MySQL - COALESCE使用结果加入

Question

我在从COALESCE

获取结果后尝试获取JOIN表

SELECT
    COALESCE(
            (
                SELECT
                      relocations.location_id
                FROM
                      relocations
                WHERE
                      parts.id=relocation.part_id
                ORDER BY
                      relocations.date_moved
                DESC
                LIMIT 
                      1
            )
            ,
            parts.location_id
       ) AS locationId,
    locations.name
FROM
    parts
JOIN
    locations ON locations.id=locationId

但是查询无效，MySQL告诉我locationId是一个未知列。我怎么能解决这个问题？

表格结构

[部件] id | LOCATION_ID

[重定位] id | location_id | date_moved

[位置] id |名称

Answer 1

如果我理解正确，您正尝试加入COALESCE列。试试这个：

SELECT a.locationId, locations.name FROM
    (SELECT
        COALESCE(
                    (
                    SELECT
                          relocations.location_id
                    FROM
                          relocations
                    WHERE
                          parts.id=relocation.part_id
                    ORDER BY
                          relocations.date_moved
                    DESC
                    LIMIT 
                          1
                )
                ,
                parts.location_id
           ) AS locationId
    FROM
        parts) a
JOIN locations ON locations.id=a.locationId