drop table t1;
create table t1 (id int,name varchar2(20));
insert into t1 values (1,'zhangsan');
insert into t1 values (2,'lisi');
insert into t1 values (3,'wangwu');
insert into t1 values (null,'zhangsi');
drop table t2;
create table t2 (id int,name varchar2(20));
insert into t2 values (1,'zhang1');
insert into t2 values (2,null);
insert into t2 values (3,null);
commit;
SQL> select a.id,b.id,a.name,b.name from t1 a left join t2 b on a.id=b.id order by a.id ;
ID ID NAME NAME
---------- ---------- -------------------- --------------------
1 1 zhangsan zhang1
2 2 lisi
3 3 wangwu
zhangsi
SQL> select * from t1 a join t2 b on a.id=b.id order by a.id ;
ID NAME ID NAME
---------- -------------------- ---------- --------------------
1 zhangsan 1 zhang1
2 lisi 2
3 wangwu 3
SQL> select a.id ,a.name,(select b.name from t2 b where b.id=a.id) bname from t1 a order by a.id;
ID NAME BNAME
---------- -------------------- --------------------
1 zhangsan zhang1
2 lisi
3 wangwu
zhangsi