drop table if exists t1,t2,t3; create table t1(id1 int not null auto_increment primary key, t char(12)); create table t2(id2 int not null, t char(12)); create table t3(id3 int not null, t char(12), index(id3)); select count(*) from t1 where id1 > 95; count(*) 5 select count(*) from t2 where id2 > 95; count(*) 25 select count(*) from t3 where id3 > 95; count(*) 250 update t1,t2,t3 set t1.t="aaa", t2.t="bbb", t3.t="cc" where t1.id1 = t2.id2 and t2.id2 = t3.id3 and t1.id1 > 90; select count(*) from t1 where t = "aaa"; count(*) 10 select count(*) from t1 where id1 > 90; count(*) 10 select count(*) from t2 where t = "bbb"; count(*) 50 select count(*) from t2 where id2 > 90; count(*) 50 select count(*) from t3 where t = "cc"; count(*) 500 select count(*) from t3 where id3 > 90; count(*) 500 delete t1.*, t2.*, t3.* from t1,t2,t3 where t1.id1 = t2.id2 and t2.id2 = t3.id3 and t1.id1 > 95; check table t1, t2, t3; Table Op Msg_type Msg_text test.t1 check status OK test.t2 check status OK test.t3 check status OK select count(*) from t1 where id1 > 95; count(*) 0 select count(*) from t2 where id2 > 95; count(*) 0 select count(*) from t3 where id3 > 95; count(*) 0 delete t1, t2, t3 from t1,t2,t3 where t1.id1 = t2.id2 and t2.id2 = t3.id3 and t1.id1 > 5; select count(*) from t1 where id1 > 5; count(*) 0 select count(*) from t2 where id2 > 5; count(*) 0 select count(*) from t3 where id3 > 5; count(*) 0 delete from t1, t2, t3 using t1,t2,t3 where t1.id1 = t2.id2 and t2.id2 = t3.id3 and t1.id1 > 0; select count(*) from t1 where id1; count(*) 0 select count(*) from t2 where id2; count(*) 0 select count(*) from t3 where id3; count(*) 0 drop table t1,t2,t3; create table t1(id1 int not null primary key, t varchar(100)) pack_keys = 1; create table t2(id2 int not null, t varchar(100), index(id2)) pack_keys = 1; delete t1 from t1,t2 where t1.id1 = t2.id2 and t1.id1 > 500; drop table t1,t2; CREATE TABLE t1 ( id int(11) NOT NULL default '0', name varchar(10) default NULL, PRIMARY KEY (id) ) TYPE=MyISAM; INSERT INTO t1 VALUES (1,'aaa'),(2,'aaa'),(3,'aaa'); CREATE TABLE t2 ( id int(11) NOT NULL default '0', name varchar(10) default NULL, PRIMARY KEY (id) ) TYPE=MyISAM; INSERT INTO t2 VALUES (2,'bbb'),(3,'bbb'),(4,'bbb'); CREATE TABLE t3 ( id int(11) NOT NULL default '0', mydate datetime default NULL, PRIMARY KEY (id) ) TYPE=MyISAM; INSERT INTO t3 VALUES (1,'2002-02-04 00:00:00'),(3,'2002-05-12 00:00:00'),(5,'2002-05-12 00:00:00'),(6,'2002-06-22 00:00:00'),(7,'2002-07-22 00:00:00'); delete t1,t2,t3 from t1,t2,t3 where to_days(now())-to_days(t3.mydate)>=30 and t3.id=t1.id and t3.id=t2.id; select * from t3; id mydate 1 2002-02-04 00:00:00 5 2002-05-12 00:00:00 6 2002-06-22 00:00:00 7 2002-07-22 00:00:00 DROP TABLE t1,t2,t3; CREATE TABLE IF NOT EXISTS `t1` ( `id` int(11) NOT NULL auto_increment, `tst` text, `tst1` text, PRIMARY KEY (`id`) ) TYPE=MyISAM; CREATE TABLE IF NOT EXISTS `t2` ( `ID` int(11) NOT NULL auto_increment, `ParId` int(11) default NULL, `tst` text, `tst1` text, PRIMARY KEY (`ID`), KEY `IX_ParId_t2` (`ParId`), FOREIGN KEY (`ParId`) REFERENCES `t1` (`id`) ) TYPE=MyISAM; INSERT INTO t1(tst,tst1) VALUES("MySQL","MySQL AB"), ("MSSQL","Microsoft"), ("ORACLE","ORACLE"); INSERT INTO t2(ParId) VALUES(1), (2), (3); select * from t2; ID ParId tst tst1 1 1 NULL NULL 2 2 NULL NULL 3 3 NULL NULL UPDATE t2, t1 SET t2.tst = t1.tst, t2.tst1 = t1.tst1 WHERE t2.ParId = t1.Id; select * from t2; ID ParId tst tst1 1 1 MySQL MySQL AB 2 2 MSSQL Microsoft 3 3 ORACLE ORACLE drop table t1, t2 ; create table t1 (n numeric(10)); create table t2 (n numeric(10)); insert into t2 values (1),(2),(4),(8),(16),(32); select * from t2 left outer join t1 using (n); n n 1 NULL 2 NULL 4 NULL 8 NULL 16 NULL 32 NULL delete t1,t2 from t2 left outer join t1 using (n); select * from t2 left outer join t1 using (n); n n drop table t1,t2 ; create table t1 (n int(10) not null primary key, d int(10)); create table t2 (n int(10) not null primary key, d int(10)); insert into t1 values(1,1); insert into t2 values(1,10),(2,20); LOCK TABLES t1 write, t2 read; DELETE t1.*, t2.* FROM t1,t2 where t1.n=t2.n; Table 't2' was locked with a READ lock and can't be updated UPDATE t1,t2 SET t1.d=t2.d,t2.d=30 WHERE t1.n=t2.n; Table 't2' was locked with a READ lock and can't be updated UPDATE t1,t2 SET t1.d=t2.d WHERE t1.n=t2.n; Table 't2' was locked with a READ lock and can't be updated unlock tables; LOCK TABLES t1 write, t2 write; UPDATE t1,t2 SET t1.d=t2.d WHERE t1.n=t2.n; select * from t1; n d 1 10 DELETE t1.*, t2.* FROM t1,t2 where t1.n=t2.n; select * from t1; n d select * from t2; n d 2 20 unlock tables; drop table t1,t2; set sql_safe_updates=1; create table t1 (n int(10), d int(10)); create table t2 (n int(10), d int(10)); insert into t1 values(1,1); insert into t2 values(1,10),(2,20); UPDATE t1,t2 SET t1.d=t2.d WHERE t1.n=t2.n; You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column set sql_safe_updates=0; drop table t1,t2; set timestamp=1038401397; create table t1 (n int(10) not null primary key, d int(10), t timestamp); create table t2 (n int(10) not null primary key, d int(10), t timestamp); insert into t1 values(1,1,NULL); insert into t2 values(1,10,NULL),(2,20,NULL); set timestamp=1038000000; UPDATE t1,t2 SET t1.d=t2.d WHERE t1.n=t2.n; select n,d,unix_timestamp(t) from t1; n d unix_timestamp(t) 1 10 1038000000 select n,d,unix_timestamp(t) from t2; n d unix_timestamp(t) 1 10 1038401397 2 20 1038401397 UPDATE t1,t2 SET 1=2 WHERE t1.n=t2.n; You have an error in your SQL syntax. Check the manual that corresponds to your MySQL server version for the right syntax to use near '1=2 WHERE t1.n=t2.n' at line 1 drop table t1,t2; set timestamp=0; set sql_safe_updates=0; create table t1 (n int(10) not null primary key, d int(10)); create table t2 (n int(10) not null primary key, d int(10)); insert into t1 values(1,1), (3,3); insert into t2 values(1,10),(2,20); UPDATE t2 left outer join t1 on t1.n=t2.n SET t1.d=t2.d; select * from t1; n d 1 10 3 3 select * from t2; n d 1 10 2 20 drop table t1,t2; create table t1 (n int(10), d int(10)); create table t2 (n int(10), d int(10)); insert into t1 values(1,1),(1,2); insert into t2 values(1,10),(2,20); UPDATE t1,t2 SET t1.d=t2.d,t2.d=30 WHERE t1.n=t2.n; select * from t1; n d 1 10 1 10 select * from t2; n d 1 30 2 20 drop table t1,t2; create table t1 (n int(10), d int(10)); create table t2 (n int(10), d int(10)); insert into t1 values(1,1),(3,2); insert into t2 values(1,10),(1,20); UPDATE t1,t2 SET t1.d=t2.d,t2.d=30 WHERE t1.n=t2.n; select * from t1; n d 1 10 3 2 select * from t2; n d 1 30 1 30 UPDATE t1 a ,t2 b SET a.d=b.d,b.d=30 WHERE a.n=b.n; select * from t1; n d 1 30 3 2 select * from t2; n d 1 30 1 30 DELETE t1, t2 FROM t1 a,t2 b where a.n=b.n; select * from t1; n d 3 2 select * from t2; n d drop table t1,t2; CREATE TABLE t1 ( broj int(4) unsigned NOT NULL default '0', naziv char(25) NOT NULL default 'NEPOZNAT', PRIMARY KEY (broj)) TYPE=MyISAM; INSERT INTO t1 VALUES (1,'jedan'),(2,'dva'),(3,'tri'),(4,'xxxxxxxxxx'),(5,'a'),(10,''),(11,''),(12,''),(13,''); CREATE TABLE t2 ( broj int(4) unsigned NOT NULL default '0', naziv char(25) NOT NULL default 'NEPOZNAT', PRIMARY KEY (broj)) TYPE=MyISAM; INSERT INTO t2 VALUES (1,'jedan'),(2,'dva'),(3,'tri'),(4,'xxxxxxxxxx'),(5,'a'); CREATE TABLE t3 ( broj int(4) unsigned NOT NULL default '0', naziv char(25) NOT NULL default 'NEPOZNAT', PRIMARY KEY (broj)) TYPE=MyISAM; INSERT INTO t3 VALUES (1,'jedan'),(2,'dva'); update t1,t2 set t1.naziv="aaaa" where t1.broj=t2.broj; update t1,t2,t3 set t1.naziv="bbbb", t2.naziv="aaaa" where t1.broj=t2.broj and t2.broj=t3.broj; drop table t1,t2,t3;