SQL Joins : in, out, shake it all about - sql-server

Question

I am performing the follwing sql to return a data where there is a match of both dob and address in tables1 & 2.
select table1.dob
, table1.address
, sum(case when person_status in ('A','B','C') then 1 else 0 end) as 'ABC_count'
, sum(case when person_status in ('D','E') then 1 else 0 end) as 'DE_Count'
, sum(case when person_status in ('F','G') then 1 else 0 end) as 'FG_Count'
from table1
inner join table2
on (table1.dob = table2.dob and table1.address = table2.address)
where table1.dob > #myDate
group by table1.dob, table1.address
order by table1.dob, table1.address
However I now want to return the data from table1 when there is no match in table2 and only that data, I thought simply changing inner join to left outer would perform what I required, it does not.
Thanks!

Answer 1

If there is no match in the join, the field from the second table are NULL, so you have to check for a NULL value in table2. Assuming dob is NOT NULL in table2, this should solve your problem:
select table1.dob
, table1.address
, sum(case when person_status in ('A','B','C') then 1 else 0 end) as 'ABC_count'
, sum(case when person_status in ('D','E') then 1 else 0 end) as 'DE_Count'
, sum(case when person_status in ('F','G') then 1 else 0 end) as 'FG_Count'
from table1
left outer join table2
on (table1.dob = table2.dob and table1.address = table2.address)
where table1.dob > #myDate and table2.dob is null
group by table1.dob, table1.address
order by table1.dob, table1.address

Answer 2

In this case thre's not a join, you should use NOT EXISTS function.

Answer 3

In my opinion LEFT JOIN is much more cleaner and you should go with that if there is no big difference between the performance of LEFT JOIN and NOT EXISTS. #JNK said "EXISTS and NOT EXISTS are ordinarily faster than joins or other operators like IN because they short circuit - the first time they get a hit they move on to the next entry", but my understanding is that NOT EXISTS and NOT IN are usually expensive as sql server has to go through all the records in the lookup table to make sure that the entry in fact does NOT EXIST, so i dont know how the short circuit would work

Answer 4

You could also use the EXCEPT keyword here.
select table1.dob
, table1.address
, sum(case when person_status in ('A','B','C') then 1 else 0 end) as 'ABC_count'
, sum(case when person_status in ('D','E') then 1 else 0 end) as 'DE_Count'
, sum(case when person_status in ('F','G') then 1 else 0 end) as 'FG_Count'
from table1
where table1.dob > #myDate
EXCEPT
select table1.dob
, table1.address
, sum(case when person_status in ('A','B','C') then 1 else 0 end) as 'ABC_count'
, sum(case when person_status in ('D','E') then 1 else 0 end) as 'DE_Count'
, sum(case when person_status in ('F','G') then 1 else 0 end) as 'FG_Count'
from table1
inner join table2
on (table1.dob = table2.dob and table1.address = table2.address)
where table1.dob > #myDate
That would get you all of the records in the first query that are not in the second query.