how can i achieve this mysql group query - mysql

Question

I have two tables A and B.
My goal is to list every row from A, while attach the SUM of 'amount' from B.
like this:
SELECT a.name,
SUM(b.amount) as amount
FROM a
LEFT JOIN b ON a.id = b.a_id
GROUP BY a.id
No probleam until this, but i also need to check whether the B table's 'shop_id' matches a value, and if does, i want that single row's 'amount', and not the SUM of all groupped rows. I hope it is understandable.
Table A
id name
----------------
1 john
2 doe
3 smith
Table B
a_id amount shop
-----------------------
1 4 1
1 3 2
2 2 2
2 7 3
3 3 3
3 1 2
Desired result with 'shop'=1:
name amount shop
---------------------
john 4 1 //no SUM, only the value of amount where shop=1
doe 9 0 //sum(7,2) because shop is not 1
smith 4 0 //sum(3,1)
I was thinking of an if statement at the SUM() selection something similar to this, but the below statement returns not the desired groupped row value
SELECT a.name,
( CASE
WHEN b.shop <> 1 THEN Sum(b.amount)
ELSE b.amount
end ) AS amount,
( CASE
WHEN b.shop <> 1 THEN 0
ELSE b.shop
end ) AS shop
FROM a
LEFT JOIN b
ON a.id = b.a_id
GROUP BY a.id
Any ideas ? Is there a way to put this condition to the GROUP like: case shop<>1 THEN .... ELSE GROUP BY a.id ??

Answer 1

You will need to Left Join twice with the table b.
First Join will enable computation of Sum.
Second Join will join only when shop = 1. So if we get some non-null value due to second join, we will consider that, else the Sum
Try the following:
SELECT
a.id,
a.name,
COALESCE(MAX(b2.amount), SUM(b1.amount)) AS amount,
COALESCE(b2.shop, 0) AS shop
FROM a
LEFT JOIN b AS b1 ON a.id = b1.a_id
LEFT JOIN b AS b2 ON a.id = b2.a_id AND b2.shop = 1
GROUP BY a.id, a.name
Result
| id | name | amount | shop |
| --- | ----- | ------ | ---- |
| 1 | john | 4 | 1 |
| 2 | doe | 9 | 0 |
| 3 | smith | 4 | 0 |
View on DB Fiddle

Answer 2

You were on the right track to use what is called conditional aggregation. Here is a version which should work:
SELECT
a.name,
CASE WHEN MAX(CASE WHEN b.shop = 1 THEN 1 ELSE 0 END) > 0
THEN SUM(CASE WHEN b.shop = 1 THEN b.amount ELSE 0 END)
ELSE SUM(b.amount) END AS amount,
MAX(CASE WHEN b.shop = 1 THEN 1 ELSE 0 END) AS shop
FROM a
LEFT JOIN b
ON a.id = b.a_id
GROUP BY
a.name;
Demo