Question

I am trying to get JOIN a table after I get a result from a SELECT with COALESCE

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

However the query is not valid and MySQL tells me locationId is an Unknown column. How could I fix this?

Table structure

[Parts]
id | location_id

[Relocations]
id | location_id | date_moved

[Locations]
id | name

Answer 1

If I understand correctly, you are trying to join on the COALESCE column. Try this:

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