update 加exists和不加的区别

SQL> select * from b1;

ID NAME
---------- --------------------
1 awen
2 steven
3 oracle

SQL> select * from b2;

ID NAME
---------- --------------------
3 awen
1 steven

SQL> update b1 set name=(select name from b2 where b2.id=b1.id);

已更新3行。

SQL> select * from b1;

ID NAME
---------- --------------------
1 steven
2
3 awen

SQL> rollback
2 ;

回退已完成。

SQL> select * from b1;

ID NAME
---------- --------------------
1 awen
2 steven
3 oracle

SQL> select * from b2;

ID NAME
---------- --------------------
3 awen
1 steven


SQL> update b1 set name =(select name from b2 where b1.id=b2.id) where exists(se
lect 1 from b2 where b2.id=b1.id);

已更新2行。

SQL> select * from b1;

ID NAME
---------- --------------------
1 steven
2 steven
3 awen

SQL> select * from b2;

ID NAME
---------- --------------------
3 awen
1 steven