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我正在运行以下查询：

    select students.name, students.rollno, students.category as cat, `t1`.`studentid` 
    AS `studentid`,
    min(`t1`.`paperid`) AS `paper1`,
    min(`t2`.`paperid`) AS `paper2`,
    min(`t3`.`paperid`) AS `paper3`,
    min(`t4`.`paperid`) AS `paper4` from  students,
    ((((`sps` `t1` left join `sps` `t2` on(((`t1`.`studentid` = `t2`.`studentid`) and
    (`t2`.`paperid` > `t1`.`paperid`)))) left join `sps` `t3` on(((`t2`.`studentid` =
   `t3`.`studentid`) and (`t3`.`paperid` > `t2`.`paperid`)))) left join `sps` `t4`
    on(((`t3`.`studentid` = `t4`.`studentid`) and (`t4`.`paperid` > `t3`.`paperid`))))
    left join sgm on sgm.studentid=t1.studentid 
    left join str on str.studentid=t1.studentid 
    left join papers on t1.paperid=papers.p_id and papers.type='optional' 
    left join `students` `stu` on((`stu`.`studentid` = `t1`.`studentid`))) 
    where ((`stu`.`course` = '1') 
    and (`stu`.`status` = 'regular')
    and students.studentid=t1.studentid) 
    and papers.type='optional'  
    and sgm.groupid='1'
    and str.semesterid='1' 
    and str.sessionid='13' 
    group by `t1`.`studentid`  
    order by name

我在这里要做的是，我正在从表中获取结果：

Students Table
| studentid  |   name |    rollno   |  category |  course |
     1           ABC        1            new         1
     2           BCD        2            old         1
     3           CDE        3            new         1

Papers Table
|  p_id  |   p_name |  type |
    p1         a       main
    p2         b       optional
    p3         c       main
    p4         d       optional
    p5         e       optional

SGM
|  id  |  studentid  | group |
    1       1            a
    2       2            a
    3       3            b

SPS
|  id  |  studentid | paper |
    1        1          p1
    2        1          p3
    3        1          p4
    4        1          p5
    5        2          p3
    6        2          p4
    7        2          p5
    8        3          p1

这些是学生表、SPS、SGM、纸。在查询中，我试图找出每个学生应该有一个结果，其中应该包含信息、姓名、rollno、cat、studentid、paper1、paper2、paper3 和 paper4。我能够做到这一点，但我坚持只获取那些类型是可选的纸张。我只需要选择可选纸张。类型是paperid，分配给每个学生的论文是sps。我怎样才能做到这一点？

如果没有分配文件，则应该有空值。

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