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