加入SQL查詢問題

Question

我有以下查詢，該查詢返回產品和找到的最低售價以及該售價的數量。 一切工作正常，直到我想要獲得在product_price表中沒有任何價格的產品。 我怎樣才能讓它返回產品價格和空值的空值？

SELECT p.*, MIN(pp.sellPrice) as sellPrice, pp.quantity FROM `product` as p
LEFT JOIN `product_price_group` as ppg ON ppg.productId = p.`id`
LEFT JOIN `product_price` as pp ON pp.priceGroupId = ppg.`id`
WHERE p.`id` = 1 AND p.`active` = 1

具有可用價格的產品的輸出：

+----+--------------+--------+--------------+--------------+-----------+----------+
| id | name         | active | sortSequence | creationDate | sellPrice | quantity |
+----+--------------+--------+--------------+--------------+-----------+----------+
|  1 | product_id_1 |      1 |            1 |   1287481220 |     22.00 |       10 |
+----+--------------+--------+--------------+--------------+-----------+----------+

沒有定價的產品的輸出

+----+------+--------+--------------+--------------+-----------+----------+
| id | name | active | sortSequence | creationDate | sellPrice | quantity |
+----+------+--------+--------------+--------------+-----------+----------+
| NULL | NULL | NULL |         NULL |         NULL |      NULL |     NULL |
+----+------+--------+--------------+--------------+-----------+----------+

所需的輸出：

+----+--------------+--------+--------------+--------------+-----------+----------+
| id | name         | active | sortSequence | creationDate | sellPrice | quantity |
+----+--------------+--------+--------------+--------------+-----------+----------+
|  2 | product_id_2 |      1 |            1 |   1287481220 |     NULL  |     NULL |
+----+--------------+--------+--------------+--------------+-----------+----------+

更新似乎我在選擇不存在的產品！ 很蠢

Answer 1

對product_price表使用LEFT OUTER JOIN怎么樣？

SELECT p.*, MIN(pp.sellPrice) as sellPrice, pp.quantity FROM `product` as p
LEFT JOIN `product_price_group` as ppg ON ppg.productId = p.`id`
LEFT OUTER JOIN `product_price` as pp ON pp.priceGroupId = ppg.`id`
WHERE p.`id` = 1 AND p.`active` = 1

這是你想要的嗎？

更新：修訂-像其他人說的那樣， LEFT JOIN = LEFT (OUTER) JOIN因此在這種情況下它對您沒有幫助...

Answer 2

我可能不正確，但是我對LEFT JOIN的理解一直是SQL語句中編寫的相等性測試中的表引用……此外，我如何編寫查詢……從表開始期望FIRST（左），第二次連接到OTHER（右）表...還要保持該關系的各自連接條件...

select from x left join y where x.fld = y.fld
   instead of 
select from x left join y where y.fld = x.fld

因此，我將按以下方式調整您的查詢

SELECT
      p.*, 
      MIN(pp.sellPrice) as sellPrice, 
      pp.quantity 
   FROM 
      product as p
         LEFT JOIN product_price_group as ppg 
            ON p.id = ppg.productId
            LEFT JOIN product_price as pp 
               ON ppg.id = pp.priceGroupId
   WHERE 
          p.id = 1 
      AND p.active = 1

另外，您可以使用IFNULL（field，0）包裝min（）和數量，以防止顯示NULL，而實際為零。