Oracle - 内连接和左连接

Question

create table t1 (v varchar2(500), n number);
Insert into T1 (V,N) values ('a',1);
Insert into T1 (V,N) values ('bb',2);
Insert into T1 (V,N) values ('c',3);
Insert into T1 (V,N) values ('d',4);

create table t2 (v varchar2(500), n number);
Insert into T2 (V,N) values ('a',1);
Insert into T2 (V,N) values ('bb',2);

select * from t1 join t2 on t1.v = t2.v 
union all
select * from t1 LEFT join t2 on t1.v = t2.v ;

Output:

    a     1     a         1
    bb    2     bb        2
    a     1     a         1
    bb    2     bb        2
    d     4     (null)  (null)
    c     3     (null)  (null)

Can we get the same above output from single scan of T1 and T2 ie from single query without UNION ALL etc? Want to re-write the above Select query so that it scans the tables T1 and T2 only once and give the same result. See the LEFT join.

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The output cant be changed as we are passing it further in the application, duplicate data is required as per the requirement.

Answer 1

" Want to re-write the above Select query so that it scans the tables T1 and T2 only once"

You could use subquery factoring . The WITH clauses read each table once and the UNION-ed queries read from them:

with cte1 as ( select * from t1 )
   , cte2 as ( select * from t2 ) 
select * from cte1 join cte2 on cte1.v = cte2.v 
union all
select * from cte1 LEFT join cte2 on cte1.v = cte2.v ;

Here is a SQL Fiddle demo .

Answer 2

You can avoid excess joins and unions by doubling the rows:

select t1.*,t2.* from t1
  left join t2 on t1.v=t2.v
  cross join (select 1 as dbl from dual 
            union select 2 as dbl from dual) dbl
where dbl=1 or t2.v is not null