SQL implementation LeetCode (185. Top three highest salaries in the department)

[LeetCode] 185. Department Top Three Salaries

The Employee table holds all employees. Every employee has an Id, and there is also a column for the department Id.

+----+-------+--------+--------------+
| Id | Name | Salary | DepartmentId |
+----+-------+--------+--------------+
| 1 | Joe | 70000 | 1 |
| 2 | Henry | 80000 | 2 |
| 3 | Sam | 60000 | 2 |
| 4 | Max | 90000 | 1 |
| 5 | Janet | 69000 | 1 |
| 6 | Randy | 85000 | 1 |
+----+-------+--------+--------------+

The Department table holds all departments of the company.

+----+----------+
| Id | Name |
+----+----------+
| 1 | IT |
| 2 | Sales |
+----+----------+

Write a SQL query to find employees who earn the top three salaries in each of the department. For the above tables, your SQL query should return the following rows.

+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT | Max | 90000 |
| IT | Randy | 85000 |
| IT | Joe | 70000 |
| Sales | Henry | 80000 |
| Sales | Sam | 60000 |
+------------+----------+--------+

This question is an extension of the previous Department Highest Salary question. It is marked as Hard and is quite difficult. It combines the knowledge points of many previous questions. First, let's look at the method of using Select Count (Distinct). We merge the two tables Employee and Department, and then find out that there are only two salaries higher than the current salary. Then the top three highest salaries can be retrieved. See the code below:

Solution 1:

SELECT d.Name AS Department, e.Name AS Employee, e.Salary FROM Employee e
JOIN Department d on e.DepartmentId = d.Id
WHERE (SELECT COUNT(DISTINCT Salary) FROM Employee WHERE Salary > e.Salary
AND DepartmentId = d.Id) < 3 ORDER BY d.Name, e.Salary DESC;

The following method replaces <3 in the above method with IN (0, 1, 2), which has the same effect:

Solution 2:

SELECT d.Name AS Department, e.Name AS Employee, e.Salary FROM Employee e, Department d
WHERE (SELECT COUNT(DISTINCT Salary) FROM Employee WHERE Salary > e.Salary
AND DepartmentId = d.Id) IN (0, 1, 2) AND e.DepartmentId = d.Id ORDER BY d.Name, e.Salary DESC;

Or we can also use Group by Having Count(Distinct ..) keyword to do it:

Solution 3:

SELECT d.Name AS Department, e.Name AS Employee, e.Salary FROM 
(SELECT e1.Name, e1.Salary, e1.DepartmentId FROM Employee e1 JOIN Employee e2 
ON e1.DepartmentId = e2.DepartmentId AND e1.Salary <= e2.Salary GROUP BY e1.Id 
HAVING COUNT(DISTINCT e2.Salary) <= 3) e JOIN Department d ON e.DepartmentId = d.Id 
ORDER BY d.Name, e.Salary DESC;

The following method is slightly more complicated and uses variables. It is the same as the method used in Solution 3 in Consecutive Numbers. The purpose is to add a rank to each person according to the salary level, and finally return the items with a rank value less than or equal to 3. See the code below:

Solution 4:

SELECT d.Name AS Department, e.Name AS Employee, e.Salary FROM 
(SELECT Name, Salary, DepartmentId,
@rank := IF(@pre_d = DepartmentId, @rank + (@pre_s <> Salary), 1) AS rank,
@pre_d := DepartmentId, @pre_s := Salary 
FROM Employee, (SELECT @pre_d := -1, @pre_s := -1, @rank := 1) AS init
ORDER BY DepartmentId, Salary DESC) e JOIN Department d ON e.DepartmentId = d.Id
WHERE e.rank <= 3 ORDER BY d.Name, e.Salary DESC;

Similar topics:

Department Highest Salary

Second Highest Salary

Combine Two Tables

References:

https://leetcode.com/discuss/23002/my-tidy-solution

https://leetcode.com/discuss/91087/yet-another-solution-using-having-count-distinct

https://leetcode.com/discuss/69880/two-solutions-1-count-join-2-three-variables-join

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