从数据库查询结果

Question

I have two tables : galleries and shared galleries.

Structure of galleries: (for storing images of individual students. One student contains multiple images)

id, student_id, classroom_id, image

Structure of shared_galleries: (for storing images which are common to all students in a classroom. One classroom contains many images):

id,classroom_id,image

Other than these two tables I have students table and classrooms table. Students table store the classroom_id.

I need to get a query so that I can display the images stored in 'galleries' for a student and those stored in shared gallery of the classroom in which that student belongs in a single page. How can I achieve this ? Something like this returns duplicated results :

select galleries.id as gid, 
       shared_galleries.id as sid,
       galleries.student_id, galleries.classroom_id 
from galleries 
     inner join shared_galleries on galleries.classroom_id=shared_galleries.classroom_id 
where galleries.student_id=31 and galleries.classroom_id=28

Answer 1

You will need to join the student to the shared_galleries along the relations to get the right results

SELECT 
    g.image AS image,
    0 AS is_shared
WHERE 
    g.student_id = :student_id
FROM
    galleries AS g
UNION
SELECT
    sg.image AS image,
    1 AS is_shared
FROM 
    shared_galleries AS sg
    LEFT JOIN classrooms AS c ON c.id = sg.classroom_id
    LEFT JOIN students AS s ON s.classroom_id = c.id
WHERE 
    s.id = :student_id

This should give you all the images for a student with :student_id , I've also added the is_shared column in case you need to know the origin of the image

Answer 2

Since your images are stored on a per-record basis (in both galleries and shared galleries, using a JOIN is not needed. Use a UNION instead:

SELECT 
    galleries.id AS ID, 
    galleries.student_ID AS StudentID, 
    galleries.classroom_id as ClassroomID, 
    galleries.image as Image 
WHERE 
    galleries.student_id = 31
UNION
SELECT
    shared_galleries.id AS ID, 
    NULL AS StudentID, 
    shared_galleries.classroom_id as ClassroomID, 
    shared_galleries.image as Image 
WHERE 
    shared_galleries.classroom_ID = 31

This will produce a list of records, with each record containing one image (since the number of images per student and the number if images per classroom is never consistent)

If you want do a little extra and ensure that your classroom images are always linked to the correct student, then declare a variable at the beginning to set the student_id value, and then use it in the second SELECT statement.