by b; t1 b count(*) t1 a 1 t1 b 1 t1 c 2 t2 c 1 t2 d 1 t2 e 1 t2 f 1 (select a,b from t1 limit 2) union all (select a,b from t2 order by a) limit 4; a b 1 a 2 b 3 c 4 d (select a,b from t1 limit 2) union all (select a,b from t2 order by a limit 1); a b 1 a 2 b 3 c (select a,b from t1 limit 2) union all (select a,b from t2 order by a limit 1) order by b desc; a b 3 c 2 b 1 a explain (select a,b from t1 limit 2) union all (select a,b from t2 order by a limit 1) order by b desc; table type possible_keys key key_len ref rows Extra t1 ALL NULL NULL NULL NULL 4 t2 ALL NULL NULL NULL NULL 4 Using filesort t1 ALL NULL NULL NULL NULL 4 (select sql_calc_found_rows a,b from t1 limit 2) union all (select a,b from t2 order by a) limit 2; a b 1 a 2 b select found_rows(); found_rows() 6 explain select a,b from t1 union all select a,b from t2; table type possible_keys key key_len ref rows Extra t1 ALL NULL NULL NULL NULL 4 t2 ALL NULL NULL NULL NULL 4 explain select xx from t1 union select 1; Unknown column 'xx' in 'field list' explain select a,b from t1 union select 1; table type possible_keys key key_len ref rows Extra t1 ALL NULL NULL NULL NULL 4 0 0 No tables used explain select 1 union select a,b from t1 union select 1; table type possible_keys key key_len ref rows Extra 0 0 No tables used t1 ALL NULL NULL NULL NULL 4 0 0 No tables used explain select a,b from t1 union select 1 limit 0; table type possible_keys key key_len ref rows Extra t1 ALL NULL NULL NULL NULL 4 0 0 Impossible WHERE select a,b from t1 into outfile 'skr' union select a,b from t2; Wrong usage of UNION and INTO select a,b from t1 order by a union select a,b from t2; Wrong usage of UNION and ORDER BY insert into t3 select a from t1 order by a union select a from t2; Wrong usage of UNION and ORDER BY create table t3 select a,b from t1 union select a from t2; The used SELECT statements have a different number of columns select a,b from t1 union select a from t2; The used SELECT statements have a different number of columns select * from t1 union select a from t2; The used SELECT statements have a different number of columns select a from t1 union select * from t2; The used SELECT statements have a different number of columns select * from t1 union select SQL_BUFFER_RESULT * from t2; Wrong usage/placement of 'SQL_BUFFER_RESULT' create table t3 select a,b from t1 union all select a,b from t2; insert into t3 select a,b from t1 union all select a,b from t2; replace into t3 select a,b as c from t1 union all select a,b from t2; drop table t1,t2,t3; CREATE TABLE t1 ( `pseudo` char(35) NOT NULL default '', `pseudo1` char(35) NOT NULL default '', `same` tinyint(1) unsigned NOT NULL default '1', PRIMARY KEY (`pseudo1`), KEY `pseudo` (`pseudo`) ) TYPE=MyISAM; INSERT INTO t1 (pseudo,pseudo1,same) VALUES ('joce', 'testtt', 1),('joce', 'tsestset', 1),('dekad', 'joce', 1); SELECT pseudo FROM t1 WHERE pseudo1='joce' UNION SELECT pseudo FROM t1 WHERE pseudo='joce'; pseudo dekad joce SELECT pseudo1 FROM t1 WHERE pseudo1='joce' UNION SELECT pseudo1 FROM t1 WHERE pseudo='joce'; pseudo1 joce testtt tsestset SELECT * FROM t1 WHERE pseudo1='joce' UNION SELECT * FROM t1 WHERE pseudo='joce' order by pseudo desc,pseudo1 desc; pseudo pseudo1 same joce tsestset 1 joce testtt 1 dekad joce 1 SELECT pseudo1 FROM t1 WHERE pseudo='joce' UNION SELECT pseudo FROM t1 WHERE pseudo1='joce'; pseudo1 testtt tsestset dekad SELECT pseudo1 FROM t1 WHERE pseudo='joce' UNION ALL SELECT pseudo FROM t1 WHERE pseudo1='joce'; pseudo1 testtt tsestset dekad SELECT pseudo1 FROM t1 WHERE pseudo='joce' UNION SELECT 1; pseudo1 testtt tsestset 1 drop table t1; drop table if exists t1,t2; create table t1 (a int); create table t2 (a int); insert into t1 values (1),(2),(3),(4),(5); insert into t2 values (11),(12),(13),(14),(15); (select * from t1 limit 2) union (select * from t2 limit 3) limit 4; a 1 2 11 12 (select * from t1 limit 2) union (select * from t2 limit 3); a 1 2 11 12 13 (select * from t1 limit 2) union (select * from t2 limit 20,3); a 1 2 set SQL_SELECT_LIMIT=2; (select * from t1 limit 1) union (select * from t2 limit 3); a 1 11 set SQL_SELECT_LIMIT=DEFAULT; drop table t1,t2; CREATE TABLE t1 ( cid smallint(5) unsigned NOT NULL default '0', cv varchar(250) NOT NULL default '', PRIMARY KEY (cid), UNIQUE KEY cv (cv) ) ; INSERT INTO t1 VALUES (8,'dummy'); CREATE TABLE t2 ( cid bigint(20) unsigned NOT NULL auto_increment, cap varchar(255) NOT NULL default '', PRIMARY KEY (cid), KEY cap (cap) ) ; CREATE TABLE t3 ( gid bigint(20) unsigned NOT NULL auto_increment, gn varchar(255) NOT NULL default '', must tinyint(4) default NULL, PRIMARY KEY (gid), KEY gn (gn) ) ; INSERT INTO t3 VALUES (1,'V1',NULL); CREATE TABLE t4 ( uid bigint(20) unsigned NOT NULL default '0', gid bigint(20) unsigned default NULL, rid bigint(20) unsigned default NULL, cid bigint(20) unsigned default NULL, UNIQUE KEY m (uid,gid,rid,cid), KEY uid (uid), KEY rid (rid), KEY cid (cid), KEY container (gid,rid,cid) ) ; INSERT INTO t4 VALUES (1,1,NULL,NULL); CREATE TABLE t5 ( rid bigint(20) unsigned NOT NULL auto_increment, rl varchar(255) NOT NULL default '', PRIMARY KEY (rid), KEY rl (rl) ) ; CREATE TABLE t6 ( uid bigint(20) unsigned NOT NULL auto_increment, un varchar(250) NOT NULL default '', uc smallint(5) unsigned NOT NULL default '0', PRIMARY KEY (uid), UNIQUE KEY nc (un,uc), KEY un (un) ) ; INSERT INTO t6 VALUES (1,'test',8); SELECT t4.uid, t5.rl, t3.gn as g1, t4.cid, t4.gid as gg FROM t3, t6, t1, t4 left join t5 on t5.rid = t4.rid left join t2 on t2.cid = t4.cid WHERE t3.gid=t4.gid AND t6.uid = t4.uid AND t6.uc = t1.cid AND t1.cv = "dummy" AND t6.un = "test"; uid rl g1 cid gg 1 NULL V1 NULL 1 SELECT t4.uid, t5.rl, t3.gn as g1, t4.cid, t4.gid as gg FROM t3, t6, t1, t4 left join t5 on t5.rid = t4.rid left join t2 on t2.cid = t4.cid WHERE t3.gid=t4.gid AND t6.uid = t4.uid AND t3.must IS NOT NULL AND t6.uc = t1.cid AND t1.cv = "dummy" AND t6.un = "test"; uid rl g1 cid gg (SELECT t4.uid, t5.rl, t3.gn as g1, t4.cid, t4.gid as gg FROM t3, t6, t1, t4 left join t5 on t5.rid = t4.rid left join t2 on t2.cid = t4.cid WHERE t3.gid=t4.gid AND t6.uid = t4.uid AND t3.must IS NOT NULL AND t6.uc = t1.cid AND t1.cv = "dummy" AND t6.un = "test") UNION (SELECT t4.uid, t5.rl, t3.gn as g1, t4.cid, t4.gid as gg FROM t3, t6, t1, t4 left join t5 on t5.rid = t4.rid left join t2 on t2.cid = t4.cid WHERE t3.gid=t4.gid AND t6.uid = t4.uid AND t6.uc = t1.cid AND t1.cv = "dummy" AND t6.un = "test"); uid rl g1 cid gg 1 NULL V1 NULL 1 drop table t1,t2,t3,t4,t5,t6;