Get overlaped time range with conditions
key | start |  end  | employee(fk)
 1    6:00    8:00     A
 2    9:00    11:00    B
 3    7:00    9:00     B
 4    7:00    11:00    C

given {A,B}   returns 2 columns 7:00  |  8:00  time range which overlaped by A and B
given {B,C}  returns  2 columns 9:00  |  11:00  time range which overlaped by B and C

SELECT T1.employee, T2.employee, T1.start, T2.end
FROM Table T1, Table T2
WHERE T1.key!=T2.key AND T1.employee!=T2.employee
      AND T1.start <= T2.end AND T1.start >= T2.start