你需要使用外部连接。
首先，您需要使用现代的join语法，这种语法从1992年就开始使用了。使用此语法可以 LEFT [OUTER] JOIN ，例如：

SELECT
  s.id AS id, s.grade_id as grade_id, g.name as grade_name, s.name AS name, 
  s.scheduled_days as scheduled_days, s.period_id as period_id,
  p.name as period_name, s.start_time as start_time, 
  s.end_time as end_time, s.teacher_id as teacher_id, 
  u.name as teacher_name, s.classroom_id as classroom_id, 
  c.name as classroom_name
FROM sections s
JOIN grades g ON s.grade_id = g.id
JOIN periods p ON s.period_id = p.id
LEFT JOIN teachers t ON s.teacher_id = t.id -- allows nulls for s.teacher_id
LEFT JOIN users u ON t.user_id = u.id -- allows nulls on t.user_id
LEFT JOIN classrooms c ON s.classroom_id = c.id -- allows nulls for s.classroom_id

这样，您将使用 WHERE 子句只包括“筛选条件”，但不再包括“联接条件”。阅读和调试会简单得多。