Untitled

Display null for column if no row found
SELECT s.first_name, s.last_name, tgs.group_identifier
FROM staff s
JOIN staff_training st
ON s.staff_id = st.staff_id
JOIN training_group_staff tgs
ON st.staff_training_id = tgs.staff_training_id
WHERE st.staff_course_id = '164'

first_name  |  last_name  |  group_identifier
 Jim        |   Jones     |   3
 Harry      |   Jones     |   6

first_name  |  last_name  |  group_identifier
 Jim        |   Jones     |   3
 Harriet    |   Smith     |   NULL   // No row in training_group_staff
 Harry      |   Jones     |   6

SELECT s.first_name, s.last_name, tgs.group_identifier
FROM staff s
JOIN staff_training st
JOIN ON s.staff_id = st.staff_id
LEFT OUTER JOIN training_group_staff tgs
ON st.staff_training_id = tgs.staff_training_id
WHERE st.staff_course_id = '164'