Limiting results in SQL query

Question

I was going trough ways to limit results in SQL and I have come across such a way:

SELECT name
FROM employees e
WHERE 2>=( SELECT COUNT(*) FROM employees e1 WHERE e1.birthdate>e.birthdate);

This query returns three youngest employees but I don't quite get how this query works. How come it is 2>= and not 2<=? Could anyone please shed the light on what this query does? Thank you!

Answer 1

Understand it with example data :

employeeId birthdate
1          12/29/2014
2          11/20/2014
3          01/01/2015
4          11/19/1991

now, evaluate the subquery for each :

employeeId birthdate   subquery result
1          12/29/2014  how many employees have birth date > 12/29/2014 = 1
2          11/20/2014  how many employees have birth date > 11/20/2014 = 2
3          01/01/2015  how many employees have birth date > 01/01/2015 = 0
4          11/19/1991  how many employees have birth date > 11/19/1991 = 3

now, apply the criteria 2>= subquery result. It is certain that it will return empid 1,2,3 not 4.

Note : just for information that there are other approaches to achieve this efficiently.