lostintheriver lostintheriver - 5 months ago 8
SQL Question

SQL query left join issue

I am making a query but it is not working properly. I have two tables one called subarea and the other student_subarea. In the subarea table I have the id and the fields of study and in the student_subarea the student choices, composed by the id (PK), the student_id and the student_subarea which is a FK to subarea which links the subarea id and the student_subarea.

What I want to acomplish is to obtain all fields of study in one column and in another column the id of the student if he is in the class. Otherwise, show null or something.

SELECT a.`id` , a.`name` , a.`area_id` , u. *
FROM `subarea` a
LEFT JOIN student_subarea u ON u.subarea_id = a.id
WHERE u.student_id =50
OR u.student_id IS NULL


Doing this is not helping at all. I tried to use functions and subqueries without any success. Could some help me.

thank you in advance

PD: I am using mysql btw.

Answer

The general rule for left join and filtering is to put the filtering clauses in the on clause for all but the first table. So this may do what you want:

SELECT a.`id` ,  a.`name` , a.`area_id` , u. * 
FROM `subarea` a LEFT JOIN
     student_subarea u
     ON u.subarea_id = a.id AND u.student_id = 50;

How do you remember this logic? A left join returns all rows from the first table even when there is no match on the second table. That appears to be what you want.

The problem with your logic is that students other than student 50 match the logic. So, those rows get filtered out.

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