Calculate change in value window function when the identifier has dropped of the current period I was wondering how can i got about finding the change in value from one period to another but in my scenario the identifier has dropped off the current period.

I'm think this could be done with a cte and a select distinct id but i'm not sure how to implement.

Some demo date below

CREATE TABLE basic_pays (
    employee_id int,
    fiscal_year INT,
    salary DECIMAL(10 , 2 ),
    PRIMARY KEY (employee_id, fiscal_year)
);

INSERT INTO basic_pays(employee_id, fiscal_year,salary) VALUES(1,2017,24000);
INSERT INTO basic_pays(employee_id, fiscal_year,salary) VALUES(2,2017,17000);
INSERT INTO basic_pays(employee_id, fiscal_year,salary) VALUES(1,2018,25920);

SELECT 
    employee_id, 
    fiscal_year,
    salary,
    salary - LAG(salary) 
        OVER (PARTITION BY employee_id ORDER BY fiscal_year) salary_change
FROM
    basic_pays
GROUP BY 
    employee_id, fiscal_year, salary
ORDER BY fiscal_year, employee_id

DROP TABLE basic_pays


The expected output would be 

| employee_id | fiscal_year | salary   | salary_change |
|-------------|-------------|----------|---------------|
| 1           | 2017        | 24000.00 | NULL          |
| 2           | 2017        | 17000.00 | NULL          |
| 1           | 2018        | 25920.00 | 1920.00       |
| 2           | 2018        | 0.00     | -17000.00     |
You could do it like this, performance wise this could hurt on bigger datasets.

;WITH CTE
AS
(
SELECT DISTINCT employee_id,a.fiscal_year
from basic_pays
CROSS APPLY
(
SELECT distinct fiscal_year 
from basic_pays
) as a)


SELECT   c.employee_id, 
    c.fiscal_year,
    CASE WHEN salary IS NULL then 0.00 else salary END as salary,
    CASE WHEN salary IS NULL then 0.00 else salary END - LAG(salary) 
        OVER (PARTITION BY c.employee_id ORDER BY c.fiscal_year) salary_change
FROM
    CTE c
LEFT JOIN basic_pays b
ON C.employee_id = b.employee_id
AND c.fiscal_year= b.fiscal_year;


Result

employee_id fiscal_year    salary       salary_change
1             2017        24000.00      NULL
1             2018        25920.00      1920.00
2             2017        17000.00      NULL
2             2018        NULL          -17000.00