CREATE TABLE departments (department_name VARCHAR, department_id VARCHAR)
CREATE TABLE employees (department_id VARCHAR)

find the name of all departments that do actually have one or more employees assigned to them.
SELECT DISTINCT T2.department_name FROM employees AS T1 JOIN departments AS T2 ON T1.department_id = T2.department_id
SELECT DISTINCT "T2"."department_name" FROM "employees" AS "T1" JOIN "departments" AS "T2" ON "T1"."department_id" = "T2"."department_id"