Question

我正在练习恢复MySQL的标准，并且使用Northlight练习数据库遇到了一个问题。我专门用这个网站进行练习：http://www.w3schools.com/sql/trysql.asp

我注意到员工Adam West（员工ID 10）在Orders表中没有出现任何事件。我正在尝试获取一个返回以下内容的列表：

FirstName   LastName    Number of Sales
Nancy       Davolio     29
Andrew      Fuller      20
Janet       Leverling   31
Margaret    Peacock     40
Steven      Buchanan    11
Michael     Suyama      18
Robert      King        14
Laura       Callahan    27
Anne        Dodsworth   6
Adam        West        0

但是，每次，底行（Adam West = 0）都不会返回。

我尝试了以下内容：

SELECT Employees.FirstName, Employees.LastName, COUNT(Employees.EmployeeID) as "Number of Sales"
FROM Employees
WHERE Employees.EmployeeID = Orders.EmployeeID
GROUP BY Orders.EmployeeID

我也尝试在LEFT JOIN Orders行和FROM行之间插入WHERE，但无济于事。

Answer 1

使用ON时，您需要将该子句添加到LEFT JOIN语句中。另外，由于Orders.EmployeeID可能不存在，您需要GROUP BY Employees.EmployeeID。

试试这个：

SELECT Employees.FirstName, Employees.LastName, COUNT(Orders.EmployeeID) as "Number of Sales"
FROM Employees
LEFT JOIN Orders ON Employees.EmployeeID = Orders.EmployeeID
GROUP BY Employees.EmployeeID

Answer 2

请从以下示例中了解：

CREATE TABLE Employees
(
EmployeeID INT,
FirstName VARCHAR(30),
LastName VARCHAR(30)
);

CREATE TABLE Orders
(
orderId INT,
EmployeeID INT
);

/ *表员工的数据* /

INSERT  INTO employees(EmployeeID,FirstName,LastName) VALUES (1,'a','z');
INSERT  INTO employees(EmployeeID,FirstName,LastName) VALUES (2,'b','y');
INSERT  INTO employees(EmployeeID,FirstName,LastName) VALUES (3,'c','x');

/ *表格订单数据* /

INSERT  INTO orders(orderId,EmployeeID) VALUES (101,1);
INSERT  INTO orders(orderId,EmployeeID) VALUES (102,2);
INSERT  INTO orders(orderId,EmployeeID) VALUES (103,1);
INSERT  INTO orders(orderId,EmployeeID) VALUES (104,1);

SELECT Employees.FirstName, Employees.LastName, COUNT(orders.EmployeeID) AS "Number of Sales"
FROM Employees LEFT JOIN Orders 
ON(Employees.EmployeeID = Orders.EmployeeID)
GROUP BY Orders.EmployeeID;

FirstName  LastName  Number of Sales  
c          X         0                
a          z         3                
b          Y         1

此处使用左连接。它将从左表中获取所有行，从右表中获取匹配的条目。这里没有订单表中的条目。

Answer 3

我有同样的问题。这是我的解决方法...

蝙蝠侠卖给名字以辅音结尾的客户有多少订单？

SELECT em.EmployeeID, em.LastName, em.FirstName, COUNT(Customers.CustomerID) as  'number of orders'
FROM (SELECT *
FROM Employees
LEFT JOIN Orders ON Employees.EmployeeID = Orders.EmployeeID
WHERE Employees.EmployeeID = 10) as em
LEFT JOIN Customers ON em.CustomerID = Customers.CustomerID
WHERE Customers.CustomerID is null 
OR (Customers.CustomerName not LIKE '%a'
    AND Customers.CustomerName not LIKE '%e'
    AND Customers.CustomerName not LIKE '%i'
    AND Customers.CustomerName not LIKE '%o'
    AND Customers.CustomerName not LIKE '%u'
    AND Customers.CustomerName not LIKE '%y')

结果：
EmployeeID LastName FirstName订单数
10西亚当0

如何选择所有给定的员工ID，即使是那些在另一个表中没有出现的员工ID？

3 个答案: