PostgreSQL: Replace value of one table by mean value of another table when criteria is met

Imagine the following two CTEs:

table_a shows the number of exempt students and the mean exam score (excluding exempt students) per category, per month.

table_b shows a breakdown of each student information (using the month of January only as an example). Note that output in table_a is obtained through information of table_b.

As exempt students were under mitigating circumstances, their exam scores won't be considered. Instead, I would like to assign to them the mean score of all students that were examined as shown below:

What is the best way of achieving this? I'm struggling to see how to join the tables when for one of the table the timestamp is actually a truncated date as it provides grouped information.

with table_a (exam_month, category, "# exempt students", "mean score (excl. exempt students")
as (values 
('2022-01-01 12:00:00'::timestamp, 'A', 2, 61.8),
('2022-02-01 12:00:00'::timestamp, 'A', 1, 63.1),
('2022-03-01 12:00:00'::timestamp, 'A', 2, 61.1),
('2022-01-01 12:00:00'::timestamp, 'B', 4, 73.8),
('2022-02-01 12:00:00'::timestamp, 'B', 3, 71.3),
('2022-03-01 12:00:00'::timestamp, 'B', 2, 72.5)
),

table_b (student_id, exam_timestamp, category, score, student_status)
as (values
(1, '2022-01-10 09:30:00'::timestamp, 'A', 13.1, 'exempt'),
(2, '2022-01-12 12:00:00'::timestamp, 'A', 63.7, 'examined'),
(3, '2022-01-14 09:00:00'::timestamp, 'A', 59.2, 'examined'),
(4, '2022-01-16 14:30:00'::timestamp, 'A', 62.8, 'examined'),
(5, '2022-01-18 12:30:00'::timestamp, 'A', 41.2, 'exempt'),
(6, '2022-01-20 12:00:00'::timestamp, 'A', 61.5, 'examined')
)

Solution

You can join by using date_trunc() and by casting to date:

with table_a (exam_month, category, "# exempt students", "mean score (excl. exempt students")
as (
 values 
  ('2022-01-01 12:00:00'::timestamp, 'A', 2, 61.8),
  ('2022-02-01 12:00:00'::timestamp, 'A', 1, 63.1),
  ('2022-03-01 12:00:00'::timestamp, 'A', 2, 61.1),
  ('2022-01-01 12:00:00'::timestamp, 'B', 4, 73.8),
  ('2022-02-01 12:00:00'::timestamp, 'B', 3, 71.3),
  ('2022-03-01 12:00:00'::timestamp, 'B', 2, 72.5)
), table_b (student_id, exam_timestamp, category, score, student_status)
as (
 values
  (1, '2022-01-10 09:30:00'::timestamp, 'A', 13.1, 'exempt'),
  (2, '2022-01-12 12:00:00'::timestamp, 'A', 63.7, 'examined'),
  (3, '2022-01-14 09:00:00'::timestamp, 'A', 59.2, 'examined'),
  (4, '2022-01-16 14:30:00'::timestamp, 'A', 62.8, 'examined'),
  (5, '2022-01-18 12:30:00'::timestamp, 'A', 41.2, 'exempt'),
  (6, '2022-01-20 12:00:00'::timestamp, 'A', 61.5, 'examined')
)
select b.student_id, b.exam_timestamp, b.category, 
       case 
         when b.student_status = 'exempt' then a."mean score (excl. exempt students"
         else b.score
       end as score,
       b.student_status
  from table_b b
       join table_a a
         on a.category = b.category
        and date_trunc('month', b.exam_timestamp)::date = a.exam_month::date;

All that said, you do not need table_a for this calculation:

with table_b (student_id, exam_timestamp, category, score, student_status)
as (
 values
  (1, '2022-01-10 09:30:00'::timestamp, 'A', 13.1, 'exempt'),
  (2, '2022-01-12 12:00:00'::timestamp, 'A', 63.7, 'examined'),
  (3, '2022-01-14 09:00:00'::timestamp, 'A', 59.2, 'examined'),
  (4, '2022-01-16 14:30:00'::timestamp, 'A', 62.8, 'examined'),
  (5, '2022-01-18 12:30:00'::timestamp, 'A', 41.2, 'exempt'),
  (6, '2022-01-20 12:00:00'::timestamp, 'A', 61.5, 'examined')
)
select b.student_id, b.exam_timestamp, b.category, 
       (case 
         when b.student_status = 'exempt' 
           then sum(case
                      when b.student_status = 'exempt' then 0
                      else b.score
                    end) over w
                / 
                sum((b.student_status = 'examined')::int) over w
         else b.score
       end)::numeric(4,1) as score,
       b.student_status
  from table_b b
window w as (partition by date_trunc('month', b.exam_timestamp), category)
;

db<>fiddle here