MySQL, get sum of two queries - mysql

Question

I have three different tables about Product having different columns and structure, assume
Product1, Product2, Product3
So, I'm trying to get sum of count(*) of three tables having same user_id, i.e. foreach user_id field.
Table - Product1
select P.user_id, count(*)
from Product1 P
group by P.user_id
Table - Product2
select P.user_id, count(*)
from Product2 P
group by P.user_id
Table - Product3
select P.user_id, count(*)
from Product3 P
group by P.user_id
They give me user_id field and count(*),
Can I add results of count(*), foreach user_id field? Thanks, in advance

Answer 1

Having three tables with the same structure is usually a sign of poor database design. You should figure out ways to combine the tables into a single table.
In any case, you can aggregate the results. One way is:
select user_id, sum(cnt)
from ((select user_id, count(*) as cnt
from product1
group by user_id
) union all
(select user_id, count(*) as cnt
from product2
group by user_id
) union all
(select user_id, count(*) as cnt
from product3
group by user_id
)
) up
group by user_id;
You want to use union all rather than a join because MySQL does not support full outer join. Union all ensures that users from all three tables are included.
Aggregating twice (in the subqueries and the outer query) allows MySQL to use indexes for the inner aggregations. That can be a performance advantage.
Also, if you are looking for a particular user or set of users, use a where clause in the subqueries. That is more efficient (in MySQL) than bringing all the data together in subqueries and then doing the filtering.

Answer 2

Combine the results using UNION and then do the addition.
Query
select t.`user_id`, sum(`count`) as `total` from(
select `user_id`, count(*) as `count`
from `Product1`
group by `user_id`
union all
select `user_id`, count(*)
from `Product2`
group by `user_id`
union all
select `user_id`, count(*)
from `Product3`
group by `user_id`
) t
group by t.`user_id`;

Answer 3

You could sum the result of union all
select user_id, sum(my_count)
from (
select P.user_id, count(*) my_count
from Product1 P
group by P.user_id
UNION ALL
select P.user_id, count(*)
from Product2 P
group by P.user_id
UNION ALL
select P.user_id, count(*)
from Product3 P
group by P.user_id ) t
group by user_id

Answer 4

Yes you can :)
SELECT SUM(userProducts) userProducts
FROM (
SELECT count(user_id) userProducts FROM Product1 WHERE user_id = your_user_id
UNION ALL
SELECT count(user_id) userProducts FROM Product2 WHERE user_id = your_user_id
UNION ALL
SELECT count(user_id) userProducts FROM Product3 WHERE user_id = your_user_id
) s

Answer 5

Please try below. Not tried in db so could get syntax error.
select p.user_id ,sum(total) from (
select P.user_id, count() total from product1 p group by P.user_id
union all
select P.user_id, count() total from product2 p group by P.user_id
union all
select P.user_id, count(*) total from product3 p group by P.user_id
) a

Answer 6

Yes, we can aggregate results from different tables using join and union based on our requirement. In your case Union All will work perfectly and can write optimised query by using count(1) instead of count(*), as it uses first Index of the table which is more often clustered Index.
select user_id, sum(cnt)
from ((select user_id, count(1) as cnt
from product1
group by user_id
) union all
(select user_id, count(1) as cnt
from product2
group by user_id
) union all
(select user_id, count(1) as cnt
from product3
group by user_id
)
) a
group by user_id;