LEFT / RIGHT JOIN中的子查询过滤器

Question

I tried following query to get the missing records in TAB1

SELECT *
FROM TAB1 T1 
RIGHT JOIN TAB2 T2 ON T1.MemNo = T2.MemID
WHERE EXISTS ( SELECT 1
               FROM TAB3 x 
               WHERE x.Col2 =  T2.SVID
               AND x.Col1 = T1.SID )
AND T1.MemNo IS NULL

But it doesn't give any results

Sample data;

TAB1

MemNo   SID

116537  S110
116537  D011
575777  D012
214438  S110
434611  D114
214438  D011
208368  D012
208368  S110

TAB2

MemID   SVID

116537  110
116537  11
214438  11
434675  114
214438  110
575788  12
208368  12
208368  110


TAB3

Col1    Col2

D011    11
S110    110
D114    114
D012    12

How should I change the query to get the expected results. Which is mentioned as below

TAB2

MemID   SVID

575788  12
434675  114

In case of INNER JOIN this works fine

Thanks

Answer 1

Here is your query, written so I understand it better:

select *
from tab2 t2 left outer join
     tab1 t1
     on t1.MemNo = t2.MemId
where exists (SELECT 1
              FROM TAB3 x 
              WHERE x.Col2 =  T2.SVID AND x.Col1 = T1.SID ) and
      T1.MemNo IS NULL

The column t1.MemNo is NULL only when the outer join fails to find a match in that table (because it is part of the join condition). When this is NULL then so is T1.SID , so the exists clause will find no rows because one of the AND conditions evaluates to NULL .

If you want to handle this case, then you need logic in the exists clause as well:

select *
from tab2 t2 left outer join
     tab1 t1
     on t1.MemNo = t2.MemId
where exists (SELECT 1
              FROM TAB3 x 
              WHERE x.Col2 =  T2.SVID AND (x.Col1 = T1.SID or t1.SID is null)) and
      T1.MemNo IS NULL;