Question

I need help in simplifying the below query.

I was able to check for ‘0’ count without using Group By/having Count clauses in the below query but with correlated subquery.

Now, I’ve been asked to simplify the below query as simple joins!.

I tried merging the query into one. But the output differs.

Could you please suggest any other idea of simplifying the query, which is checking for ‘0’ count.

select distinct tab1.col1
  from tab1
  where tab1.col2 = 'A'
  And 0 = (select count(tab2.col1)
            from tab2
            where tab2.col2 = 'B'
            and tab2.col1 = tab1.col1)

Answer 1

This sort of thing would normally be written as a NOT EXISTS

SELECT distinct tab1.col1
  FROM tab1
 WHERE tab1.col2 = 'A'
   AND NOT EXISTS( 
      SELECT 1
        FROM tab2
       WHERE tab2.col2 = 'B'
         AND tab2.col1 = tab1.col1 )

However you could also write

SELECT tab1.col1, count(tab2.col1)
  FROM (SELECT * FROM tab1 WHERE col2 = 'A') tab1,
       (SELECT * FROM tab2 WHERE col2 = 'B') tab2 
 WHERE tab1.col1 = tab2.col2(+)
 GROUP BY tab1.col1
HAVING count(tab2.col1) = 0