2020-04-10-（2）

分组比较

Employee 表包含所有员工信息，每个员工有其对应的 Id, salary 和 department Id。

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问题：
编写一个 SQL 查询，找出每个部门工资最高的员工。例如，根据上述给定的表格，Max 在 IT 部门有最高工资，Henry 在 Sales 部门有最高工资。

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Solution

SELECT DepartmentId, max( Salary ) 
FROM Employee 
GROUP BY DepartmentId 

输出：分部门选出最高
{"headers": ["DepartmentId", "max( Salary )"], 
   "values": [[1, 90000], [2, 80000]]}

Select Department.Name As DepartmentId, Employee.Name As Employee, Salary
From Employee,Department
Where Employee.DepartmentId = Department.Id 

输出：内连接
{"headers": ["DepartmentId", "Employee", "Salary"], 
"values": [["IT", "Joe", 70000], 
                ["IT", "Jim", 90000], 
                ["Sales", "Henry", 80000], 
                ["Sales", "Sam", 60000],
                ["IT", "Max", 90000]]}

子查询（*And ID，Salary in*）
Select Department.Name As Department, Employee.Name As Employee, Salary
From Employee,Department
Where Employee.DepartmentId = Department.Id 
And (Employee.DepartmentId, Salary) IN (
    Select DepartmentId, max( Salary ) 
    From Employee 
    Group By DepartmentId)

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