Result of query as column value - sql

Question

I've got three tables:
Lessons:
CREATE TABLE lessons (
id SERIAL PRIMARY KEY,
title text NOT NULL,
description text NOT NULL,
vocab_count integer NOT NULL
);
+----+------------+------------------+-------------+
| id | title | description | vocab_count |
+----+------------+------------------+-------------+
| 1 | lesson_one | this is a lesson | 3 |
| 2 | lesson_two | another lesson | 2 |
+----+------------+------------------+-------------+
Lesson_vocabulary:
CREATE TABLE lesson_vocabulary (
lesson_id integer REFERENCES lessons(id),
vocabulary_id integer REFERENCES vocabulary(id)
);
+-----------+---------------+
| lesson_id | vocabulary_id |
+-----------+---------------+
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 2 |
| 2 | 4 |
+-----------+---------------+
Vocabulary:
CREATE TABLE vocabulary (
id integer PRIMARY KEY,
hiragana text NOT NULL,
reading text NOT NULL,
meaning text[] NOT NULL
);
Each lesson contains multiple vocabulary, and each vocabulary can be included in multiple lessons.
How can I get the vocab_count column of the lessons table to be calculated and updated whenevr I add more rows to the lesson_vocabulary table. Is this possible, and how would I go about doing this?
Thanks

Answer 1

You can use SQL triggers to serve your purpose. This would be similar to mysql after insert trigger which updates another table's column.
The trigger would look somewhat like this. I am using Oracle SQL, but there would just be minor tweaks for any other implementation.
CREATE TRIGGER vocab_trigger
AFTER INSERT ON lesson_vocabulary
FOR EACH ROW
begin
for lesson_cur in (select LESSON_ID, COUNT(VOCABULARY_ID) voc_cnt from LESSON_VOCABULARY group by LESSON_ID) LOOP
update LESSONS
set VOCAB_COUNT = LESSON_CUR.VOC_CNT
where id = LESSON_CUR.LESSON_ID;
end loop;
END;

Answer 2

It's better to create a view that calculates that (and get rid of the column in the lessons table):
select l.*, lv.vocab_count
from lessons l
left join (
select lesson_id, count(*)
from lesson_vocabulary
group by lesson_id
) as lv(lesson_id, vocab_count) on l.id = lv.lesson_id
If you really want to update the lessons table each time the lesson_vocabulary changes, you can run an UPDATE statement like this in a trigger:
update lessons l
set vocab_count = t.cnt
from (
select lesson_id, count(*) as cnt
from lesson_vocabulary
group by lesson_id
) t
where t.lesson_id = l.id;

Answer 3

I would recommend using a query for this information:
select l.*,
(select count(*)
from lesson_vocabulary lv
where lv.lesson_id = l.lesson_id
) as vocabulary_cnt
from lessons l;
With an index on lesson_vocabulary(lesson_id), this should be quite fast.
I recommend this over an update, because the data remains correct.
I recommend this over a trigger, because it is simpler.
I recommend this over a subquery with aggregation because it should be faster, particularly if you are filtering on the lessons table.