postgresql group by和inner join

Question

我希望SQL中的查詢同時執行INNER JOIN和GROUP BY 。 我嘗試了以下不起作用：

SELECT customer.first_name, SUM(payment.amount)
FROM customer
GROUP BY customer.customer_id
INNER JOIN payment
ON payment.customer_id = customer.customer_id;

先感謝您！

Answer 1

首先， GROUP BY出現在查詢的末尾（如果你有一些，則在order by或者having子句之前）。

然后，select中不在聚合函數中的所有字段必須在group by子句中。

所以

SELECT customer.first_name, SUM(payment.amount)
FROM customer
INNER JOIN payment
ON payment.customer_id = customer.customer_id
GROUP BY  customer.first_name;

但具有相同first_name的客戶將被分組，這可能不是您想要的。

所以相反

SELECT  customer.first_name, SUM(payment.amount)
FROM customer
INNER JOIN payment
ON payment.customer_id = customer.customer_id
GROUP BY  customer.first_name, customer.customer_id;

Answer 2

您想按customer_id分組，但獲取first_name？

SELECT customer.first_name, SUM(payment.amount)
FROM customer
INNER JOIN payment
ON payment.customer_id = customer.customer_id
GROUP BY customer.customer_id, customer.first_name;

您也可以在派生表中進行聚合，然后您可以從客戶那里獲得其他列：

SELECT customer.first_name, SumPayment
FROM customer
INNER JOIN 
 (
   SELECT customer_id,
          SUM(payment.amount) AS SumPayment
   FROM payment
   GROUP BY customer_id
 ) AS payment
ON payment.customer_id = customer.customer_id