Select count of rows in two other tables - sql

Question

I have 3 tables. The main one in which I want to retrieve some information and two others for row count only.
I used a request like this :
SELECT A.*,
COUNT(B.id) AS b_count
FROM A
LEFT JOIN B on B.a_id = A.id
WHERE A.id > 50 AND B.ID < 100
GROUP BY A.id
from Gerry Shaw's comment here. It works perfectly but only for one table.
Now I need to add the row count for the third (C) table. I tried
SELECT A.*,
COUNT(B.id) AS b_count
COUNT(C.id) AS c_count
FROM A
LEFT JOIN B on B.a_id = A.id
LEFT JOIN C on C.a_id = A.id
GROUP BY A.id
but, because of the two left joins, my b_count and my c_count are false and equal to each other. In fact my actual b_count and c_count are equal to real_b_count*real_c_count. Any idea of how I could fix this without adding a lot of complexity/subqueries ?
Data sample as requested:
Table A (primary key : id)
id | data1 | data2
------+-------+-------
1 | 0,45 | 0,79
----------------------
2 | -2,24 | -0,25
----------------------
3 | 1,69 | 1,23
Table B (primary key : (a_id,fruit))
a_id | fruit
------+-------
1 | apple
------+-------
1 | banana
--------------
2 | apple
Table C (primary key : (a_id,color))
a_id | color
------+-------
2 | blue
------+-------
2 | purple
--------------
3 | blue
expected result:
id | data1 | data2 | b_count | c_count
------+-------+-------+---------+--------
1 | 0,45 | 0,79 | 2 | 0
----------------------+---------+--------
2 | -2,24 | -0,25 | 1 | 2
----------------------+---------+--------
3 | 1,69 | 1,23 | 0 | 1

Answer 1

There are two possible solutions. One is using subqueries behind SELECT
SELECT A.*,
(
SELECT COUNT(B.id) FROM B WHERE B.a_id = A.id AND B.ID < 100
) AS b_count,
(
SELECT COUNT(C.id) FROM C WHERE C.a_id = A.id
) AS c_count
FROM A
WHERE A.id > 50
the second are two SQL queries joined together
SELECT t1.*, t2.c_count
FROM
(
SELECT A.*,
COUNT(B.id) AS b_count
FROM A
LEFT JOIN B on B.a_id = A.id
WHERE A.id > 50 AND B.ID < 100
GROUP BY A.id
) t1
JOIN
(
SELECT A.*,
COUNT(C.id) AS c_count
FROM A
LEFT JOIN C on C.a_id = A.id
WHERE A.id > 50
GROUP BY A.id
) t2 ON t1.id = t2.id
I prefer the second syntax since it clearly shows the optimizer that you are interested in GROUP BY, however, the query plans are usually the same.

Answer 2

If tables B & C also have their own key fields, then you can use COUNT DISTINCT on the primary key rather than foreign key. That gets around the multi-line problem you see on link to several tables. If you can post the table structures then we can advise further.

Answer 3

Try something like this
SELECT A.*,
(SELECT COUNT(B.id) FROM B WHERE B.a_id = A.id) AS b_count,
(SELECT COUNT(C.id) FROM C WHERE C.a_id = A.id) AS c_count
FROM A

Answer 4

That is the easier way I can think:
Create table #a (id int, data1 float, data2 float)
Create table #b (id int, fruit varchar(50))
Create table #c (id int, color varchar(50))
Insert into #a
SELECT 1, 0.45, 0.79
UNION ALL SELECT 2, -2.24, -0.25
UNION ALL SELECT 3, 1.69, 1.23
Insert into #b
SELECT 1, 'apple'
UNION ALL SELECT 1, 'banana'
UNION ALL SELECT 2, 'orange'
Insert into #c
SELECT 2, 'blue'
UNION ALL SELECT 2, 'purple'
UNION ALL SELECT 3, 'orange'
SELECT #a.*,
(SELECT COUNT(#b.id) FROM #b where #b.id = #a.id) AS b_count,
(SELECT COUNT(#c.id) FROM #c where #c.id = #a.id) AS b_count
FROM #a
ORDER BY #a.id
Result:
id data1 data2 b_count b_count
1 0,45 0,79 2 0
2 -2,24 -0,25 1 2
3 1,69 1,23 0 1

Answer 5

If table b and c have unique id, you can try this:
SELECT A.*,
COUNT(distinct B.fruit) AS b_count,
COUNT(distinct C.color) AS c_count
FROM A
LEFT JOIN B on B.a_id = A.id
LEFT JOIN C on C.a_id = A.id
GROUP BY A.id
See SQLFiddle MySQL demo.