Question

I am trying to pull up some data.

Here is the setup:

A [school] can have multiple [semester]. Only one [semester] can be active per [school].

The simplified fields per table:

• [school] has ‘id’,’title’
• [school_semester] has ‘id’,’school_id’ (fk), ‘semester_id’ (fk), ‘active‘, ‘start_date’, ‘end_date’
• [semester] has ‘id’, ‘title’

The “school_semester” table holds information for it’s specific school/semester. (startdate,enddate,etc).

Anyway, I am simply trying to pull up all schools, and in this query I want to also see that school’s current active semester.

Here is my query so far:

SELECT *, `school`.`name` as school_name
FROM (`school`)
LEFT JOIN `school_semester` ON `school`.`id` = `school_semester`.`school_id`
LEFT JOIN `semester` ON `semester`.`id` = `school_semester`.`semester_id`
ORDER BY `school_semester`.`active`

The problem:

This works if a school has semester that is active, but if it does not, it will show one which may be marked as inactive. If I add the statement WHERE school_semester.active = 1, it excludes schools that do not have an active semester..

Any pointers?

Answer 1

move the condition on the joining :

SELECT *, `school`.`name` as school_name
FROM (`school`)
LEFT JOIN `school_semester` ON 
        `school`.`id` = `school_semester`.`school_id` AND
        `school_semester`.`active`
LEFT JOIN `semester` ON `semester`.`id` = `school_semester`.`semester_id`

This way, only the active semesters will be joined to the result.