Database-data query

Query the names of all students in the grade, and sort them in ascending order by number
Select sid, sname
From STUDENTS
Where grade='1'
Order by sid;
Query the course scores for which the student's elective scores are qualified, and convert the scores into points
Select sid,cid,(score-60)*0.1+1 as jidian
From CHOICES
Where score>=60; –Excluding the elective records with null scores
Query class is the name of the course or
the Select CNAME
the From COURSES
the Where hour or hour = 30 = 60;
Query all course numbers that contain "number" in the course name
Select cid
From COURSES
Where cname Like'%数%';
Query the course numbers of all course selection records (not repeated display)
select distinct cid
from CHOICES;
Statistics of the average salary of all teachers
Select AVG(salary) as avgSalary
From TEACHERS;
Query the number, name and average score of all students, and sort them in descending order of the total average score.
Select sid, Avg(score) as avgScore
From CHOICES
Group by sid
Order by avgScore Desc;
Count the number of electives and average grades of each course
Select cid, COUNT(sid) as numOfPerson, AVG(score) as avgScore
From CHOICES
Group by cid;
Query the number of students who have taken at least two courses
Select sid
From CHOICES
Group by sid
Having COUNT(sid)>=2;
Query the course names and grades of all courses selected by students with the number S244157
Select cname, score
From COURSES,CHOICES
Where COURSES.cid=CHOICES.cid and sid='S244157';
Query the numbers of all students who have selected the database
Select sid
From CHOICES, COURSES
Where CHOICES.cid=COURSES.cid and cname='database';
Find the pairs of students who have chosen the same course
Select A.sid as stu1,B.sid as stu2
From CHOICES A,CHOICES B
Where A.cid=B.cid and A.no<B.no;
Find the number of the course selected by at least two students
Select cid
From CHOICES
Group by cid
Having COUNT(*)>=2;
Query the student ID of a course selected by the student who has taken the number S244157 (including the student S244157)
Select distinct A.sid-If a student has taken multiple courses selected by S244157, the student ID will appear more Times, so use distinct
From CHOICES A,CHOICES B
Where A.cid = B.cid and B.sid='S244157';
Inquire about students' basic information and elective course numbers and grades
Select STUDENTS.*,cid,score
From STUDENTS Left Outer Join CHOICES On (CHOICES.sid=STUDENTS.sid);
Query the name of the student with the student number S244157 and the name and grade of the elective course
Select sname, cname, score
From STUDENTS,CHOICES,COURSES
Where STUDENTS.sid=CHOICES.sid and CHOICES.cid=COURSES.cid
and STUDENTS.sid='S244157 ';
Query the data of all students in the same grade of the student with student number S244157
Select A.*
From STUDENTS A, STUDENTS B
Where A.grade=B.grade and B.sid='S244157';
Query the detailed information of all students who have selected courses
Select *
From STUDENTS
Where sid In(select sid
from CHOICES);
or:
Select distinct STUDENTS.* --If you are querying student numbers, write the name as (only write a distinct)
From STUDENTS, CHOICES-distinct STUDENTS.sid,STUDENTS.sname
Where STUDENTS.sid=CHOICES.sid; --Use connection to remove student records for unselected courses
Query the number of courses that no students choose
Select cid
From COURSES
Where cid Not In(
Select cid
From CHOICES);
Query the student number and name of the students who have taken the elective course named C language
Select STUDENTS.sid, STUDENTS.sname
From STUDENTS,CHOICES, COURSES
Where STUDENTS.sid=CHOICES.sid and CHOICES.cid=COURSES.cid
and cname='C language ';
Find the elective record with the worst elective course score
Select *
From CHOICES
Where score=(
Select MIN(score)
From CHOICES);
Find the elective record with the worst grades in elective courses
Select *
From CHOICES
Where score=(
Select MIN(score) --null returns unknown when compared with any number, so it is not the minimum value
From CHOICES);
– or
Select *
From CHOICES
Where score<=All(
Select score - the query result contains null, there is no value less than null,
From CHOICES - ∴ must be removed to find the minimum value
Where score is not null);
error:
Select *
From CHOICES
Where score= MIN(score);-the aggregation should not appear in the WHERE clause, unless the aggregation is in the HAVING clause or the subquery contained in the select list, and the column to be aggregated is an external reference
Find out the same course name as the course hour of the course "C language" or course "operating system" (including both)
Select cname
From COURSES
Where hour =some( –or change "=some" to in
Select hour
From COURSES
Where cname ='C language' or cname='operating system');
Inquire about the names of all the students taking the course number C153488.
Select sname
From STUDENTS,CHOICES
Where STUDENTS.sid=CHOICES.sid and cid='C153488';
Query the names of students who have taken all courses.
Select sname
From STUDENTS
Where Not Exists(
Select *
From COURSES as X
Where Not Exists(
Select *
From CHOICES as Y
Where Y.sid=STUDENTS.sid and Y.cid=X.cid
)
);
Use set operations to query the student number of the elective course "data structure" or course "database".
Select sid
From CHOICES
Where cid=(
Select cid
From COURSES
Where cname='Data Structure')
Union
Select sid
From CHOICES
Where cid=(
Select cid
From COURSES
Where cname='Database');
Realize set intersection operation, query the number of students who take both the elective course "Computer Network" and the elective course "C Language".
Select sid
From CHOICES
Where cid=(
Select cid
From COURSES
Where cname='Computer Network')
Intersect
Select sid
From CHOICES
Where cid=(
Select cid
From COURSES
Where cname='C Language');
Realize the set subtraction operation, query the number of the students who take the elective course C language but not the elective course "computer network".
Select sid
From CHOICES
Where cid=(
Select cid
From COURSES
Where cname='C Language')
Except
Select sid
From CHOICES
Where cid=(
Select cid
From COURSES
Where cname='Computer Network');

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