PHP, Mysql sub query with where clause - php

Question

I have a query that display all my CLASS details, now I want a sub query that will count all enrolled student in that class.
I tried using two separate query and trying to merge them but I'm not successful doing so.
This is my query so far:
$str = "SELECT
class.id,
class.code AS classcode,
section.name AS sectionname,
subject.code,
class.units,
sched.name AS schedule,
class.slots,
class.dissolved,
(SELECT
Count(enrolldet.enrollno)
FROM
enrolldet
Inner Join enroll ON enrolldet.enrollno = enroll.enrollno
Inner Join class ON enrolldet.class = class.id
WHERE
enroll.validated = '1' AND
class.id = class.id) AS enrolled
FROM
class
Left Join sched ON class.sched = sched.id
Left Join section ON class.section = section.id
Left Join subject ON class.subject = subject.id
";

Answer 1

You don't need all those joins in the subquery. This should suffice:
(SELECT Count(*)
FROM enrolldet ed JOIN
enroll e
ON ed.enrollno = e.enrollno
WHERE e.validated = 1 AND
ed.class = c.id
) AS enrolled
That is, you don't need class in the subquery. And, I'm guessing that validated is a number so I removed the single quotes around "1".

Answer 2

You can't have the class table of both the main query and the subquery named class, because the subquery has access to both tables and this results in the SQL engine being confused due to the ambiguity.
To fix your query, give an alias, like cls, the inner one and change the AND clause to:
AND `class`.`id` = `cls`.`id`
Also, note that your subquery can be simplified by removing the inner class table altogether, since the subquery can access the class table of the main query, as already mentioned.
Code:
SELECT
`class`.`id`,
`class`.`code` AS `classcode`,
`section`.`name` AS `sectionname`,
`subject`.`code`,
`class`.`units`,
`sched`.`name` AS `schedule`,
`class`.`slots`,
`class`.`dissolved`,
(
SELECT COUNT(`enrollno`)
FROM `enrolldet`
INNER JOIN `enroll`
ON `enrolldet`.`enrollno` = `enroll`.`enrollno`
WHERE `enroll`.`validated` = '1'
AND `enrolldet`.`class` = `class`.`id`
) AS `enrolled`
FROM `class`
LEFT JOIN `sched` ON `class`.`sched` = `sched`.`id`
LEFT JOIN `section` ON `class`.`section` = `section`.`id`
LEFT JOIN `subject` ON `class`.`subject` = `subject`.`id`

Answer 3

$str = "SELECT
c.id,
c.code AS classcode,
section.name AS sectionname,
subject.code,
c.units,
sched.name AS schedule,
c.slots,
c.dissolved,
(SELECT
Count(e.enrollno)
FROM
enrolldet AS e
Inner Join enroll ON e.enrollno = enroll.enrollno
Inner Join class ON e.class = class.id
WHERE
enroll.validated = '1' AND
class.id = c.id) as enrolled
FROM
class AS c
Left Join sched ON c.sched = sched.id
Left Join section ON c.section = section.id
Left Join subject ON c.subject = subject.id";
i got the correct answer using this. for future reference. i just added aliases for each table