简单的2张表演示

1.左连接

SELECT CustomerPro.ID,CustomerPro.CustomerName,Employee.EmployeeName
FROM CustomerPro
LEFT JOIN Employee 
on CustomerPro.EmployeeID=Employee.ID

2.右连接

SELECT CustomerPro.ID,CustomerPro.CustomerName,Employee.EmployeeName
FROM CustomerPro
RIGHT  JOIN Employee 
on CustomerPro.EmployeeID=Employee.ID

3.内连接

SELECT CustomerPro.ID,CustomerPro.CustomerName,Employee.EmployeeName
FROM CustomerPro,Employee 
WHERE CustomerPro.EmployeeID=Employee.ID
这2种其实都是内连接的写法
SELECT CustomerPro.ID,CustomerPro.CustomerName,Employee.EmployeeName
FROM CustomerPro
inner JOIN Employee
on CustomerPro.EmployeeID=Employee.ID