Best approach to SQL Server query using distinct

Question

I've two tables: TableA and TableB, joined by TableA.TableA_Id->1..n<-TableB.TableA_Id. A simple PK-FK.

I need to extract distinct TableA records given a certain condition on TableB. Here's my 1st approach:

SELECT * FROM TableA A INNER JOIN TableB B ON A.idA = B.IdA AND B.Date = '2009-01-10' ORDER BY A.Id;

This is nice, but it doesn't give me "distinct" records. Some records on table B may be duplicate and hence I could get the same records more than once.

So I decided to perform a subselect (performance is not an issue given that the subselect will probably end up with 20/30 records max):

SELECT * FROM TableA WHERE TableA.Id IN ( SELECT DISTINCT IdA FROM TableB WHERE Date = '20090110' ) ORDER BY TableA.IdA;

This works fine.

Now the question is: how can I use the Inner Join and still get the distinct values? Is this possible in one pass or the nested query is a must? What am I missing?

Answer 1

use a derived table

SELECT * FROM TableA 
JOIN
(SELECT DISTINCT IdA FROM TableB WHERE Date = '20090110') a
ON a.IDA = TAbleA.IDA
ORDER BY TableA.IdA

Answer 2

I think a normal exists statement is what you need:

SELECT * 
FROM TableA A 
WHERE Exists( select B.IdA from TableB B where A.IdA = B.IdA and B.Date = '2009-01-10' )
ORDER BY A.Id;

Performance-wise it should be the best approach.

If you need values from the other table, and to avoid using distinct, you could join to a sub query that is grouped like so:

Select TableA.*, groupedTableB.OtherData 
From TableA
Inner join 
(
    select TableB.IdA, Sum(TableB.Data) SummaryData
    from TableB where TableB.Date = '2009-01-10'
    group by TableB.IdA

) groupedTableB on groupedTableB.IdA = TableA.IdA

Answer 3

And the problem using

SELECT DISTINCT * FROM TableA A INNER JOIN TableB B ON A.idA = B.IdA 
AND B.Date = '2009-01-10' ORDER BY A.Id;

is?

If it's that it does return duplicate idA values, that's because you are selecting too many columns, if you can't reduce what you select, you need the subquery.

Answer 4

How about this :

select * from TableA a 
  where a.TableA_Id in
(select TableA_Id from TableB where [Date] = '2009-01-10')
order by a.TableA_Id

You could add distinct to the subquery if you want, I'm not sure whether this will improve or reduce preformance offhand.