postgresql 分组和内部连接

Question

我想要一个同时执行INNER JOIN 和GROUP BY 的SQL 查询。我尝试了以下不起作用：

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

提前谢谢你！

Answer 1

您想按 customer_id 分组，但获得 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进行聚合，然后您可以从客户那里获得额外的列：

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

Answer 2

首先，GROUP BY 出现在查询的末尾（如果有的话，就在 order by 或 having 子句之前）。

那么，select中所有不在聚合函数中的字段都必须在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;

但是具有相同名字的客户将被分组，这可能不是您真正想要的。

相当

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;