LEFT JOIN count from another table - mysql

Question

I am trying to count how many transactions have been complete for a course, I am trying to left join the training_transactions with a count of all the rows where the training_transaction_course = course_id and where training_transaction_status = 'completed' Here's the code I have so far:
SELECT training.*,
Count(DISTINCT training_transactions.training_transaction_course) AS completed_training_payments
left JOIN users
ON training.course_user = users.user_id
LEFT JOIN training_transactions
ON training.course_user = training_transactions.training_transaction_user
FROM training
WHERE course_id = ?
AND training_transactions.training_transaction_status = 'complete'
AND course_enabled = 'enabled'
My tables:
training transactions
CREATE TABLE IF NOT EXISTS `training_transactions` (
`training_transaction_id` int(11) NOT NULL,
`training_transaction_user` int(11) NOT NULL,
`training_transaction_course` int(11) NOT NULL,
`training_transaction_status` varchar(50) NOT NULL,
`training_transaction_enabled` varchar(50) NOT NULL DEFAULT 'enabled',
`training_transaction_date` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
training
CREATE TABLE IF NOT EXISTS `training` (
`course_id` int(11) NOT NULL,
`course_user` int(11) NOT NULL,
`course_type` varchar(255) NOT NULL,
`course_name` varchar(255) NOT NULL,
`course_location` varchar(255) NOT NULL,
`course_duration` varchar(255) NOT NULL,
`course_fitness_type` varchar(255) NOT NULL,
`course_instructor_name` varchar(255) NOT NULL,
`course_price` int(15) NOT NULL,
`course_start_date` date NOT NULL,
`course_max_attendees` int(8) NOT NULL,
`course_accommodation` varchar(255) NOT NULL,
`course_accommodation_price` varchar(255) NOT NULL,
`course_status` varchar(50) NOT NULL,
`course_enabled` varchar(10) NOT NULL DEFAULT 'enabled'
) ENGINE=InnoDB AUTO_INCREMENT=24 DEFAULT CHARSET=latin1;
As you can see I am trying to get the count of completed transactions as a count to deduct from course_max_attendees, and then I can check if there's any places left.

Answer 1

You want to select trainings. So select from training. You want to show the transaction count with it, which you can do in a subquery in your select clause:
select
t.*,
(
select count(*)
from training_transactions tt
where tt.training_transaction_user = t.course_user
and tt.training_transaction_status = 'complete'
) as completed_training_payments
from training t
where t.course_id = ?
and t.course_enabled = 'enabled';
And here is the same with a join:
select
t.*, coalesce(tt.cnt, 0) as completed_training_payments
from training t
left join
(
select training_transaction_status, count(*) as cnt
from training_transactions
where training_transaction_status = 'complete'
group by training_transaction_status
) tt on tt.training_transaction_user = t.course_user
where t.course_id = ?
and t.course_enabled = 'enabled';

Answer 2

First, if you want to know how many completed transactions have taken place for each course, you can't get the User table involved. You will be aggregating away any user information.
Then, you must start with the course table, which looks like you have named training. Now you want to count every completed transaction for each course. A left join works just about perfectly for this:
select t.Name, count( * ) as completed_training_payments
from training t
left join training_transactions tt
on tt.user = t.course_user
and tt.status = 'complete'
where t.course_status = 'enabled'
group by t.Name;
The problem with this is that it will give a count value of "1" for every course with one completed transaction but also those with no completed transactions at all! So every row with a count of "1" would be suspect. The solution is to count keys, not rows. This is done with the sum rather than the count function.
select t.Name, sum( case when tt.course_user is null then 0 else 1 end ) as completed_training_payments
from training t
left join training_transactions tt
on tt.user = t.course_user
and tt.status = 'complete'
where t.course_status = 'enabled'
group by t.Name;
Since tt.course_user will only be null when there are no completed transactions at all, that course will show a "count" of "0".