select count items each group - mysql

Question

I have two tables category and adverts, i need to select all categories and the number of adverts it has in the adverts table that has at least count greater than zero
the category table
cat_id | Name
----------------
1 | Toys
2 | fashion
3 | electronics
The adverts table
cat_id | title
----------------
1 | a
2 | b
2 | c
1 | d
2 | e
what i expect
cat_id | count | Name
-----------------------
1 |2 | a
2 |3 | b
The query i tried
Select
c.name, c.cat_id,c.parent_id, #count:= (Select Count(av.cat_id) From adsview av Where av.cat_id = c.cat_id)
from
category c WHERE #count > 0
i am getting and empty result, what am i doing wrong?

Answer 1

If you want to make sure that the cat_id from category table are in adverts table you need to join as
select
c.cat_id,
c.Name,
count(a.cat_id) as `count`
from category c
join adverts a on a.cat_id = c.cat_id
group by c.cat_id ;

Answer 2

select cat_id, count(*)
from adverts
group by cat_id;
So the mySQL query engine will grab every single row from the adverts table, it'll put them into neat piles where all rows in the pile have the same category, it'll count the number of rows in each pile, and then it'll return to you a result row for each pile with the pile's id and the number of rows.
Now lets add something: we want to also get the category's name. So we indicate that in the select clause, and add a join to the from clause. The join says "for every row in table a, consider it alongside every row in table b. if some condition holds, put this combined row into the from set". You can see that joins are actually quite slow in SQL (relatively).
select c.cat_id, count(*) as count, c.name
from adverts as a join categories as c on a.cat_id = c.cat_id
group by c.cat_id;
Note also that I've aliased the tables as a and c respectively, so as to remove the ambiguity over the column name cat_id (otherwise the mySQL query engine may get confused).

Answer 3

You can try this, mate:
SELECT
c.cat_id,
COUNT(a.cat_id) AS count,
a.title
FROM
category c
LEFT JOIN adverts a ON a.cat_id = c.cat_id
GROUP BY
c.cat_id
HAVING
count > 0;
or this:
SELECT
c.cat_id,
COUNT(a.cat_id) AS count,
a.title
FROM
category c
INNER JOIN adverts a ON a.cat_id = c.cat_id
GROUP BY
c.cat_id;

Answer 4

You have to use group by function like below
select cat_id, count(*) as count
from adverts
group by cat_id;