Find the names of nurses who are on call.
SELECT DISTINCT T1.name FROM nurse AS T1 JOIN on_call AS T2 ON T1.EmployeeID  =  T2.nurse
names of nurses who are on call refers to joining the nurse table with the on_call table using the nurse's EmployeeID matching the nurse field in the on_call table; DISTINCT is used to ensure each nurse's name is listed once.