Question

What is the difference between the following pieces of SQL code:

select count(*)
from A
left join B
on a.id = b.id
where a.status = 2 and
b.id is NULL

and

select count(*)
from A
left join B
on a.id = b.id
and a.status =2
where b.id is NULL

? I read this : Semantic difference between join queries but I still don’t know which one is better to use

I have spend the past hour reading this and grasping it all answers added value but I understood the code example together with “never encountered this” the best

Answer 1

Rewriting the 2 queries without the LEFT JOIN / IS NULL so it’s obvious that they can return different data sets in certain cases:

---Query 1

SELECT COUNT(*)
FROM a
WHERE a.status = 2
  AND NOT EXISTS 
      ( SELECT *
        FROM b
        WHERE a.id = b.id
      )

and

---Query 2

SELECT COUNT(*)
FROM a
WHERE NOT ( a.status = 2
        AND EXISTS 
            ( SELECT *
              FROM b
              WHERE a.id = b.id
            )
          )

--- or:

---Query 2
SELECT COUNT(*)
FROM a
WHERE a.status <> 2
   OR NOT EXISTS 
      ( SELECT *
        FROM b
        WHERE a.id = b.id
      )