create table A(     id number primary key,     act varchar2(100) ); create table B(     id number,     actn_numb integer,     primary key(id,actn_numb) ); insert into A(id,act) values(1,'art'); insert into A(id,act) values(2,'music'); insert into A(id,act) values(3,'sport'); insert into B(id,actn_numb) values(1,1); insert into B(id,actn_numb) values(1,2); insert into B(id,actn_numb) values(1,3); insert into B(id,actn_numb) values(1,4); insert into B(id,actn_numb) values(1,5); insert into B(id,actn_numb) values(2,1); insert into B(id,actn_numb) values(2,2); insert into B(id,actn_numb) values(2,3); insert into B(id,actn_numb) values(3,1);select * from A;select * from B;    --列不能外部连接到子查询    select A.*, B.*      from A      left join B        on A.id = B.id       and B.actn_numb = (select max(B.actn_numb) from B where B.id = a.id);       --解决办法       select A.*,C.ACTN_NUMB from A     left join (    select * from (        select B.*,(row_number() over(partition by id order by actn_numb desc)) rn         from B)        where rn=1 ) C     on (C.id=A.id);        --    select * from a;    select * from b;        select *      from a      left join (select b.* from b where b.actn_numb = 1) c        on a.id = c.id;