Class Table has class_id and class_name columns with the ff values:
class_id | class_Name
1 | Algebra
2 | History
3 | PE
Student table has student_id and student_name column:
student_id | student_name
1 | Kylo Ren
2 | Rey
The third table which is a junction table for both table has the following columns class_id and student_id
class_id | student_id
1 | 1
1 | 2
2 | 1
3 | 2
How can I find the number of students enrolled per subjects?
Output should be like this
Class Name | Number of Students
Algebra | 2
History | 1
PE | 1
Then after that edit the query so that it would only show the class name and number of students greater than 2
Thanks!
Aggregate the junction table by student, and then assert that a matching student has all courses:
WITH cte AS (
SELECT student_id
FROM junction
GROUP BY student_id
HAVING COUNT(class_id) = (SELECT COUNT(*) FROM class)
)
SELECF COUNT(*) AS num_all_classes FROM cte;
Maybe I got this wrong...
SELECT
c.class_name "Class Name",
count(distinct j.student_id) "Number of Students"
FROM
class c
LEFT JOIN junction j ON j.class_id = c.class_id
GROUP BY
1
Related
I have the following tables:
Student: Student_ID (PK), Student_Name
Course: Course_ID (PK), Course_Name
Enrollment: Student_ID (FK), Course_ID (FK)
I need 2 queries:
A query that computes for each student id in the Student table the total number of different (Unique) classmates that this student has across all courses. If the student is not enrolled in any courses 0 should be returned.
For example if Student_ID 123 is enrolled in 3 courses and he has 10 different classmates in each of those courses, I should get the following result:
Student_ID Num_of_classmates
-----------------------------------
123 30
A SQL query that returns all students with all of their classmates. classmate of a student is another student who is enrolled in at least one same class.
It is unclear which of these you want:
Total students in all the classes (which would include a given student).
Total distinct students, since some classmates might be in multiple classes.
Total distinct students not including the given student ("I am not my own classmate").
In any case, the idea is basically two joins and aggregation:
select s.student_id,
count(ec.student_id) as total_classmates,
count(distinct s.student_id) as distinct_classmates,
count(distinct case when ec.student_id <> s.student_id then ec.student_id end) as distinct_other_classmates
from student s left join
enrollment e
on e.student_id = s.student_id left join
enrollment ec
on ec.class_id = e.class_id;
group by s.student_id;
Here I only give a solution to part 2) as #Gordon Linoff has done part 1) and you have also fixed the -1 problem.
A point: use inner join instead of left join here to avoid NULL in classmate names. Hopefully this also adds a little bit of help :)
Test dataset
if object_id('tempdb..#Student') is not null drop table #Student;
create table #Student (
Student_ID int PRIMARY key,
Student_Name varchar(50)
)
insert into #Student(Student_ID, Student_Name)
values (1,'Alice'), (2,'Bob'),(3,'Claire'),(4,'Danny'),(5,'Eve'),(6,'Frank');
if object_id('tempdb..#Course') is not null drop table #Course;
create table #Course (
Course_ID int PRIMARY key,
Course_Name varchar(50)
)
insert into #Course(Course_ID, Course_Name)
values (1,'Algorithm'), (2,'Bash'),(3,'Compiler'),(4,'Design Pattern'),(5,'Exception Handling');
if object_id('tempdb..#Enrollment') is not null drop table #Enrollment;
create table #Enrollment (
Student_ID int,
Course_ID int
)
insert into #Enrollment(Student_ID, Course_ID)
values (1,1),(1,3),
(2,2),(2,3),
(3,3),(3,4),
(4,1),(4,4),
(5,1),
(6,5); -- This Frank guy has no classmate
-- select * from #Student;
-- select * from #Course;
-- select * from #Enrollment;
Solution to 2)
select distinct
A.Student_Name as Student_Name,
D.Student_Name as Classmate_Name
from #Student as A
-- Student (A) -> Enrolled Courses (B)
inner join #Enrollment as B
on A.Student_ID = B.Student_ID
-- Enrollment Courses (B) -> Enrolled Students in that Course (C)
inner join #Enrollment as C
on B.Course_ID = C.Course_ID
and B.Student_ID <> C.Student_ID -- exclude self
-- Classmate's names
inner join #Student as D
on C.Student_ID = D.Student_ID
order by Student_Name, Classmate_Name;
Output
N.B. Frank has no classmates and do not show a NULL value.
| Student_Name | Classmate_Name |
|--------------|----------------|
| Alice | Bob |
| Alice | Claire |
| Alice | Danny |
| Alice | Eve |
| Bob | Alice |
| Bob | Claire |
| Claire | Alice |
| Claire | Bob |
| Claire | Danny |
| Danny | Alice |
| Danny | Claire |
| Danny | Eve |
| Eve | Alice |
| Eve | Danny |
I have two tables
Student
StudentId | StudentName
---------- | --------------
1 | John
2 | Susan
3 | Andy
4 | Joe
Department
StudentId | DepartmentId
---------- | ------------
1 | 123
1 | 234
2 | 123
2 | 456
3 | 123
4 | 456
Each student can be in multiple departments but I have to find those students which are only in one department like student 3 and 4
Any help?
Use GROUP BY and HAVING
SELECT s.StudentId,s.StudentName
FROM Department d
JOIN Student s ON s.StudentId=d.StudentId
GROUP BY s.StudentId,s.StudentName
HAVING COUNT(d.DepartmentId)=1
And if you also want to show students which not in Department table
SELECT s.StudentId,s.StudentName
FROM Department d
RIGHT JOIN Student s ON s.StudentId=d.StudentId
GROUP BY s.StudentId,s.StudentName
HAVING COUNT(d.DepartmentId)<=1
Variant with LEFT JOIN
SELECT s.StudentId,s.StudentName
FROM Student s
LEFT JOIN Department d ON s.StudentId=d.StudentId
GROUP BY s.StudentId,s.StudentName
HAVING COUNT(d.DepartmentId)<=1
Try this,
SELECT st.StudentId, St.StudentName
FROM student st
JOIN Department dep ON dep.StudentId = st.StudentId
GROUP BY st.StudentId
HAVING COUNT(dep.DepartmentId) = 1;
You can use IN with department table group by StudentId alongwith having count(Student_Id) = 1 in sub-query as below.
SELECT *
FROM student s
WHERE s.StudentId IN (
SELECT d.StudentId
FROM Department d
GROUP BY d.StudentId
HAVING count(d.DepartmentId) = 1
);
Result:
+-----------+-------------+
| studentid | studentname |
+-----------+-------------+
| 3 | Andy |
| 4 | Joe |
+-----------+-------------+
DEMO
Simply join them & aggregate them by Group by clause with having.. count() function
SELECT s.StudentId, s.StudentName
FROM student s
JOIN Department d ON d.StudentId = s.StudentId
GROUP BY s.StudentId, s.StudentName
HAVING COUNT(d.DepartmentId) = 1;
Result :
studentid studentname
3 Andy
4 Joe
Another approach is to eliminate the students having multiple departments at the beginning by using a cte table, then use the table with IN keyword like:
with cte as (
select StudentId
from Department
group by StudentId
having count(StudentId)=1
)
select *
from Students
where StudentId in (select StudentId from cte)
Is there any way to display two person name in same row based on their role.
I having 4 tables which is person, student, teacher and TourAttend. Person is the parent of the two table student and teacher.
Person
-------
personID
Name
Role
Student
--------
StudentID
personID reference person
Teacher
-------
teacherID
personID reference person
TourAttend
-----------
TourID reference to Tour table (Which i didnt reveal)
TourAttendID primary key
PersonID reference person
example values of
PersonID | Name | Role
-----------------------
1 | student1| student
2 | student2| student
3 | teacher1| teacher
4 | teacher2| teacher
5 | student3| student
example values for TourAttend
TourID | tourAttend | PersonID
1 | 1 | 1
1 | 2 | 2
1 | 3 | 3
2 | 4 | 4
2 | 5 | 5
i would to get a query result that is each student is accompanied by who (teacher) Based on the Tour
Student Name | Teacher Name
student 1 | teacher 1
student 2 | teacher 1
student 3 | teacher 2
so far i only tried is
select person.name as 'student name' , person.name as 'Teacher name' from person
and i am stuck because the person.name is used for both. how do i resolve this problem?
This should work for you
select
t1.name as student_name, t2.name as teacher_name
from
(
select
p.name, t.TourID
from
Person p
inner join
TourAttend t
on
p.PersonID = t.PersonID
where
p.role = 'student'
) t1
inner join
(
select
p1.name, t3.TourID
from
Person p1
inner join
TourAttend t3
on
p1.PersonID = t3.PersonID
where
p1.role = 'teacher'
) t2
on
t1.TourID = t2.TourID;
I'm using SQL Server. I find myself doing complex queries in the WHERE clause with the following syntax:
SELECT ..
WHERE StudentID IS NULL OR StudentID NOT IN (SELECT StudentID from Students)
was wondering if there's a better approach/more cleaner way to replace it with because this is a small example of the bigger query I'm doing which includes multiple conditions like that.
As you can see I'm trying to filter for a specific column the rows which its column value is null or not valid id.
EDIT
Courses:
|CourseID | StudentID | StudentID2|
|-----------------------------------|
| 1 | 100 | NULL |
| 2 | NULL | 200 |
| 3 | 1 | 1 |
Students
|StudentID | Name |
|--------------------
| 1 | A |
| 2 | B |
| 3 | C |
Query:
SELECT CourseID
FROM Courses
WHERE
StudentID IS NULL OR StudentID NOT IN (SELECT * FROM Students)
OR StudentID2 IS NULL OR StudentID2 NOT IN (SELECT * FROM Students)
Result:
| CourseID |
|-----------|
| 1 |
| 2 |
As you can see, course 1 and 2 has invalid students.
Alain was close, except the studentID2 column is associated with the courses table. Additionally, this is joining each studentID column to an instance of the students table and the final WHERE is testing if EITHER of the student ID's fail, so even if Student1 is valid and still fails on Student2, it will capture the course as you are intending.
SELECT
C.CourseID
FROM
Courses C
LEFT JOIN Students S
ON C.StudentId = S.StudentId
LEFT JOIN Students S2
OR C.StudentId2 = S2.StudentId
WHERE
S.StudentId IS NULL
OR S2.StudentID IS NULL
this is not a sure shot but i have had experience that this is better performer than the question one:
SELECT CourseID from Courses WHERE
Courses.StudentID NOT exists (SELECT 1 FROM Students where Students.StudentID=nvl(Courses.StudentID,-1));
Also Create an index on StudentId in the Students Table.
And if your data model supports create a primary key foreign key relationship between the 2 tables. That way u definitely avoid invalid values in the courses table.
After your update:
SELECT CourseID from Courses WHERE
Courses.StudentID NOT exists (SELECT 1 FROM Students where Students.StudentID=nvl(Courses.StudentID1,-1) or Students.StudentID=nvl(Courses.StudentID2,-1));
The NOT EXISTS pattern is fine, however, you have several ways to do that.
You should check here and here
For example with LEFT JOIN (two left joins since two variables are checked)
SELECT *
from Courses
LEFT JOIN Students Student1
on Courses.StudentId = Student1.StudentId
LEFT JOIN Students Student2
on Courses.StudentId2 = Student2.StudentId
WHERE
-- No matching Student
student1.StudentId IS NULL
and student2.StudentId IS NULL
Hi I have an "Student" table with below records
INSERT into Student(studId,name)
Values(1,A)
values(2,B)
values(3,C)
values(4,D)
I have a "Department" table with below records
INSERT into dept(deptId,Deptname,Emp Id)
Values(D1,Phy,1)
values(D2,Maths,2)
values(D3,Geo,3)
How can i find the student who does not belong to any department i.e. in this case the result should be "D".
I know the left outer join would return all the records from the student table but i am only interested to get 1 record i.e.: of student "D".
SELECT *
FROM Student
WHERE studId NOT IN (SELECT EmpId FROM dept)
Find all Student.studId entries that do not already exist in the dept.EmptId column as an entry. (Assuming I read your table correctly)
Ideally though, you should probably break out the "Student" and "Department" and creating a joining table (maybe called) "Student_Department" that links the keys of each table.
+--------------+ +--------------------+ +--------------+
| Student | | Student_Department | | Dept |
|--------------| |--------------------| |--------------|
| studId | <-----| studId | .-> | deptId |
| name | | deptId | --' | name |
| ... | +--------------------+ | ... |
+--------------+ +--------------+
This allows you to only have to define a student and department once, but can assign one student to multiple departments, one department to multiple students, or any combination therein.
Pseudo-code:
select *
from students
where id not in(select a.id from students a
inner join department b where b.id in('D1','D2','D3'))
While I agree with some commenters that this sounds like a homework problem, I'll answer with a question...
I know the left outer join would return all the records from the student table but i am only interested to get 1 record i.e.: of student "D".
OK, let's say you run the following query:
SELECT * FROM Student
LEFT OUTER JOIN dept ON Student.studId = dept.EmpId
You'd get the results:
studId name deptId deptName EmpId
1 A D1 Phy 1
2 B D2 Maths 2
3 C D3 Geo 3
4 D NULL NULL NULL
Can you add a WHERE clause to this query that will filter out only the data you want? :)
SELECT s.name
FROM Student s
LEFT JOIN Dept d ON d.empId = s.studId
WHERE d.empId IS NULL