MySQL：将分组数据连接到表-仅连接第一行

Question

我在连接具有以下内容的表时遇到问题：

表RingOrderItem ：

+----+-------------+--------+
| ID | ID_RingType | Amount |
+----+-------------+--------+
|  1 | A           |    100 |
|  2 | B           |     50 |
|  3 | A           |    500 |
|  4 | C           |    100 |
+----+-------------+--------+

分组表的Rings SELECT min(Rings.Number) AS Number, ID_RingType FROM Rings GROUP BY ID_RingType语句：

+--------+-------------+
| Number | ID_RingType |
+--------+-------------+
|      1 | A           |
|      1 | B           |
+--------+-------------+

我想从RingOrderItem检索所有记录，并从分组表Rings到它们的RingOrderItem号，为此我使用了以下查询：

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;

由于某种原因，我仅从RingOrderItem表中获得第一行：

+----+--------------+-------------+--------+-----------+---------+
| ID | ID_RingOrder | ID_RingType | Amount | FromValue | ToValue |
+----+--------------+-------------+--------+-----------+---------+
|  1 |            1 | A           |    100 |         1 |     100 |
+----+--------------+-------------+--------+-----------+---------+

我想要所有行，并且如果数据不能被连接（ ID_RingType值C ）， ID_RingType返回NULL 。

谢谢Zbynek

Answer 1

我认为您在主查询上不需要两个min（）函数，因为您已经在子查询中获取了min值。

另外，对可能为NULL的列进行数学运算并不是一个好主意

尝试这个：

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;

另外，将您的左连接ON子句切换为将第一个表列在第一位。