oracle 多表关联更新为什么要加exists

SQL> create table t1(id int,name varchar2(10));

Table created.

SQL> create table t2(id int,name varchar2(10));

Table created.

SQL> insert into t1 values(1,'a');

1 row created.

SQL> insert into t1 values(2,'b');

1 row created.

SQL> insert into t2 values(3,'c');

1 row created.

SQL> insert into t2 values(2,'d');

1 row created.

SQL> select * from t1;

ID NAME
---------- ----------
1 a
2 b

SQL> select * from t2;

ID NAME
---------- ----------
3 c
2 d

SQL> update t1 set name =(select name from t2 where t1.id=t2.id);

2 rows updated.

SQL> select * from t1;

ID NAME
---------- ----------
1
2 d

SQL> delete from t1;

2 rows deleted.

SQL> delete from t2;

2 rows deleted.

SQL> insert into t1 values(1,'a');

1 row created.

SQL> insert into t1 values(2,'b');

1 row created.

SQL> insert into t2 values(3,'c');

1 row created.

SQL> insert into t2 values(2,'d');

1 row created.

SQL> update t1 set name =(select name from t2 where t1.id=t2.id) where exists(select 1 from t2 where t1.id=t2.id);

1 row updated.

SQL> select * from t1;

ID NAME
---------- ----------
1 a
2 d