由于您的图像存储在每个记录的基础上（在库和共享库中），因此不需要使用连接。改用联合体：

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

这将产生一个记录列表，每个记录包含一个图像（因为每个学生的图像数量和每个教室的图像数量从不一致）
如果您想做一些额外的工作，并确保教室图像始终链接到正确的学生，那么请在开头声明一个变量来设置student\u id值，然后在第二个select语句中使用它。

你需要和这个学生一起参加考试 shared_galleries 沿着关系得到正确的结果

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

这应该给你所有的图像为一个学生与 :student_id ，我还添加了 is_shared 列，以防您需要知道图像的来源