Dont you think, this is a very common requirement for most of the Database Developer.

Yes, MySQL Developer can visit this article.

Find top N records for each group in MySQL

In PostgreSQL, We can get a first record for each GROUP using different options like:

• Using DISTINCT ON
• Using LATERAL
• CTE with ROW_NUMBER()
• CTE with LATERAL
• Subquery with ROW_NUMBER()
• Using array_agg()

In this demonstration, I am going to give two optimized solutions to get one record per each GROUP.

Create a table with Sample records:

CREATE TABLE tbl_Employees
(
	EmpID INT
	,EmpName CHARACTER VARYING 	
	,EmpDepartment CHARACTER VARYING
	,EmpSalary INT
);

INSERT INTO tbl_Employees
VALUES 
(1,'Anvesh','Database',90000)
,(2,'Jenny','JAVA',65000)
,(3,'Martin','PHP',85000)
,(4,'Roy','PHP',94000)
,(5,'Eric','PHP',70000)
,(6,'Rajesh','Animation',50000)
,(7,'Mahi','Database',40000)
,(8,'Sofia','JAVA',60000);

First Solution using DISTINCT ON:
As per the Postgres official document,

The DISTINCT ON expression(s) must match the leftmost ORDER BY expression(s).
The ORDER BY clause will normally contain additional expression(s) that determine the desired precedence of rows within each DISTINCT ON group.

SELECT DISTINCT ON (EmpDepartment) *       
FROM   tbl_Employees
ORDER  BY EmpDepartment
	,EmpSalary DESC;

SELECT DISTINCT T.*
FROM   tbl_Employees e
,LATERAL 
(
   SELECT *
   FROM   tbl_Employees
   WHERE  EmpDepartment = e.EmpDepartment
   ORDER  BY EmpSalary DESC
   LIMIT  1
) AS T;

The Result of both the solution :

empid | empname | empdepartment | empsalary
-------+---------+---------------+-----------
     6 | Rajesh  | Animation     |     50000
     1 | Anvesh  | Database      |     90000
     2 | Jenny   | JAVA          |     65000
     4 | Roy     | PHP           |     94000