drop table if exists t1,t2,t3; 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 (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 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 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;