where子句中的MYSQL别名

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));