Question

I am writing a query to grab the items that a specific user_id was the first to use. Here is some sample data –

item_id used_user_id    date_used
1       1               2012-08-25
1       2               2012-08-26               
1       3               2012-08-27
2       2               2012-08-27
3       1               2012-08-27
4       1               2012-08-21
4       3               2012-08-24
5       3               2012-08-23

query

select item_id as inner_item_id, ( select used_user_id 
                                   from test 
                                   where test.item_id = inner_item_id 
                                   order by date_used asc 
                                   limit 1 ) as first_to_use_it 
from test 
where used_user_id = 1 
group by item_id

It returns the correct values

inner_item_id   first_to_use_it
1               1
3               1
4               1

but the query is VERY slow on a giant table. Is there a certain index that I can use or a better query that I can write?

Answer 1

i can’t get exactly what you mean because in your inner query you have sorted it by their used_user_id and and on your outer query you have filtered it also by their userid. Why not do this directly?

SELECT DISTINCT item_id AS inner_item_id,
       used_user_id AS first_to_use_it 
FROM   test
WHERE  used_user_id = 1 

UPDATE 1

SELECT  b.item_id, 
        b.used_user_id AS first_to_use_it
FROM
    (
        SELECT item_ID, MIN(date_used) minDate
        FROM tableName
        GROUP BY item_ID
    ) a
        INNER JOIN tableName b
            ON a.item_ID = b.item_ID AND
                a.minDate = b.date_used
WHERE   b.used_user_id = 1