啓用所有條目的SQL查詢

Question

我正在開發一個數據庫，我有一個表（可以說材料）。我有第二張桌子讓我保存我買的材料讓我們說purchase_elements。例如

材質ID：MAT1>量：3

材質ID：MAT2>數量：4

材質ID：Mat5>數量：2

材質ID：MAT1>量：5

當我想要查看我用查詢購買的材料時

SELECT materials.id, 
     materials.name, 
     Sum(purchase_elements.quantity) AS QUANTITY 
FROM materials 
    INNER JOIN purchase_elements 
ON materials.id= purchase_elements.id 
GROUP BY materials.id, 
      materials.name; 

That sho WS

Id/name/QUANTITY 
1 Mat1 8 
2 Mat2 4 
5 Mat5 2 

有什麼辦法查詢告訴我，不僅材料我已經做出購買，但他們都

Id/name/QUANTITY 
1 Mat1 8 
2 Mat2 4 
3 Mat3 
4 Mat4 
3 Mat5 2 

Answer 1

使用的left join。

SELECT materials.id, 
    materials.name, 
    Sum(purchase_elements.quantity) AS QUANTITY 
FROM materials 
LEFT JOIN purchase_elements 
ON materials.id= purchase_elements.[Υλικά IID] 
GROUP BY materials.id, 
     materials.name; 

Answer 2

你需要做一個LEFT OUTER JOIN，因爲INNER JOIN限行只有你已購買並過濾不屬於purchase_elements表的零件材料的材料。您將需要修改您的查詢，如下所示：

SELECT M.id, M.name, SUM(PE.quantity) AS QUANTITY 
FROM materials AS M 
    LEFT OUTER JOIN purchase_elements AS PE ON M.id= PE.MatID 
GROUP BY M.id, M.name;