Question

我想在我的数据库中计算客户的“余额”。

Customer Table
-----------------
1 - Frank
2 - Bob

Invoice Table
---------------------------------------------------
id | customer_id | amount_due | credit_due
---------------------------------------------------
1  | 1           | $10        | 
---------------------------------------------------
2  | 1           |            | $6
---------------------------------------------------

这是查找客户信息的典型sql语句。

SELECT id, name FROM CUSTOMER WHERE id = ?

如果我还要检索余额，我这样做：

SELECT a.*, b.* FROM 
(SELECT id, name FROM CUSTOMER WHERE id = ?) AS a, 
(SELECT SUM(amount_due - credit_due) AS balance FROM INVOICE WHERE cus_id = ?) AS b

运行此结果：

--------------------
id | name | balance 
---------------------
1 | Frank | $4

这在检索一个客户时效果很好，但是如果我想进行更一般的搜索怎么办？喜欢加载所有客户？

这是我想要的输出

--------------------
id | name | balance 
---------------------
1 | Frank | $4
---------------------
2 | Bob   | $0

Answer 1

我认为你只会使用聚合：

select c.customer_id, c.name,
       ( coalesce(sum(i.amount_due), 0) - coalesce(sum(i.credit_due), 0) ) as balance
from customer c join
     invoice i
     on i.customer_id = c.customer_id
group by c.customer_id, c.name;

我应该注意，如果金额字段是NULL，那么表达式sum(amount_due - credit_due)就没有达到预期效果。

MS SQL：每个结果的表计算

1 个答案: