Question

I have a system where users can complete two types of tests. I need to get a list of DISTINCT users sorted by the date of their latest test completion.

For this purpose the tables may as well be set out as:

Test_Users
 - Test_User_Id (int)

Test_1_Results
 - Test_1_Result_Id (int)
 - Test_User_Id (int)
 - Date_Of_Completion (datetime)

Test_2_Results
 - Test_2_Result_Id (int)
 - Test_User_Id (int)
 - Date_Of_Completion (datetime)

I can happily get a list of all the different Test_User_Id‘s using a UNION as follows:

SELECT Test_User_Id FROM Test_1_Results
UNION
SELECT Test_User_Id FROM Test_2_Results

And I could easily sort the results from one of these tables using a ORDER BY Date_Of_Completion DESC but i do not know how to do this with the UNION or if this is even the best way to proceed.

Ultimately i would like to be able to wrap this query within others like:

SELECT *
FROM Test_Users
WHERE Test_User_Id IN (
    //The query i am asking about
)
WHERE some_criteria

But so far i haven’t had any luck doing this while using a UNION and i am not sure what i am doing wrong.

The reason i am trying to do this is that the admin user needs to be able to view a list of everyone who has completed their tests with the most recent completions at the top.

I am fairly familiar with SQL, but i have never really had to use the UNION operator before.

Answer 1

This should work

 SELECT Test_User_Id, max(Date_Of_Completion) AS doc
 FROM ( 
     SELECT Test_User_Id, Date_Of_Completion FROM Test_1_Results
       UNION ALL
     SELECT Test_User_Id , Date_Of_Completion FROM Test_2_Results 
 ) AS temptab
 GROUP BY Test_User_Id
 ORDER BY doc DESC