Question

让我们说我们有表格Student，Subject和Marks表格。

结构和数据如下

CREATE MULTISET TABLE tmp_work_db.student ,FALLBACK ,
     NO BEFORE JOURNAL,
     NO AFTER JOURNAL,
     CHECKSUM = DEFAULT,
     DEFAULT MERGEBLOCKRATIO
     (
     student_id     integer,  
  student_name      varchar(50) 

       )
PRIMARY INDEX  ( student_id );

insert into tmp_work_db.student
values(1,'Tim');
insert into tmp_work_db.student
values(2,'John');
insert into tmp_work_db.student
values(3,'Roy');


CREATE MULTISET TABLE tmp_work_db.subject ,FALLBACK ,
     NO BEFORE JOURNAL,
     NO AFTER JOURNAL,
     CHECKSUM = DEFAULT,
     DEFAULT MERGEBLOCKRATIO
     (
     subject_id     integer,  
  subject_name      varchar(50) 

       )
PRIMARY INDEX  ( subject_id );

insert into tmp_work_db.subject
values(1,'English');
insert into tmp_work_db.subject
values(2,'Maths');
insert into tmp_work_db.subject
values(3,'Hindi');


CREATE MULTISET TABLE tmp_work_db.marks ,FALLBACK ,
     NO BEFORE JOURNAL,
     NO AFTER JOURNAL,
     CHECKSUM = DEFAULT,
     DEFAULT MERGEBLOCKRATIO
     (
     student_id     integer, 
     subject_id     integer,  
     marks      integer 

       )
PRIMARY INDEX  ( student_id,subject_id );


insert into tmp_work_db.marks 
values(1,1,50);
insert into tmp_work_db.marks 
values(1,2,100);
insert into tmp_work_db.marks 
values(1,3,40);

insert into tmp_work_db.marks 
values(2,1,50);
insert into tmp_work_db.marks 
values(2,2,30);

我们需要编写一个查询，该查询将生成针对特定学科不及格的学生的报告。

我在下面的查询中尝试了让某个特定科目不及格的学生

sel student_name,subject_name,marks from tmp_work_db.student st 
inner join
(
sel student_id,subject_id,marks from tmp_work_db.marks where marks < 35
) mrk
on st.student_id=mrk.student_id
inner join tmp_work_db.subject sub
on sub.subject_id=mrk.subject_id;

我无法招募未出现在特定学科中的学生。

Answer 1

如果您想要的是没有为任何学科打分的学生姓名，那么您需要在cross join和student之间的subject和left join至{ {1}}：

marks

对于未能通过3个表并将条件select s.student_name, t.subject_name not_appeared from student s cross join subject t left join marks m on m.student_id = s.student_id and m.subject_id = t.subject_id where m.student_id is null放在marks < 35子句中的学生：

WHERE

您可以将两个查询合并为一个：

select s.student_name, t.subject_name failed
from student s 
inner join marks m on m.student_id = s.student_id
inner join subject t on m.subject_id = t.subject_id
where m.marks < 35

请参见demo。
结果：

select  
  s.student_name, t.subject_name,
  case when m.marks is null then 'not appeared' else 'failed' end result
from student s cross join subject t 
left join marks m 
on m.student_id = s.student_id and m.subject_id = t.subject_id
where coalesce(m.marks, 0) < 35
order by s.student_name, t.subject_name

对于组合查询：

> student_name | not_appeared
> :----------- | :-----------
> John         | Hindi       
> Roy          | English     
> Roy          | Maths       
> Roy          | Hindi       

> student_name | failed
> :----------- | :-----
> John         | Maths

查询生成失败并且没有出现在考试中

1 个答案: