如何将每个“一个”的一对多联接查询的结果减少到一个signle对象中？

Question

Suppose I have 2 tables:

Applicants:
- id
- full_name
- address

Educations:
- id
- applicant_id
- institute
- address

After I did an inner join, I want to loop through the data on a template. But first I want to convert all educational records for an applicant into an array and attach it to the applicant record.

applicant_a:
- id
- full_name
- address
- educations: [ OBJECTS HERE ]

What's the way to do so? Can I do it on the database side via SQL? Or do I have to do it on PHP side?

Answer 1

This is a simple draft of how I would do it in your case.
I'm not saying this is the best or even the only way to do it.
This specific is not tested, though I used that logic often before.
Note, that this is only about the logic here.... but this should give you what you want!

$applicants = array();
$old_applicant_id=null;

while ($row=$db->fetch()) {

    // new applicant
    if($row['applicant_id']!=$old_applicant_id) {
        // save the education to the old one - if there is one
        if(isset($applicant)) {
            $applicant['education'] = $educations;
            $applicants[] = $applicant;
        }
        // then (and in first round)
        $applicant = array();
        $applicant['fullName'] = $row['fullName'];  
        // repeat for other values of applicant

        $educations = array(); // initialize educations
        $education = array();
        $education['id'] = $row['edu_id'];
        // repeat for institute, etc
        $educations[] = $education;
    } else {
        // already existing applicant, so only add education
        $education = array();
        $education['id'] = $row['edu_id'];
        // repeat for institute, etc
        $educations[] = $education;
    }
    // set old applicant
    $old_applicant_id = $row['applicant_id'];
}
// finally you have to save the last one to the array
$applicant['education'] = $educations;
$applicants[] = $applicant;

Another way would be to have two seperate queries and merge them in two loops.

This question of mine was related. I was asking about the spead of the two versions. Might be interesting.

Answer 2

MySQL does not return arrays, so this kind of thing is usually best done in client code (in this case PHP).

But if you are just going to concatenate the educations into a string of some sort you may want to look into the GROUP_CONCAT aggregate function.