How to remove count values "0" from result table? - sql

Question

I'm trying to write a query that returns a table with columns name and numberOfClasses. This table includes the name of all students and the amount of classes the student follows. I use the following database tables:
╔══════════════╦═══════════╦══════════╗
║ TakesClasses ║ ║ ║
╠══════════════╬═══════════╬══════════╣
║ id ║ person_id ║ class_id ║
║ 99 ║ 1 ║ 40 ║
║ 98 ║ 1 ║ 41 ║
║ 97 ║ 1 ║ 42 ║
║ 96 ║ 1 ║ 43 ║
║ 95 ║ 2 ║ 44 ║
║ 94 ║ 2 ║ 45 ║
║ 93 ║ 2 ║ 46 ║
╚══════════════╩═══════════╩══════════╝
╔═════════╦═══════╦══╗
║ Persons ║ ║ ║
╠═════════╬═══════╬══╣
║ id ║ name ║ ║
║ 1 ║ Bart ║ ║
║ 2 ║ David ║ ║
║ 3 ║ Dani ║ ║
║ 4 ║ Erik ║ ║
╚═════════╩═══════╩══╝
I used the following query:
SELECT
name,
COUNT(T.person_id) AS numberOfClasses
FROM
Persons P
LEFT OUTER JOIN
TakesClasses T ON P.id = T.person_id
GROUP BY
P.id, P.name
And this is the output:
╔═══════╦═════════════════╗
║ name ║ numberOfClasses ║
╠═══════╬═════════════════╣
║ Bart ║ 4 ║
║ Dani ║ 3 ║
║ David ║ 0 ║
║ Erik ║ 0 ║
╚═══════╩═════════════════╝
How can I remove the entries with 0 from my the results table?
Thanks for the help in advance!

Answer 1

Just use JOIN instead of LEFT JOIN
select name, count(T.person_id) as amountOfClasses
from Persons P join
TakesClasses T
on P.id = T.person_id
group by P.id, P.name

Answer 2

If you want to filter on aggregated data like what comes out of GROUP BY you can use the HAVING clause
SELECT name,COUNT(T.person_id) as amountOfClasses
FROM Persons P LEFT OUTER JOIN TakesClasses T
on P.id = T.person_id
GROUP BY P.id, P.name
HAVING COUNT(T.person_id) > 0;
sqlFiddle: http://sqlfiddle.com/#!9/1108c5/8