LEFT JOIN with OR clause without UNION - sql

Question

I know this shouldn't happen in a database, but it happened and we have to deal with it. We need to insert new rows into a table if they don't exist based on the values in another table. This is easy enough (just do LEFT JOIN and check for NULL values in 1st table). But...the join isn't very straight forward and we need to search 1st table on 2 conditions with an OR and not AND. So basically if it finds a match on either of the 2 attributes, we consider that the corresponding row in 1st table exists and we don't have to insert a new one. If there are no matches on either of the 2 attributes, then we consider it as a new row. We can use OR condition in the LEFT JOIN statement but from what I understand, it does full table scan and the query takes a very long time to complete even though it yields the right results. We cannot use UNION either because it will not give us what we're looking for.
Just for simplicity purpose consider the scenario below (we need to insert data into tableA).
If(OBJECT_ID('tempdb..#tableA') Is Not Null) Begin
Drop Table #tableA End
If(OBJECT_ID('tempdb..#tableB') Is Not Null) Begin
Drop Table #tableB End
create table #tableA ( email nvarchar(50), id int )
create table #tableB ( email nvarchar(50), id int )
insert into #tableA (email, id) values ('123#abc.com', 1), ('456#abc.com', 2), ('789#abc.com', 3), ('012#abc.com', 4)
insert into #tableB (email, id) values ('234#abc.com', 1), ('456#abc.com', 2), ('567#abc.com', 3), ('012#abc.com', 4), ('345#abc.com', 5)
--THIS QUERY IS CORRECTLY RETURNING 1 RECORD
select B.email, B.id
from #tableB B
left join #tableA A on A.email = B.email or B.id = A.id
where A.id is null
--THIS QUERY IS INCORRECTLY RETURNING 3 RECORDS SINCE THERE ARE ALREADY RECORDS WITH ID's 1 & 3 in tableA though the email addresses of these records don't match
select B.email, B.id
from #tableB B
left join #tableA A on A.email = B.email
where A.id is null
union
select B.email, B.id
from #tableB B
left join #tableA A on B.id = A.id
where A.id is null
If(OBJECT_ID('tempdb..#tableA') Is Not Null) Begin
Drop Table #tableA End
If(OBJECT_ID('tempdb..#tableB') Is Not Null) Begin
Drop Table #tableB End
The 1st query works correctly and only returns 1 record, but the table size is just few records and it completes under 1 sec. When the 2 tables have thousands or records, the query may take 10 min to complete. The 2nd query of course returns the records we don't want to insert because we consider them existing. Is there a way to optimize this query so it takes an acceptable time to complete?

Answer 1

You are using an anti join, which is another way of writing the straight-forward NOT EXISTS:
where not exists
(
select null
from #tableA A
where A.email = B.email or B.id = A.id
)
I.e. where not exists a row in table A with the same email or the same id. In other words: where not exists a row with the same email and not exists a row with the same id.
where not exists (select null from #tableA A where A.email = B.email)
and not exists (select null from #tableA A where B.id = A.id)
With the appropriate indexes
on #tableA (id);
on #tableA (email);
this should be very fast.

Answer 2

It's hard to tune something you can't see. Another option to get the data is to:
SELECT B.email
, B.id
FROM #TableB B
EXCEPT
(
SELECT B.email
, B.id
FROM #tableB B
INNER JOIN #tableA A
ON A.email = B.email
UNION ALL
SELECT B.email
, B.id
FROM #tableB B
INNER JOIN #tableA A
ON B.id = A.id
)
This way you don't have to use OR, you can use INNER JOIN rather than LEFT JOIN and you can use UNION ALL instead of UNION (though this advantage may well be negated by the EXCEPT). All of which may help your performance. Perhaps the joins can be more efficient when replaced with EXISTS.
You didn't mention how this problem occurred (where the data from both tables is coming from, and why they are out of sync when they shouldn't be), but it would be preferable to fix it at the source.

Answer 3

No the query returns correctly 3 rows
because
select B.email, B.id
from #tableB B
left join #tableA A on A.email = B.email
where A.id is null
Allone reurns the 3 rows.
For your "problemm"
select B.email, B.id
from #tableB B
left join #tableA A on A.email = B.email or B.id = A.id
where A.id is null
will che3kc for every row, if it is true to be included
So for example
('123#abc.com', 1) ('234#abc.com', 1)
as the Ids are the same it will be joined
but when you join by the emails the condition is false and so is included in the result set
You can only use the UNION approach, when you are comparing only the emails or the ids, but with both the queries are not equivalent