[MySQL practice] query statement demonstration

Simple query

Example of query statement of student course selection database ：

• select form where

use  Students' course selection ;

SELECT sno,sname FROM student
WHERE sdept = ' Statistics College '

SELECT DISTINCT  sno FROM SC

SELECT sno,grade FROM SC
WHERE cno = 10001
ORDER BY grade DESC,sno ASC

SELECT sno,grade*0.8 FROM SC
WHERE cno = 10001 AND (grade < 70 OR grade >60)

SELECT* FROM student
WHERE sdept IN (' Statistics College ',' Graduate School of Engineering ')
ORDER BY sdept DESC

SELECT sno,cno FROM SC
WHERE grade IS NULL

SELECT SC.sno,sname,cname,grade FROM SC,Course,student
WHERE SC.sno = student.sno AND SC.cno =Course.cno
ORDER BY grade DESC

SELECT* FROM Course
WHERE cno IN (SELECT cno FROM SC  WHERE sno IN(SELECT sno FROM student WHERE sdept = ' Statistics College ' ) 
AND ccredit > 2)

SELECT student.*,SC.* FROM student,Sc
WHERE sc.sno=student.sno

SELECT X.sno,sname,grade FROM student X,SC Y
WHERE X.sno=Y.sno AND Y.cno = '10001'

SElECT FIRST.*,SECOND.cpno  Antecedent course   FROM Course FIRST,Course SECOND
WHERE (FIRST.cpno=SECOND.cname AND SECOND.cpno IS NOT NULL)

nested queries

• left join Left connection

-- nesting 
/*
1. There is no... In the elective course UML Therefore, it is amended as follows java
 Checked the elective course java Student ID and name of the course 
*/
--method1.
select sno ,sname from student where sno in
(select sno from SC  where cno in (select cno from Course where cname = 'java'))

--method2.

select student.sno ,sname from student ,SC,Course where student.sno=SC.sno and SC.cno = Course.cno and Course.cname = 'java'

/*
2. Query the student ID and name of students older than Wang Hua .
*/
select sno,sname  from student where  sage > 

(select sage from student where sname =' Wang Hua ')  

/*
3. Inquire about c1 The grade of the course is lower than Zhang San's student number and grade 
*/
select SC.sno ,grade from SC  where grade < 
(select grade from SC,student where (SC.sno=student.sno and student.sname='zeze' and  SC.cno='c1'))

/*
4. Check the students younger than the students in other colleges 
*/
select sname from student where sage >all (select sage from student where sdept =' Statistics College ') AND sdept!=' Statistics College '

/*
5 Check the elective course C2 Name of the student of the course 
*/
--method1
select student.sname from student,sc where  student.sno=SC.sno and SC.cno= 'c1'
--method2
select student.sname from student left  join SC on student.sno=SC.sno left join Course on Course.cno=SC.cno where Course.cno='c1 '

/*
6. The names of students who have taken all courses have been queried 
*/
select sname
from student
where not exists (select *
                            from Course
                            where not exists( select *
                                                        from SC
                                                        where SC.sno = student.sno and SC.cno = Course.cno))

/*
7. The query has at least selected students as "S2" The name of the student ID of all the courses selected by the students 
*/
select sname
from student
where not exists(select *
                           from SC
                           where SC.sno = 'S2' and not exists(select *
                                                              from Course
                                                              where SC.sno = student.sno and SC.cno = Course.cno))

/*
9. Inquire about   Names of students who have taken both data structure and database principle and application courses （ps: Because there is no , So there was no result ）
*/
select sname from student
left join SC sc
on sc.sno = student.sno
left join Course c1
on sc.cno = c1.cno
where c1.cname = ' data structure '
intersect
select sname from student
left join SC sc
on sc.sno = student.sno
left join Course c1
on sc.cno = c1.cno
where c1.cname = ' Database principle '

Combination query

• intersection intersect
• Combine union
• remove expcept
• grouping group by
• Filter polymerization having And group by Use it together

-- Combine 
/*
1. Use set operation to query the names of students who have taken both the course of data structure and the course of database principle and application .
*/
select sname
from student
where sno in( select sno
                       from SC
                       where cno in (select cno
                                              from Course
                                              where cname = ' data structure '))
intersect

select sname
from student
where sno in( select sno
                       from SC
                       where cno in (select cno
                                              from Course
                                              where cname = ' Database principle '))
/*
2. Use set operation to query the student number who has taken the course of data structure or the course of database principle and application .
*/
select sno
from SC
where cno in (select cno
                       from Course
                       where cname = ' data structure ')
union

select sno
from SC
where cno in (select cno
                       from Course
                       where cname = ' Database principle ')

/*
3. Use set operation to query the student number of students who have taken the data structure course instead of the database principle and application course .
*/
select sno
from SC
where cno in (select cno
                       from Course
                       where cname = ' data structure ')
except

select sno
from SC
where sno in (select cno
                       from Course
                       where cname = ' Database principle ')
/*
4. Count the number of students who have taken courses .
*/
select count(distinct sno) as status
from SC

/*
5. The number of elective courses with qualified results exceeds  4  Student no. of students above class 、 Total score .
（ps：having  It is also a kind of filtration , Is aggregate filtering .
where  For filtering , Filter the result set before production .
having  stay  where Filter the aggregated results after filtering the source input .
HAVING Clause allows us to filter groups of data after grouping ． 
WHERE Clause filters records before aggregation ． That is to say, it works on GROUP BY  Clause and HAVING Before clause ． 
 and  HAVING Clause filters group records after aggregation . ）
*/
select SC.sno,SUM(grade) as ' Total score ' 
from SC  
where  SC.grade > =60 
group by sno 
having count(cno)>4
/*
6. Count the number of students in each department .

*/
select sdept ,count(sdept)  as total from student  
group by  sdept
/*
7. Count the number of students of all ages 

*/
select sage ,count(sage) as everyone from student 
group by sage 

/*
8. Count the number of elective courses and average score of each student 
*/
select sno,count(cno) as ' Number of courses ',AVG(grade) as ' Average score '
from SC
group by sno 

/*
9. Query the details of each course and the number of students selected .
*/
select Course.*,count(SC.cno) as ' The number of students who choose courses '
from Course,SC
where SC.cno = Course.cno
group by Course.cno,ccredit,cname,cpno,ctech 

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