mySQL-仅选择连续出现的记录

我正在制作一张处理学生记录的表格。基本上，我只需要选择至少连续3个未注册学期的学生的记录

样本数据：

StudentID|Unregistered|Semester
10000000 |Y           |1
10000000 |Y           |2
10000000 |Y           |6
10000001 |Y           |2
10000001 |Y           |8
10000001 |Y           |9
10000001 |Y           |10
10000002 |Y           |1
10000002 |Y           |2
10000002 |Y           |3
10000002 |Y           |10

---

我只想从学生ID 10000001和10000002中选择记录。

以下查询将为您提供连续学期的计数：

select s.id,
if(s.semester = @previousSem + 1 and @previousId = s.id, 
   @count := @count + 1, @count := 1) as count,
   @previousSem := s.semester as sem,
   @previousId := s.id as previousId
from sample s, (SELECT 
        @count := 1, 
        @previousSem := -1,
        @previousId := 0) a
order by s.id, s.semester

现在，您需要将此

查询

与主表（

self-join

）连接起来，并过滤出具有3个或更多计数的记录，例如：

select s.id
from sample s join (select s.id,
if(s.semester = @previousSem + 1 and @previousId = s.id, 
   @count := @count + 1, @count := 1) as count,
   @previousSem := s.semester as sem,
   @previousId := s.id as previousId
from sample s, (SELECT 
        @count := 1, 
        @previousSem := -1,
        @previousId := 0) a
order by s.id, s.semester) s1
on s.id = s1.id
where s1.count >= 3
group by s.id;

这是

---

p.S.删除

unregistered

列为简洁起见，您可以在实际查询中添加它，并在内部查询中添加

WHERE

条件（

WHERE registered='Y'

）。而且它不使用会话变量（它们是邪恶的）：

更灵活的解决方案是：

set @num = 3;

select distinct r1.StudentID
from registrations r1
join registrations r2 
  on  r2.StudentID = r1.StudentID
  and r2.Semester  > r1.Semester
  and r2.Semester <= r1.Semester + @num - 1
where r1.Unregistered = 'Y'
  and r2.Unregistered = 'Y'
group by r1.StudentID, r1.Semester
having count(*) = @num - 1

set@num=3；
选择不同的r1.StudentID
来自注册r1
加入注册r2
在r2.StudentID=r1.StudentID上
r2.学期>r1.学期
和r2。我们可以看到（幸运的是，它们都未注册）的可能副本
set @num = 3;

select distinct r1.StudentID
from registrations r1
join registrations r2 
  on  r2.StudentID = r1.StudentID
  and r2.Semester  > r1.Semester
  and r2.Semester <= r1.Semester + @num - 1
where r1.Unregistered = 'Y'
  and r2.Unregistered = 'Y'
group by r1.StudentID, r1.Semester
having count(*) = @num - 1