579_Find Cumulative Salary of an Employee

The Employee table holds the salary information in a year.

Write a SQL to get the cumulative sum of an employee's salary over a period of 3 months but exclude the most recent month.

The result should be displayed by 'Id' ascending, and then by 'Month' descending.

Example Input

| Id | Month | Salary |
|----|-------|--------|
| 1  | 1     | 20     |
| 2  | 1     | 20     |
| 1  | 2     | 30     |
| 2  | 2     | 30     |
| 3  | 2     | 40     |
| 1  | 3     | 40     |
| 3  | 3     | 60     |
| 1  | 4     | 60     |
| 3  | 4     | 70     |

Output

| Id | Month | Salary |
|----|-------|--------|
| 1  | 3     | 90     |
| 1  | 2     | 50     |
| 1  | 1     | 20     |
| 2  | 1     | 20     |
| 3  | 3     | 100    |
| 3  | 2     | 40     |

Explanation

Employee '1' has 3 salary records for the following 3 months except the most recent month '4': salary 40 for month '3', 30 for month '2' and 20 for month '1' So the cumulative sum of salary of this employee over 3 months is 90(40+30+20), 50(30+20) and 20 respectively.

| Id | Month | Salary |
|----|-------|--------|
| 1  | 3     | 90     |
| 1  | 2     | 50     |
| 1  | 1     | 20     |

Employee '2' only has one salary record (month '1') except its most recent month '2'.

| Id | Month | Salary |
|----|-------|--------|
| 2  | 1     | 20     |

Employ '3' has two salary records except its most recent pay month '4': month '3' with 60 and month '2' with 40. So the cumulative salary is as following.

| Id | Month | Salary |
|----|-------|--------|
| 3  | 3     | 100    |
| 3  | 2     | 40     |

Solution

SELECT 
    E0.Id, 
    E1.Month, 
    IFNULL(E1.Salary, 0) + IFNULL(E2.Salary, 0) + IFNULL(E3.Salary, 0) AS Salary
FROM 
    (SELECT Id, MAX(Month) AS Month
    FROM Employee
    GROUP BY Id
    HAVING COUNT(Month) > 1) AS E0
    LEFT JOIN Employee AS E1 ON (E0.Id = E1.Id AND E0.Month > E1.Month)
    LEFT JOIN Employee AS E2 ON (E1.Id = E2.Id AND E1.Month - E2.Month = 1)
    LEFT JOIN Employee AS E3 ON (E2.Id = E3.Id AND E2.Month - E3.Month = 1)
ORDER BY Id, Month DESC