SQL - Select distinct on two column - sql

Question

I have this table 'words' with more information:
+---------+------------+-----------
| ID |ID_CATEGORY | ID_THEME |
+---------+------------+-----------
| 1 | 1 | 1
| 2 | 1 | 1
| 3 | 1 | 1
| 4 | 1 | 2
| 5 | 1 | 2
| 6 | 1 | 2
| 7 | 2 | 3
| 8 | 2 | 3
| 9 | 2 | 3
| 10 | 2 | 4
| 11 | 2 | 4
| 12 | 3 | 5
| 13 | 3 | 5
| 14 | 3 | 6
| 15 | 3 | 6
| 16 | 3 | 6
And this query that gives to me 3 random ids from different categories, but not from different themes too:
SELECT Id
FROM words
GROUP BY Id_Category, Id_Theme
ORDER BY RAND()
LIMIT 3
What I want as result is:
+---------+------------+-----------
| ID |ID_CATEGORY | ID_THEME |
+---------+------------+-----------
| 2 | 1 | 1
| 7 | 2 | 3
| 14 | 3 | 6
That is, repeat no category or theme.

Answer 1

When you use GROUP BY you cannot include in the select list a column which is not being ordered. So, in your query it's impossible to inlcude Id in the select list.
So you need to do something a bit more complex:
SELECT Id_Category, Id_Theme,
(SELECT Id FROM Words W
WHERE W.Id_Category = G.Id_Category AND W.Id_Theme = G.Id_Theme
ORDER BY RAND() LIMIT 1
) Id
FROM Words G
GROUP BY Id_Category, Id_Theme
ORDER BY RAND()
LIMIT 3
NOTE: the query groups by the required columns, and the subselect is used to take a random Id from all the possible Ids in the group. Then main query is filtered to take three random rows.