Question

我无法理解为什么。请帮帮我我有Customer表，有很多订单。所以客户（id），订单（id，customer_id）。我希望得到所有订单少于100的客户。所以我对postgres进行sql查询（我使用Rails和ActiveRecord），但查询是

SELECT customers.*, COUNT(*) AS c_c FROM "customers" LEFT OUTER JOIN orders ON orders.customer_id = customers.id GROUP BY "customers"."id" HAVING c_c < 100 OR orders.id IS NULL

我有错误

ActiveRecord::StatementInvalid: PG::UndefinedColumn: ERROR:  column "c_c" does not exist

为什么呢？我怎么能这样做？

Answer 1

查询本身看起来无效。您不能从HAVING子句引用别名列。你的COUNT（*）也不明确。您可以在c_c中将COUNT(orders.id)替换为HAVING，然后在SELECT中添加COUNT(orders.id)来修复：

SELECT customers.*, COUNT(orders.id) AS c_c FROM "customers" LEFT OUTER JOIN orders ON orders.customer_id = customers.id GROUP BY "customers"."id" HAVING COUNT(orders.id) < 100 OR orders.id IS NULL

修改

对于ActiveRecord查询，请尝试：

Customer.joins("LEFT JOIN orders ON orders.customer_id = customers.id").group("customers.id").having("COUNT(orders.id) < 100")

确保您在has_many :orders
中定义了Customer

Answer 2

你不能使用c_c

SELECT customers.*, (
  SELECT COUNT(*)
  FROM orders
  WHERE customers.id = orders.customer_id
) AS c_c 
FROM "customers" 
LEFT OUTER JOIN orders 
ON orders.customer_id = customers.id 
GROUP BY "customers"."id" 
HAVING (SELECT COUNT(*)
  FROM orders
  WHERE customers.id = orders.customer_id
) < 100 OR orders.id IS NULL

当我有PG :: UndefinedColumn：ERROR时

2 个答案: