SQL Inner Join On Null Values

Question

I have a Join

SELECT * FROM Y
INNER JOIN X ON ISNULL(X.QID, 0) = ISNULL(y.QID, 0) 

Isnull in a Join like this makes it slow.

Answer 1

You could also use the coalesce function. I tested this in PostgreSQL, but it should also work for MySQL or MS SQL server.

INNER JOIN x ON coalesce(x.qid, -1) = coalesce(y.qid, -1)

This will replace NULL with -1 before evaluating it. Hence there must be no -1 in qid.

Answer 2

This article has a good discussion on this issue. You can use

SELECT * 
FROM Y
INNER JOIN X ON EXISTS(SELECT X.QID 
                       INTERSECT 
                       SELECT y.QID);

Answer 3

You have two options

INNER JOIN x
   ON x.qid = y.qid OR (x.qid IS NULL AND y.qid IS NULL)

or easier

INNER JOIN x
  ON x.qid IS NOT DISTINCT FROM y.qid

Answer 4

If you want null values to be included from Y.QID then Fastest way is

SELECT * FROM Y LEFT JOIN X ON y.QID = X.QID

Note: this solution is applicable only if you need null values from Left table i.e. Y (in above case).

Otherwise INNER JOIN x ON x.qid IS NOT DISTINCT FROM y.qid is right way to do

Answer 5

Are you committed to using the Inner join syntax?

If not you could use this alternative syntax:

SELECT * 
FROM Y,X
WHERE (X.QID=Y.QID) or (X.QUID is null and Y.QUID is null)

Answer 6

I'm pretty sure that the join doesn't even do what you want. If there are 100 records in table a with a null qid and 100 records in table b with a null qid, then the join as written should make a cross join and give 10,000 results for those records. If you look at the following code and run the examples, I think that the last one is probably more the result set you intended:

create table #test1 (id int identity, qid int)
create table #test2 (id int identity, qid int)

Insert #test1 (qid)
select null
union all
select null
union all
select 1
union all
select 2
union all
select null

Insert #test2 (qid)
select null
union all
select null
union all
select 1
union all
select 3
union all
select null


select * from #test2 t2
join #test1 t1 on t2.qid = t1.qid

select * from #test2 t2
join #test1 t1 on isnull(t2.qid, 0) = isnull(t1.qid, 0)


select * from #test2 t2
join #test1 t1 on 
 t1.qid = t2.qid OR ( t1.qid IS NULL AND t2.qid IS NULL )


select t2.id, t2.qid, t1.id, t1.qid from #test2 t2
join #test1 t1 on t2.qid = t1.qid
union all
select null, null,id, qid from #test1 where qid is null
union all
select id, qid, null, null from #test2  where qid is null