下面三条语句从最终结果上看是等效的：
(1)update t1 set c3= 'XXX'
where exists ( select 1 from t2 where t2.id=t1.id and t2.username=t1.username);
create index t1_idx on t1(id,username);
(2) update t1 set c3= 'YYY'
where (id,username) IN (select id,username from t2 );
(3)
merge into t1
using t2
on (t1.id =t2.id and t1.username =t2.username)
when matched then
update set c3= 'ZZZ';
SQL> select * from t1;
ID USERNAME C3
---------- -------------------- --------------------
1 tom1
2 tom2
3 tom3
4 tom4
SQL> select * from t2;
ID USERNAME C3
---------- -------------------- --------------------
2 tom2
4 tom4
5 tom5
(1)update t1 set c3= 'XXX'
where exists ( select 1 from t2 where t2.id=t1.id and t2.username=t1.username);
create index t1_idx on t1(id,username);
(2) update t1 set c3= 'YYY'
where (id,username) IN (select id,username from t2 );
(3)
merge into t1
using t2
on (t1.id =t2.id and t1.username =t2.username)
when matched then
update set c3= 'ZZZ';
SQL> select * from t1;
ID USERNAME C3
---------- -------------------- --------------------
1 tom1
2 tom2
3 tom3
4 tom4
SQL> select * from t2;
ID USERNAME C3
---------- -------------------- --------------------
2 tom2
4 tom4
5 tom5
来自 “ ITPUB博客 ” ，链接：https://blog.itpub.net/27042095/viewspace-769055/ ，如需转载，请注明出处，否则将追究法律责任。
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