MYSQL alias in where clause - php

Question

i have a query and i want to filter the result set using an ALIASED column in my WHERE condition, ive read that you cannnot do it but i dont have yet an alternative way to do it efficiently since im working on thousands of records.
$str = "SELECT
mainclass.id AS classid,
mainclass.code AS classcode,
Sum(CASE WHEN enroll.acctok = '1' AND enroll.assessed = '1'
THEN enroll.assessed
ELSE 0 end) AS assessedinclass,
Sum(enroll.validated = '1') AS validatedinclass,
section.name AS sectionname,
subject.code AS subcode,
subject.subdesc,
mainclass.units,
sched.name AS schedule,
mainclass.tutorial,
mainclass.dissolved,
mainclass.slots,
mainclass.ismother,
mergeclass.code AS mothercode,
mergeclass.id AS mothercodeid,
mergeclass.slots AS mothercodeslots,
mainclass.mergein,
mainclass.inst,
instructor.lname,
instructor.fname,
instructor.mname,
instructor.suffix
FROM
class AS mainclass
Left Join enrolldet ON mainclass.id = enrolldet.class
Left Join enroll ON enrolldet.enrollno = enroll.enrollno
Inner Join period ON mainclass.period = period.id
Inner Join section ON mainclass.section = section.id
Inner Join subject ON mainclass.subject = subject.id
Left Join sched ON mainclass.sched = sched.id
Left Join class AS mergeclass ON mainclass.mergein = mergeclass.id
Left Join instructor ON mainclass.inst = instructor.userid
Left Join course ON section.course = course.id
WHERE
(period.id = :period OR period.code = :period)";
if($level != ''){
$str .= " AND course.level = '".$level."'";
}
if($dept != ''){
$str .= " AND course.dept = '".$dept."'";
}
if($display != ''){
switch ($display) {
case 'open':
$str .= " AND mainclass.slots > assessedinclass";
break;
case 'dissolved':
$str .= " AND mainclass.dissolved = 1";
break;
case 'tutorial':
$str .= " AND mainclass.tutorial = 1";
break;
case 'closed':
$str .= " AND mainclass.slots <= assessedinclass";
break;
}
}
$str .= "GROUP BY
mainclass.id,
mainclass.code";
$str .= " ORDER BY subject.code, mainclass.id";
return $this->_db->select($str,array(':period' => $period));
im trying to filter it using the ASSESSEDINCLASS WHERE mainclass.slots > ASSESSEDINCLASS means the subject is OPEN
while if mainclass.slots <= ASSESSEDINCLASS it will be considered CLOSED

Answer 1

You can try below. Write an outer select which will have all the required columns for you to process filters.
select
classid,
classcode,
assessedinclass,
validatedinclass,
sectionname,
subcode,
subdesc,
units,
schedule,
tutorial,
dissolved,
slots,
ismother,
mothercode,
mothercodeid,
mothercodeslots,
mergein,
inst,
lname,
fname,
mname,
suffix
from
(select
mainclass.id AS classid,
mainclass.code AS classcode,
Sum(case when enroll.acctok = '1' and enroll.assessed = '1'
then enroll.assessed
else 0
end) as assessedinclass,
Sum(enroll.validated = '1') AS validatedinclass,
section.name AS sectionname,
subject.code AS subcode,
subject.subdesc,
mainclass.units,
sched.name AS schedule,
mainclass.tutorial,
mainclass.dissolved,
mainclass.slots,
mainclass.ismother,
mergeclass.code AS mothercode,
mergeclass.id AS mothercodeid,
mergeclass.slots AS mothercodeslots,
mainclass.mergein,
mainclass.inst,
instructor.lname,
instructor.fname,
instructor.mname,
instructor.suffix
from
class as mainclass
left join
enrolldet on mainclass.id = enrolldet.class
left join
enroll on enrolldet.enrollno = enroll.enrollno
inner join
period on mainclass.period = period.id
inner join
section on mainclass.section = section.id
inner join
subject on mainclass.subject = subject.id
left join
sched on mainclass.sched = sched.id
left join
class as mergeclass on mainclass.mergein = mergeclass.id
left join
instructor on mainclass.inst = instructor.userid
left join
course on section.course = course.id)
where
(period.id = :period OR period.code = :period);
Your final query would be -
select
<req_cols>
from
(select
<agg_cols>
from
table
group by
<grpCols>)
where
<filter on agg cols>

Answer 2

$str = "SELECT
mainclass.id AS classid,
mainclass.code AS classcode,
Sum(CASE WHEN enroll.acctok = '1' AND enroll.assessed = '1'
THEN enroll.assessed
ELSE 0 end) AS assessedinclass,
Sum(enroll.validated = '1') AS validatedinclass,
section.name AS sectionname,
subject.code AS subcode,
subject.subdesc,
mainclass.units,
sched.name AS schedule,
mainclass.tutorial,
mainclass.dissolved,
mainclass.slots,
mainclass.ismother,
mergeclass.code AS mothercode,
mergeclass.id AS mothercodeid,
mergeclass.slots AS mothercodeslots,
mainclass.mergein,
mainclass.inst,
instructor.lname,
instructor.fname,
instructor.mname,
instructor.suffix
FROM
class AS mainclass
Left Join enrolldet ON mainclass.id = enrolldet.class
Left Join enroll ON enrolldet.enrollno = enroll.enrollno
Inner Join period ON mainclass.period = period.id
Inner Join section ON mainclass.section = section.id
Inner Join subject ON mainclass.subject = subject.id
Left Join sched ON mainclass.sched = sched.id
Left Join class AS mergeclass ON mainclass.mergein = mergeclass.id
Left Join instructor ON mainclass.inst = instructor.userid
Left Join course ON section.course = course.id
WHERE
(period.id = :period OR period.code = :period)";
if($level != ''){
$str .= " AND course.level = '".$level."'";
}
if($dept != ''){
$str .= " AND course.dept = '".$dept."'";
}
if($display != ''){
switch ($display) {
case 'open':
$str .= " AND mainclass.slots > assessedinclass";
break;
case 'dissolved':
$str .= " AND mainclass.dissolved = 1";
break;
case 'tutorial':
$str .= " AND mainclass.tutorial = 1";
break;
case 'closed':
$str .= " AND mainclass.slots <= assessedinclass";
break;
}
}
$str .= "GROUP BY
mainclass.id,
mainclass.code";
$str .="HAVING <your condition>";
$str .= " ORDER BY subject.code, mainclass.id";
return $this->_db->select($str,array(':period' => $period));