Question

当我执行一些SQL命令时，我不知道为什么结果会发生变化，所以我一直坐在W3学校并且搔痒我的大脑。

所以这是W3学校的例子：

SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders
FROM Orders
INNER JOIN Employees ON Orders.EmployeeID = Employees.EmployeeID
where LastName = 'Davolio' OR LastName = 'Fuller'
GROUP BY LastName
HAVING COUNT(Orders.OrderID) > 25;

输出结果为：Davolio 29

所以我的问题是，当我将代码更改为

时

SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders
FROM Orders
INNER JOIN Employees ON Orders.EmployeeID = Employees.EmployeeID
and LastName = 'Davolio' OR LastName = 'Fuller'
GROUP BY LastName
HAVING COUNT(Orders.OrderID) > 25;

输出结果为：Davolio 29 Fuller 196

这里发生了什么，我还没有找到一个正确的解释，为什么只有196个订单的富勒才会将它改为AND，为什么会有差异呢？

Answer 1

第二种情况的逻辑是：

ON (Orders.EmployeeID = Employees.EmployeeID and LastName = 'Davolio') OR
   (LastName = 'Fuller')

您可以使用括号获得相同的结果：

ON Orders.EmployeeID = Employees.EmployeeID and
   (LastName = 'Davolio' OR LastName = 'Fuller')

或更简单地使用IN：

ON Orders.EmployeeID = Employees.EmployeeID and
   LastName IN ('Davolio', 'Fuller')

什么是连接语句中AND和WHERE之间的区别

1 个答案: