Question

我的桌子

CREATE TABLE Customers (
  id        SERIAL PRIMARY KEY,
  firstname VARCHAR(50),
  lastname  VARCHAR(50)
);

CREATE TABLE Payments (
  id          SERIAL PRIMARY KEY,
  amount      INT,
  customer_id INT,
  CONSTRAINT fk_CustomerPayment FOREIGN KEY (customer_id) REFERENCES Customers (id)
);

我希望获得所有客户的总付款金额。这是我的尝试：

SELECT SUM(p.amount)
FROM Customers c
  JOIN Payments p
    ON c.id = p.customer_id
GROUP BY p

Answer 1

select sum(p.amount) as total
from
    customers c
    inner join
    payments p on c.id = p.customer_id

如果payments.customer_id中可以有空值，则连接条件将排除它们。

没有加入会更便宜：

select sum(amount) as total
from payments
where customer_id is not null

Answer 2

从查询中删除分组..

  SELECT SUM(p.amount)
FROM Customers c
  JOIN Payments p
    ON c.id = p.customer_id

SQL：如何从连接表中检索总金额？

2 个答案: