即使看起来正确，SQL也会出错

Question

发生错误，我试图看出什么是错的，无济于事。请帮忙

 SELECT UserId, AVG(Total) AS AvgOrderTotal
FROM Invoices
HAVING COUNT(OrderId) >= 1

Answer 1

你错过了GROUP BY：

SELECT UserId, AVG(Total) AS AvgOrderTotal
FROM Invoices
GROUP BY UserId
HAVING COUNT(OrderId) >= 1

假设OrderId永远不是NULL，HAVING是多余的，所以也许这就足够了：

SELECT UserId, AVG(Total) AS AvgOrderTotal
FROM Invoices
GROUP BY UserId;