Question

I have a table something like this:

----------------------------------------
|uID|responseDate|field_01|field_02|etc|
+---+------------+--------+--------+---+
| 1 | 2011-12-02 |   yes  |    no  |   |
| 2 | 2011-11-25 |   no   |   yes  |   |
| 1 | 2012-01-02 |   no   |   yes  |   |
| 2 | 2012-12-01 |   no   |    no  |   |
| 3 | 2010-01-02 |   yes  |    no  |   |
+---+------------+--------+--------+---+

I would like to get uIDs and responseDates for whom the latest response to field_01 is ‘yes’ – so my query should return:

------------------
|uID|responseDate|
+---+------------+
| 3 | 2010-01-02 |
+---+------------+

I’m using an inner join, but incorrectly. Here is my query:

SELECT f.uID, f.responseDate
FROM form_05 as f
INNER JOIN (
    SELECT uID, max(responseDate) AS latest_date
    FROM form_05
    WHERE field_01 = 'yes'
    GROUP BY uID ) dmax
 ON dmax.uID = f.uID and dmax.latest_date = f.responseDate
 ORDER BY f.uID ASC;

What this returns, however, is the latest entries for each uID where field_01 is yes, i.e.:

------------------
|uID|responseDate|
+---+------------+
| 1 | 2011-12-02 | 
| 3 | 2010-01-02 |
+---+------------+    

But I don’t want that. I’d like to make it so that only the latest entry for each uID is eligible for the test. How can I restructure the query? Any pointers are greatly appreciated.

Answer 1

Give this a try and let me know if it doesn’t work:

select t2.uid, t2.responseDate from t2
left join (
    select t1.uid, max(t1.responseDate) as MaxDate from t1
    group by t1.uid
) as SubQuery on t2.uid = SubQuery.uid and t2.responseDate = SubQuery.MaxDate
where MaxDate is not null and field_01 = "yes"