php mysql use subquery result in WHERE clause in the same query - php

Question

is it possible to use a subquery result as a criteria in WHERE clause in the same query. i have this sql code. i want to compare the ASSESSEDINCLASS which is a result in the subquery and C.SLOTS to determine it the slot is full or not. is this even possible in a single query? thanks in advance
$str = "SELECT
c.id,
c.code AS classcode,
section.name AS sectionname,
subject.code,
subject.subdesc,
c.units,
sched.name AS schedule,
c.slots,
c.dissolved,
c.tutorial,
c.inst,
instructor.lname,
instructor.fname,
instructor.mname,
instructor.suffix,
(SELECT
Count(e.enrollno)
FROM
enrolldet AS e
Inner Join enroll ON e.enrollno = enroll.enrollno
Inner Join class ON e.class = class.id
WHERE
enroll.validated = '1' AND
class.id = c.id) as validatedinclass,
(SELECT
Count(e.enrollno)
FROM
enrolldet AS e
Inner Join enroll ON e.enrollno = enroll.enrollno
Inner Join class ON e.class = class.id
WHERE
enroll.assessed = '1' AND
class.id = c.id) as assessedinclass,
(SELECT
q.id
FROM
merged
Inner Join class AS q ON merged.mothercode = q.id
WHERE
merged.mergefrom = c.id) AS mergedto_mothercode
FROM
class AS c
Left Join sched ON c.sched = sched.id
Left Join section ON c.section = section.id
Left Join subject ON c.subject = subject.id
Left Join instructor ON c.inst = instructor.userid
Left Join course ON section.course = course.id
Inner Join period ON c.period = period.id
WHERE
(period.id = '".$period."' OR period.code = '".$period."')";
if($status == 'open'){
$str .= " AND c.slots < assessedinclass";
}
$str .= " ORDER BY subject.subdesc";

Answer 1

To answer your question yes you could by using Having clause, Make you use it after where clause
having assessedinclass = someval
Or
$str .= " HAVING c.slots < assessedinclass";
But what i prefer don't use dependent sub queries instead use join for these as sub clause to your main query somewhat like below
SELECT ....,
COALESCE(aic.assessedinclass, 0) AS assessedinclass,
FROM class AS c
JOINS....
LEFT JOIN (SELECT class.id,
COUNT(e.enrollno) AS assessedinclass
FROM enrolldet AS e
INNER JOIN enroll
ON e.enrollno = enroll.enrollno
INNER JOIN class
ON e.class = class.id
WHERE enroll.assessed = '1'
GROUP BY class.id) aic
ON aic.id = c.id
WHERE ( period.id = '".$period."' OR period.code = '".$period."' )
AND c.slots < COALESCE(aic.assessedinclass, 0)