左联接“选择”

Question

美好的一天，

有人可以向我解释以下语法在做什么吗？我了解它之前要连接的表的所有内容，但是在这种情况下，它不是要连接的表，它看起来像是一个选择，我对它要选择的内容感到非常困惑。

LEFT OUTER JOIN (SELECT PARTRACKINFO.INVLOGID AS INVLOGID, GROUP_CONCAT(PARTRACKINFO.TRACKVALUE) AS TRACKVALUE
    FROM (SELECT TIINVENTORYLOG.INVENTORYLOGID AS INVLOGID, CONCAT(PARTTRACKING.ABBR, ': ',
            GROUP_CONCAT((CASE

                WHEN PARTTRACKING.TYPEID = 20 THEN SUBSTRING(TIINVENTORYLOG.INFODATE FROM 1 FOR 11)
                WHEN PARTTRACKING.TYPEID = 30 THEN SUBSTRING(TIINVENTORYLOG.INFODATE FROM 1 FOR 11)
                WHEN PARTTRACKING.TYPEID = 40 THEN TIINVENTORYLOGSN.SERIALNUM
                WHEN PARTTRACKING.TYPEID = 50 THEN CAST(TIINVENTORYLOG.INFODOUBLE AS DECIMAL(18,2))
                WHEN PARTTRACKING.TYPEID = 60 THEN CAST(TIINVENTORYLOG.INFODOUBLE AS DECIMAL(18,5))
                WHEN PARTTRACKING.TYPEID = 70 THEN TIINVENTORYLOG.INFOINTEGER
                WHEN PARTTRACKING.TYPEID = 80 THEN TIINVENTORYLOG.INFOINTEGER
                ELSE TIINVENTORYLOG.INFO
            END ))) AS TRACKVALUE
        FROM TIINVENTORYLOG
            INNER JOIN PARTTRACKING ON (TIINVENTORYLOG.PARTTRACKINGID = PARTTRACKING.ID)
            LEFT OUTER JOIN TIINVENTORYLOGSN ON (TIINVENTORYLOG.ID = TIINVENTORYLOGSN.TIINVENTORYLOGID)
            GROUP BY TIINVENTORYLOG.INVENTORYLOGID, PARTTRACKING.ABBR, PARTTRACKING.SORTORDER
        ORDER BY PARTTRACKING.SORTORDER) AS PARTRACKINFO
    GROUP BY PARTRACKINFO.INVLOGID) TRACKINFO ON (INVENTORYLOG.ID = TRACKINFO.INVLOGID)

谢谢