Re: SQL query problem of a Quiz program

Did you try DISTINCT ON?

postgres=# table x;
id | qid | uid
----+-----+--------
1 | 25 | 1
2 | 25 | 1
3 | 25 | 1
4 | 26 | 1
5 | 26 | 1
6 | 27 | 1
7 | 27 | 1
8 | 25 | 2
9 | 25 | 2
10 | 25 | 2
11 | 26 | 2
12 | 26 | 2
13 | 27 | 2
14 | 27 | 2
15 | 25 | <NULL>
16 | 25 | <NULL>
17 | 25 | <NULL>
18 | 26 | <NULL>
19 | 26 | <NULL>
20 | 27 | <NULL>
21 | 27 | <NULL>
(21 rows)

postgres=# select distinct on (qid, uid) * from x order by uid, qid, id
desc;
id | qid | uid
----+-----+--------
3 | 25 | 1
5 | 26 | 1
7 | 27 | 1
10 | 25 | 2
12 | 26 | 2
14 | 27 | 2
17 | 25 | <NULL>
19 | 26 | <NULL>
21 | 27 | <NULL>
(9 rows)

Is that what you need?

On Sat, Dec 17, 2016 at 4:25 PM, Arup Rakshit <aruprakshit1987(at)outlook(dot)com>
wrote:

> Hi,
>
> Here is a sample data from table "quiz_results":
>
> id | question_id | user_id
> ----+-------------+------------
> 2 | 25 | 5142670086
> 3 | 26 |
> 4 | 26 |
> 5 | 27 |
> 6 | 25 | 5142670086
> 7 | 25 | 5142670086
> 8 | 25 | 5142670086
> 9 | 26 |
> 10 | 40 | 5142670086
> 11 | 29 | 5142670086
>
>
> As you see above question id 25 appeared more than once. This is basically
> a quiz result table where for users as they answered. question_id 25 always
> the first questions. Any user can go though the quiz N number of time. So,
> I want to find the last occurrence of the question_id 25 for any specific
> user in the table, and select that and all answers the users gave after
> this till the end of the quiz. Any idea how to solve it in a single
> efficient query. My all try didn't work out.
>
>
>
> --------------------
> Regards,
> Arup Rakshit
>
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