MySQL: join grouped data to table - only first row joined - mysql

Question

I have problem joining tables with following content:
Table RingOrderItem:
+----+-------------+--------+
| ID | ID_RingType | Amount |
+----+-------------+--------+
| 1 | A | 100 |
| 2 | B | 50 |
| 3 | A | 500 |
| 4 | C | 100 |
+----+-------------+--------+
Grouped table Rings - result of SELECT min(Rings.Number) AS Number, ID_RingType FROM Rings GROUP BY ID_RingType statement:
+--------+-------------+
| Number | ID_RingType |
+--------+-------------+
| 1 | A |
| 1 | B |
+--------+-------------+
I want to retrieve all records from RingOrderItem and join number from grouped table Rings to them, for which I used this query:
SELECT
roi.ID,
roi.ID_RingOrder,
roi.ID_RingType,
roi.Amount,
min(r.Number) AS `FromValue`,
min(r.Number) + roi.Amount - 1 AS `ToValue`
FROM
RingOrderItem AS roi
LEFT JOIN
(SELECT min(Rings.Number) AS Number, ID_RingType FROM Rings
GROUP BY ID_RingType)
AS r ON r.ID_RingType = roi.ID_RingType;
For some reason, I get only the first row from RingOrderItem table:
+----+--------------+-------------+--------+-----------+---------+
| ID | ID_RingOrder | ID_RingType | Amount | FromValue | ToValue |
+----+--------------+-------------+--------+-----------+---------+
| 1 | 1 | A | 100 | 1 | 100 |
+----+--------------+-------------+--------+-----------+---------+
I want all rows, and if the data can not be joined (value C in ID_RingType), than return simply NULL.
Thanks,
Zbynek

Answer 1

I don't think you need the two min() functions on the main query since you are already getting the min values in the sub query.
Also, it's not really a good idea to do math to a column that might be NULL
Try this:
SELECT
roi.ID,
roi.ID_RingOrder,
roi.ID_RingType,
roi.Amount,
r.Number AS FromValue,
COALESCE(r.Number, 0) + roi.Amount - 1 AS ToValue
FROM
RingOrderItem AS roi
LEFT JOIN
(
SELECT
MIN(Rings.Number) AS Number,
ID_RingType
FROM
Rings
GROUP BY
ID_RingType
) AS r ON roi.ID_RingType = r.ID_RingType;
Also, switch your left join ON clause to have the first table listed first.