如何根据某些条件省略表连接？

Question

=======================

SELECT *
      FROM PRODUCT P
      JOIN PRODUCT_LOC PL
        ON P.PRODUCT_ID = PL.PRODUCT_ID

      JOIN PRODUCT_LOC_DEF PLD AND PL.LOC_ID = PLD.LOC_ID
      JOIN PRODUCT_CURRENT_LOC PCL
        ON PLD.LOC_ID = PCL.LOC_ID AND P.PRODUCT_ID = PCL.PRODUCT_ID

我应该如何修改查询以获得附加快照中给出的案例1和案例2中的预期结果？

优选的： 我正在寻找相同查询中的修改，而不是进行两次查询并将它们联合起来。

Answer 1

只需使用left join代替内部联接：

   SELECT *
     FROM PRODUCT P
LEFT JOIN PRODUCT_LOC PL ON P.PRODUCT_ID = PL.PRODUCT_ID
LEFT JOIN PRODUCT_LOC_DEF PLD AND PL.PRODUCT_LOC_ID = PLD.LOC_ID
LEFT JOIN PRODUCT_CURRENT_LOC PCL ON PLD.LOC_ID = PCL.LOC_ID

Answer 2

你应该试试这个：

SELECT * FROM 
    PRODUCT p
INNER JOIN 
    PRODUCT_CURRENT_LOC pcl
ON
    p.PRODUCT_ID = pcl.PRODUCT_ID
INNER JOIN 
    PRODUCT_LOC pc
ON
    pc.LOC_ID = pcl.LOC_ID
UNION
SELECT * FROM 
    PRODUCT p
LEFT OUTER JOIN 
    PRODUCT_CURRENT_LOC pcl
ON
    p.PRODUCT_ID = pcl.PRODUCT_ID
    AND 
    pcl.PRODUCT_ID IS NULL
LEFT OUTER JOIN 
    PRODUCT_LOC pc
ON
    pc.LOC_ID = pcl.LOC_ID
    AND 
    pc.LOC_ID IS NULL