Question

I have 2 tables to join in a specific way. I think my query is right, but not sure.

select t1.userID, t3.Answer Unit, t5.Answer Demo   FROM         table1 t1       inner join (select * from table2) t3 ON t1.userID = t3.userID        inner join (select * from table2) t5 ON t1.userID = t5.userID   where       NOT EXISTS (SELECT * FROM table1 t2 WHERE t2.userID = t1.userID AND t2.date > t1.date)       and NOT EXISTS (SELECT * FROM table2 t4 WHERE t4.userID = t3.userID and t4.counter > t3.counter)       and NOT EXISTS (SELECT * FROM table2 t6 WHERE t6.userID = t5.userID and t6.counter > t5.counter)       and t1.date_submitted >'1/1/2009'       and t3.question = Unit       and t5.question = Demo   order by       t1.userID   

From table1 I want distinct userID where date > 1/1/2009

table1       userID    Date 1         1/2/2009   1         1/2/2009   2         1/2/2009   3         1/2/2009   4         1/1/2008   

So The result I want from table1 should be this:

userID   1   2   3   

I then want to join this on userID with table2, which looks like this:

table2  userID    question   answer   counter 1         Unit       A        1 1         Demo       x        1 1         Prod       100      1 2         Unit       B        1 2         Demo       Y        1 3         Prod       100      1 4         Unit       A        1 1         Unit       B        2 1         Demo       x        2 1         Prod       100      2 2         Unit       B        2 2         Demo       Z        2 3         Prod       100      2 4         Unit       A        2 

I want to join table1 with table2 with this result:

userID    Unit    Demo 1         B       X 2         B       Z 

In other words,
select distinct userID from table2 where question = Unit for the highest counter
and then
select distinct userID from table2 where question = Demo for the highest counter.

I think what I’ve done is 3 self-joins then joined those 3 together.

Do you think it’s right?

Answer 1

SELECT  du.userID, unit.answer, demo.answer FROM    (         SELECT  DISTINCT userID         FROM    table1         WHERE   date > '1/1/2009'         ) du LEFT JOIN         table2 unit ON      (userID, question, counter) IN         (         SELECT  du.userID, 'Unit', MAX(counter)         FROM    table2 td         WHERE   userID = du.userID                 AND question = 'Unit'         ) LEFT JOIN         table2 demo ON      (userID, question, counter) IN         (         SELECT  du.userID, 'Demo', MAX(counter)         FROM    table2 td         WHERE   userID = du.userID                 AND question = 'Demo'         ) 

Having an index on table2 (userID, question, counter) will greatly improve this query.

Since you mentioned SQL Server 2005, the following will be easier and more efficient:

SELECT  du.userID,         (         SELECT  TOP 1 answer         FROM    table2 ti         WHERE   ti.user = du.userID                 AND ti.question = 'Unit'         ORDER BY                 counter DESC         ) AS unit_answer,         (         SELECT  TOP 1 answer         FROM    table2 ti         WHERE   ti.user = du.userID                 AND ti.question = 'Demo'         ORDER BY                 counter DESC         ) AS demo_answer FROM    (         SELECT  DISTINCT userID         WHERE   date > '1/1/2009'         FROM    table1         ) du 

To aggregate:

SELECT  answer, COUNT(*) FROM    (         SELECT  DISTINCT userID         FROM    table1         WHERE   date > '1/1/2009'         ) du JOIN    table2 t2 ON      t2.userID = du.userID         AND t2.question = 'Unit' GROUP BY         answer