Question

假设我有一个具有以下结构的数据库：

Customers  
ID Name  
1  Bob  
2  Jim  
3  Sally  

Products  
ID Description  
10 Pencil  
11 Ruler  
12 Eraser  

Purchases  
ID Customer_ID Product_ID  
1 1 10  
2 1 11    
3 2 10  
4 3 12

是否可以（不针对每个客户，针对每个产品）编写查询以获得如下格式：

Customer Pencil Ruler Eraser  
1 1 1 0  
2 1 0 0  
3 0 0 1

我在哪里，但我想避免这种结构，因为它会变得冗长，产品的数量会发生变化。当客户确实购买产品10时，我也会收到重复的行：

  SELECT DISTINCT CUSTOMER.CUSTOMER_ID, (case when a.CUSTOMER_ID = 
    CUSTOMER.CUSTOMER_ID THEN 1 ELSE 0 END) as product10
    from 
    (SELECT PURCHASES.CUSTOMER_ID from PURCHASES
    WHERE PURCHASES.PRODUCT_ID = 10) a, CUSTOMER

CUSTOMER_ID product10  
1 1  
1 0  
2 1  
2 0  
3 0

谢谢！

Answer 1

您可以执行条件聚合：

select c.id, sum(case when p.Description = 'Pencil' then 1 else 0 end) as Pencil,
             sum(case when p.Description = 'Ruler' then 1 else 0 end) as Ruler,
             sum(case when p.Description = 'Eraser' then 1 else 0 end) as Eraser
from Purchases prc inner join 
     Customers c
     on c.id = prc.Customer_ID inner join
     Products p 
     on p.id = prc.Product_ID  
group by c.id;

Answer 2

你可以试试这个：

SELECT DISTINCT t1.ID, t3.ID 
FROM Customers t1 
INNER JOIN Purchases t2 
   ON t1.ID = t2.Customer_ID 
INNER JOIN Products t3 
   ON t2.Product_ID = t3.ID;

它不会返回客户在一行中拥有的所有内容，但它会返回客户与产品之间的关系不存在重复的所有行。

用于汇总三个表的SQL查询

2 个答案: