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Formatted question description: https://leetcode.ca/all/184.html

Table: Employee

+--------------+---------+
| Column Name  | Type    |
+--------------+---------+
| id           | int     |
| name         | varchar |
| salary       | int     |
| departmentId | int     |
+--------------+---------+
id is the primary key column for this table.
departmentId is a foreign key of the ID from the Department table.
Each row of this table indicates the ID, name, and salary of an employee. It also contains the ID of their department.

 

Table: Department

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| id          | int     |
| name        | varchar |
+-------------+---------+
id is the primary key column for this table. It is guaranteed that department name is not NULL.
Each row of this table indicates the ID of a department and its name.

 

Write an SQL query to find employees who have the highest salary in each of the departments.

Return the result table in any order.

The query result format is in the following example.

 

Example 1:

Input: 
Employee table:
+----+-------+--------+--------------+
| id | name  | salary | departmentId |
+----+-------+--------+--------------+
| 1  | Joe   | 70000  | 1            |
| 2  | Jim   | 90000  | 1            |
| 3  | Henry | 80000  | 2            |
| 4  | Sam   | 60000  | 2            |
| 5  | Max   | 90000  | 1            |
+----+-------+--------+--------------+
Department table:
+----+-------+
| id | name  |
+----+-------+
| 1  | IT    |
| 2  | Sales |
+----+-------+
Output: 
+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT         | Jim      | 90000  |
| Sales      | Henry    | 80000  |
| IT         | Max      | 90000  |
+------------+----------+--------+
Explanation: Max and Jim both have the highest salary in the IT department and Henry has the highest salary in the Sales department.

Algorithm

Join the two tables together, and then mark all the columns required in the result table, and then find the highest salary, using the Max keyword.

A pair (a,b) in ( block returning tuple)

Code

SQL

SELECT
    Department.name AS 'Department',
    Employee.name AS 'Employee',
    Employee.Salary
FROM
    Employee
JOIN -- default is inner join
    Department
ON Employee.DepartmentId = Department.Id
WHERE
    (Employee.DepartmentId , Employee.Salary) IN -- so that if tie, both will appear in result
    (
        SELECT
            DepartmentId, MAX(Salary)
        FROM
            Employee
        GROUP BY DepartmentId
    )
;

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