Will this work?
SELECT employee_id FROM employees
WHERE company_id=5
and employee_id IN (2,3,4,5)
and exists (select * from employees where company_id = 5 and employee_id IN (2,3,4,5))
union all
SELECT employee_id FROM employees
WHERE company_id=5
and Not exists (select * from employees where company_id = 5 and employee_id IN (2,3,4,5))
Only one of the exists/not exists can fire. So this will do one select or the other, but without the use of If or Case statements. The first exists is probably unnecessary... so
SELECT employee_id FROM employees
WHERE company_id=5
and employee_id IN (2,3,4,5)
union all
SELECT employee_id FROM employees
WHERE company_id=5
and Not exists (select * from employees where company_id = 5 and employee_id IN (2,3,4,5))