SQL query to print the name of the employee getting max % hike in the annual salary from 2018 to 2019

Question

I am very much new to SQL and have been practicing straight forward queries since a few days. I cam across this question which I would request help for. So there is a table with employee name, salary and salary date as columns. Date is simply Year from 2015 to 2019. I am trying to write a query where I can get employee name with the maximum % hike in salary from 2018 to 2019. I wrote the below query but stuck for hours at the same.

    CREATE TABLE data (
      salary INTEGER NOT NULL,
      emp_name TEXT NOT NULL,
      Sal_Date YEAR NOT NULL
    );
    

INSERT INTO data VALUES (10000, 'Ryan', 2015);
INSERT INTO data VALUES (12000, 'Bryan', 2016);
INSERT INTO data VALUES (11000, 'Manthan', 2016);
INSERT INTO data VALUES (15000, 'Susan', 2017);
INSERT INTO data VALUES (16000, 'Alien', 2017);
INSERT INTO data VALUES (10000, 'Ryan', 2018);
INSERT INTO data VALUES (12000, 'Bryan', 2018);
INSERT INTO data VALUES (11000, 'Manthan', 2018);
INSERT INTO data VALUES (15000, 'Susan', 2018);
INSERT INTO data VALUES (16000, 'Alien', 2018);
INSERT INTO data VALUES (11000, 'Ryan', 2019);
INSERT INTO data VALUES (13000, 'Bryan', 2019);
INSERT INTO data VALUES (15000, 'Manthan', 2019);
INSERT INTO data VALUES (18000, 'Susan', 2019);
INSERT INTO data VALUES (32000, 'Alien', 2019);

SELECT salary  from data
    group by  ;

I am getting no logic to solve this. Can anybody please help with the query and logic explanation. I'll be grateful. Thanks

Answer 1

SELECT * , (salary - LAG(salary,1,salary) over (partition by emp_name order by sal_date)) /salary * 100 as promotionPercentage
from data 
order by emp_name , sal_date

db<>fiddle here

Answer 2

You can use a Self-join to join a table to itself and return the salary for the previous year, then order by the % change in salary and return the first one

SELECT TOP 1 t1.emp_name
FROM data t1
JOIN data t2 ON t1.emp_name = t2.emp_name AND t1.Sal_date = t2.Sal_Date + 1
WHERE t1.Sal_Date = 2019
ORDER BY (t1.Sal_Date - t2.Sal_Date) / t2.Sal_Date * 100

Alternatively, you can use the LAG analytic function but I am not sure it is present in all DBMS? I am using MSSQL for this one

Answer 3

You can self-join the table with the condition that:

• same emp_name
• the first table is for 2018, the other one 2019

Then, you can compare the salary in 2018 and 2019 in the same row. Here is a query example. You may need to modify a bit depending on your SQL engine.

SELECT
  d18.emp_name,
  cast(d19.salary AS FLOAT) / d18.salary - 1 AS hike_18_to_19
FROM
  data d18 
INNER JOIN
  data d19
ON
  d18.emp_name = d19.emp_name
  AND d18.Sal_Date = 2018
  AND d19.Sal_Date = 2019
ORDER BY 
  hike_18_to_19 DESC

You get something like this:

  emp_name  hike_18_to_19
0    Alien       1.000000
1  Manthan       0.363636
2    Susan       0.200000
3     Ryan       0.100000
4    Bryan       0.083333