Complex sql quary
===

There are 2 tables:
- employees
- groups

Employee
(id_employee, worker_name)
1, 'worker 1'
2, 'worker 2'
3, 'worker 3'
4, 'worker 4'

Groups
(id_group, id_employee, group_name)
1, 1, 'a1'
2, 2, 'a1'
3, 3, 'a2'
4, 4, 'a2'
5, 1, 'b1'
6, 2, 'b1'
7, 3, 'b2'
8, 4, 'b2'


I need a SELECT that for given id_group
will return all groups that have exactly the same employees

for example:
SELECT for 'a1'
-> should return 'b1'
because in both groups there are: worker 1 & worker 2

SELECT for 'a2'
-> should return 'b2'
because in both groups there are: worker 3 & worker 4

Note that---
Groups need to be exactly the same, all members of a1 need to be in b1 and size of both groups need to be the same as well.