SELECT sch.id, cname, tname, status," . "(SELECT COUNT(*) FROM election WHERE id = sch.id) num" . "FROM schedule sch, course c, teacher t WHERE sch.cn = c.cn " . "AND sch.tn = t.tn AND dept = '$dept' AND term = '$term'

这是一条 SQL 查询语句，用于从数据库中获取与指定部门和学期相关的课程计划信息。该查询语句联结了三个表：schedule、course和teacher，通过它们的关联字段进行联结。查询结果包含课程计划的id、课程名称、教师姓名、状态和选课人数。其中，选课人数是通过子查询语句实现的，该语句统计了与课程计划id相关的选课记录数。

相关问题

SELECT course.Cname,teacher.Tname,sc.CId,student.Ssex FROM course JOIN teacher ON course.CId = teacher.TId LEFT JOIN sc JOIN student ON sc.SId=student.SId;这个语句有什么问题

这个 SQL 语句存在一些问题：

1. 在 LEFT JOIN sc JOIN student ON sc.SId=student.SId 中，缺少 ON 关键字，应该改为 LEFT JOIN sc ON sc.CId = course.CId AND sc.TId = teacher.TId JOIN student ON sc.SId = student.SId。这样才能正确指定 LEFT JOIN 的条件。
2. SELECT 语句中没有指定使用哪个表的 CId 字段，应该改为 SELECT course.CId, course.Cname, teacher.Tname, student.Ssex。
3. JOIN 多个表时，应该明确指定每个表的关联条件，否则会出现笛卡尔积的情况。

因此，正确的 SQL 语句应该是：

SELECT course.CId, course.Cname, teacher.Tname, student.Ssex
FROM course
JOIN teacher
  ON course.CId = teacher.TId
LEFT JOIN sc
  ON sc.CId = course.CId AND sc.TId = teacher.TId
JOIN student
  ON sc.SId = student.SId;

select c.id as cid, c.name as cname, s.id as sid, s.code , s.name as sname, s.gender, s.age, t.id as id, t.name as name -- from student s,teacher t,class c where s.class_id=c.id -- and t.id=(select c.teacher_id from class c where c.id=#{id}) from class c left outer join student s on c.id=s.class_id left OUTER join teacher t on t.id=c.teacher_id where c.id = 1用嵌套方式怎么写

以下是使用子查询的方式来嵌套查询：

SELECT c.id as cid, c.name as cname, s.id as sid, s.code, s.name as sname, s.gender, s.age, t.id as tid, t.name as tname 
FROM (
    SELECT * FROM class WHERE id = 1
) c 
LEFT OUTER JOIN student s ON c.id = s.class_id 
LEFT OUTER JOIN teacher t ON c.teacher_id = t.id;

注意，这里的子查询 `SELECT * FROM class WHERE id = 1` 返回的结果只有一行，因为我们指定了 `id = 1`，所以外层的 `LEFT OUTER JOIN` 只会返回这个班级的所有学生和对应的老师信息。