Two tables:
table_a
-------
table_a_id: (primary, int)
table_b
-------
table_a_id: (index, int, from table_a)
table_b_value: (varchar)
A one to many relationship between table_a_id and table_b_value.
Given a query like this:
SELECT DISTINCT(table_a_id) FROM table_a
JOIN table_b ON table_a.table_a_id=table_b.table_a_id
I want to order by the number of occurrences of table_a_id in table_b. I'm not really sure how to write this in MySQL.
SELECT COUNT(table_b.table_a_id) AS count
FROM table_a
JOIN table_b ON table_b.table_a_id = table_a.id
GROUP BY table_b.table_a_id
ORDER_BY count DESC
Why do you need to join the tables at all in this case?
SELECT table_a_id FROM table_b GROUP BY table_a_id order by count(table_a_id) desc
SELECT a.table_a_id, COUNT(b.table_a_id) AS a_count FROM table_a AS a LEFT JOIN table_b AS b ON a.table_a_id = b.table_a_id GROUP BY a.table_a_id ORDER BY a_count
SELECT table_a_id, COUNT(*)
FROM table_b
GROUP BY table_a_id
If you want a table like:
post_id comments_count
1 20
2 125
3 43
If you might need other information from the A table, you can:
select p.id, count(c.*) comments_count
from posts p
join comments c on c.post_id = p.id
group by p.id
If you don't need anything from the A table, you can simplify:
select post_id, count(*)
from comments
group by post_id