从列列表中删除选择子查询到主查询

Question

Query 1 (Before):

select ta.C1,
       (SELECT tb.C1 from T2 tb WHERE  tb.C2 = ta.C2)
from T1 ta
WHERE ta.C3=30025239;

I want to remove the subquery from column level. I modified the code to add join

Query 2 (After):

select ta.C1, tb.C1
from T1 ta left outer join
     T2 tb 
     on tb.C2 = ta.C2
WHERE ta.C3=30025239;

But if subquery returns blank (no value) then Query 1 returns data for ta.C1 and null for tb.C1 whereas Query 2 will return blank (no result). I want result of Query 2 as same as Query 1

Answer 1

Why do you think your two queries are giving different responses? They are equivalent:

WITH t1 AS (SELECT 1 c1, 10 c2, 30025239 c3 FROM dual UNION ALL
            SELECT 2 c1, 20 c2, 30025239 c3 FROM dual UNION ALL
            SELECT 3 c1, 30 c2, 30025238 c3 FROM dual),
     t2 AS (SELECT 100 c1, 10 c2 FROM dual UNION ALL
            SELECT 300 c1, 30 c2 FROM dual)
select 'query 1' qry, ta.C1 ta_c1, (SELECT tb.C1 from T2 tb WHERE  tb.C2 = ta.C2) tb_c1
from T1 ta WHERE ta.C3=30025239
UNION ALL
select 'query 2' qry, ta.C1, tb.C1
from T1 ta left outer join T2 tb on tb.C2 = ta.C2
WHERE ta.C3=30025239;

QRY          TA_C1      TB_C1
------- ---------- ----------
query 1          1        100
query 1          2 
query 2          1        100
query 2          2 

The only difference with query 2 is that you're no longer benefiting from the scalar subquery caching you're getting in query 1. That may not matter if the ta.c2 column is unique or doesn't contain many repeated values.

Answer 2

Both the queries are same. However in question queries and their output columns are different. There may be more conditions in your query that changes the output.