previously i thought that it's easy to store exams data in simple table like this :
id
subject_id
student_id
mark
date
but i noticed that it is very stupid idea cuz it makes database very huge
let's assume that we have school with 5000 students and there are 12 subjects for each students per month ... it means that our exams table will contain 5000*12*10 = 600000 row per year ,, ok what about if we decided to know student history from joining date to current date
.. i think it's very stupid and i think also there are optimum solutions to make another design to fit my needs ....
i need to store exams data in another lighter way
thank's in advance :)
Here is another way.
Table Student stores name, etc
Table Subject has subject names such as Reading, Writing and Arithmetic, and maybe a MinimumPassingMark.
Table Exam has ExamId, ExamDate, and SubjectID. It may also have PercentageOfOverallGrade
Table StudentExam has StudentID, ExamId, and mark.
Subjects and exams have a one to many relationship (one subject can have many exams, each exam has one subject. Students and exams have a many to many relationship.
So if you want to know if Johnny can read, it's as simple as.
select case
when sum(mark) * PercentageOfOverallGrade >= then 'pass' else 'no' end result
from Student s join StudentExam se on s.StudentId = se.StudentId
join Subject su on se.SubjectId = su.SubjectId
where s.name = 'Johnny'
and su.name = 'Reading'
Related
I'm working on my SQL project using the Oracle database for class, and I'm asked a question that I see far too often.
You have three tables:
STUDENT: SNO, SNAME
CLASS: CNO, CNAME
ATTENDANCE: SNO, CNO, Grade
The question I keep finding is of a similar type: Find the names of the students that attend in all of the classes that "John" (or anyone else) attends.
John attends three classes, so I have to find the students that also attend those three classes (could be more, but those three must be there). However, I won't always know how many classes John (or whoever) attends, so it can't be hardcoded like that.
SELECT jclass.CNO
FROM attendance jclass
INNER JOIN student on jclass.SNO = student.SNO
WHERE student.SNAME = 'John';
This gets me the classes that John attends. I tried to add the identifier for the other students:
SELECT student.SNAME
FROM student
INNER JOIN attendance on student.SNO = attendance.SNO
INNER JOIN class on attendance.CNO = class.CNO
WHERE student.SNAME <> 'John'
AND class.CNO IN (SELECT jclass.CNO
FROM attendance jclass
INNER JOIN student on jclass.SNO = student.SNO
WHERE student.SNAME = 'John');
However, this only gets me the students that appear in at least one of John's classes, rather than all of them. I can see why it's doing this, but I'm not sure how to fix it. It's the one big struggle I'm having with SQL.
Here is one way - assuming SNO is primary key in the first table, CNO is primary key in the second table, and (SNO, CNO) is (composite) primary key in the third table, and that the input student is given by a unique identifier (first name is distinctly NOT a unique identifier, so the problem stated in terms of giving "John" as the input makes no sense). Here I assume the "special" student is identified by SNO = 1001; you can make 1001 into a variable, or change it to a subquery that selects a (unique!!) SNO based on some other inputs.
I didn't try to make the query as efficient as possible, or use features you most likely haven't seen in your class. Rather, I tried to make it as elementary and as readable as possible.
select sno
from attendance
where cno in (select cno from attendance where sno = 1001)
group by sno
having count(*) = (select count(*) from attendance where sno = 1001)
;
The strategy is simple: the subquery in the in condition finds the classes attended by the "special" student, then from the attendance table we select only rows for those classes. Group by student, and count. Keep only the students for whom the count is equal to the total count for the "special" student. Note the last condition is about groups, not about input rows, so it belongs in the having clause.
When learing about joins, our instructor says to not skip tables.
For example, lets do a query that Selects the Last_Name, First_name, and Numeric_Grade.
I would write
Select Last_Name, First_Name, Numeric_Grade
From Student
Join Grade
Using(Student_id)
He says to write
Select Last_Name, First_Name, Numeric_Grade
From Student
Join Enrollment
Using(Student_id)
Join Grade
Using(Student_id)
Im confused because as long as long as i can link them through similar fields, i dont see the point of going enrollment.
He has not given me a reason for going through enrollment, other than its what the Diagram shows. Follow the diagram.
Do I have to go through Enrollment? Is it the safe way to do it, or does it not matter because Grade and Student have a Student_id primary key?
Quoting Alice Rischert in Oracle SQL By Example, lab 7.2:
The second choice is to join the STUDENT_ID from the GRADE table directly to the STUDENT_ID of the STUDENT table, thus skipping the ENROLLMENT table entirely. - - This shortcut is perfectly acceptable, even if it does not follow the primary key/foreign key relationship path. In this case, you can be sure not to build a Cartesian product because you can guarantee only one STUDENT_ID in the STUDENT table for every STUDENT_ID in the GRADE table. In addition, it also eliminates a join; thus, the query executes a little faster and requires fewer resources. The effect is probably fairly negligible with this small result set.
The only reason to go through the Enrollment table would be if you need information (fields) from that table. If both the Enrollment and Grade table have a Student_id field then you wouldn't need to go through Enrollment to get there.
In your example it looks like you are looking for First and Last Name, which should both come from the Student table and Numeric_Grade which should come from the Grade table. In this instance, there would be no need for the Enrollment table. If there were a WHERE clause that required something from the Enrollment table then yes you would need to include it, but your example I would say it is not needed.
If this is a question on a test or assignment and the teacher is requesting you go through the Enrollment table too I would do it just to appease him, but knowing that you don't actually need to do it to get the information that you require.
Depend on your tables. Sometimes you can but sometimes dont.
For example imagine in enrollment you have something like student_quit_course
Then you may only want grade of student actually finish the course and you need all three table
For this particular case you will have a GRADE for several section_id but to know what is that section you need [Section] and [Course] both join using [Enrollment]
I have a table of students and schools. How do I select the total students per school?
I'm sure this is a very simple query, however I'm not sure how to proceed on from this:
SELECT tblSchools.name
FROM tblStudentDetails
INNER JOIN tblSchools
ON tblStudentDetails.schoolId = tblSchools.id
Group by the school and use count() to count the students
SELECT s.name, count(d.id) as students_count
FROM tblSchools s
INNER JOIN tblStudentDetails d ON d.schoolId = s.id
GROUP BY s.name
I want to add on to the accepted answer as well. Working for a school district and continuously having to pull counts of students there are a few additional things to keep in mind. What students are you looking for?
Do you want active students, inactive students, or active and inactive students.
Do you want to include students that have been no showed (were going to come to your school, but ended up not coming for even a day, this is recorded in most student information systems.
Is the student attending multiple schools, in which case you want to exclude them from counting in their second or third school.
I've built the script with the idea of a normalized school district database, where things are broken out by school year, and enrollment.
Often a basic script for me looks a little like this.
SELECT s.SCHOOL_NAME, COUNT(stu.STUDENT_GU) AS STUDENT_COUNT
FROM STUDENT stu
JOIN STUDENT_YEAR sy ON sy.STUDENT_ID = stu.STUDENT_ID
JOIN SCHOOL_YEAR scy ON scy.SCHOOL_YEAR_ID = sy.SCHOOL_YEAR_ID
JOIN SCHOOL s ON s.SCHOOL_ID = scy.SCHOOL_ID
JOIN YEAR y ON y.YEAR_ID = sy.YEAR_ID
WHERE y.SCHOOL_YEAR = 2017
AND (sy.CONCURRENT IS NULL OR sy.CONCURRENT OR != 'Not Concurrent')
AND sy.ENTER_DATE IS NOT NULL
AND sy.STATUS IS NULL
GROUP BY s.SCHOOL_NAME
Because each school year is a new year, students, and schools usually have a table for the basic data that doesn't change. But also tables that are school year specific. In my example STUDENT_YEAR and SCHOOL_YEAR is where we get into the specifics regarding which kids we are actually getting, and where they currently are. I utilize the YEAR table to identify which school year I want to look at.
The STUDENT_YEAR table is where we store the students concurrency flag, enter date, and so within that table I can use in the WHERE clause a way to filter out Inactive Students, Concurrent Students, and ensure each student is counted only once.
If those year values aren't included, at least in my database, I would get all students ever enrolled for every year we've got stored.
I’ve been hours trying to build this query and I need your help so I can make it.
This is table Students (made out of inner joins):
SpecialtyChosenID StudentID Subject SubjectSpecialtyID
5ABFB416-8137 15 Math A1EBF3CB-E899
5ABFB416-8137 15 English A1EBF3CB-E899
The info in it means that a student with id no. 15 has chosen an specialty with id 5ABFB416-8137
The two subjects he has passed (Math and English) belong to a specialty with id A1EBF3CB-E899
What would be the query to know if the passed subjects belong to the specialty chosen by the student??
Counting the number of subjects with the same SubjectSpecialtyID as SpecialtyChosenID and vice versa could do.
Thanks a lot
You can do a self join. This finds the number of subjects taken by the student that match the student's chosen specialities.
SELECT l.SpecialtyChosenID, l.StudentID, Count(Distinct r.Subject) FROM Students l
LEFT JOIN Students r ON (l.StudentID=r.StudentID AND l.SpecialityChosenID=r.SubjectSpecialityID)
GROUP BY l.SpecialtyChosenID, l.StudentID
However, this is quite inefficient using the table structure given. If you have a table listing students with their specialities, and another with subjects and specialities, and a third relating students with subjects, it would be better to build this query from the base data, rather than from the derived data.
SELECT * FROM Students WHERE SpecialtyChosenID = SubjectSpecialtyID
If you only need the list of matching subjects and you have the SpecialtyChosenID you can do something like
SELECT * FROM Students
WHERE SubjectSpecialtyID = SpecialityChosenID
CASE WHEN SpecialtyChosenID = SubjectSpecialtyID THEN 1 ELSE 0 END AS specialty
I'm trying to keep track my students, and those that have not yet booked a lesson with me (I'm a music teacher as well as a CS student) but my attempt is actually giving me all those that HAVE already booked lessons, what is my mistake?
SELECT DISTINCT student.person_id FROM student, lesson
WHERE lesson.student = student.person_id
AND (select count(*)
FROM lesson
WHERE student = student.person_id
AND start_time > NOW()) = 0
Try:
SELECT DISTINCT s.person_id
FROM STUDENT s
WHERE NOT EXISTS(SELECT NULL
FROM LESSONS l
WHERE l.student = s.person_id
AND l.start_time > NOW())
Robert,
Making reasonable assumptions about how you might be storing information, your query selects those students for whom your database lists no future scheduled lesson. It doesn't matter whether a student has a past lesson or not. I'm not sure what you mean by "not yet booked a lesson," because that English phrase is somewhat awkward. Is it different from "not/never booked a lesson"? The words "not yet" indicate not before the present time, suggesting that you keep track of both booked lessons that have already been booked and also booked lessons that are "not yet booked" (whatever that means). Without a crystal ball, I don't know how you keep track of lessons that students have booked, but just not yet.
In any case, if for a particular student your database lists no lessons that start in the future for that student, you will see that student in your results (regardless of any lessons you list for that student that start now or in the past).
There's also an odd thing about your query, and that's the fact that you are joining the tables "lesson" and "student" on lesson.student = student.person_id before restricting the results based on a count. It seems to me that if you want a list of students satisfying some condition, you'd only need "student" in the FROM clause of the outer query, as in rexem's query.
If you want to see those students for whom you have no record of any lesson (regardless of lessons' start times), rexem's query should work. DISTINCT is unnecessary if the table "student" lists each student only once. The advantage of using EXISTS is that with EXISTS, as opposed to COUNT, you don't suggest needing to count up the total number of future lessons for each student before comparing the count with zero. With EXISTS, you suggest no need to keep looking for a student's future lessons once any such lesson is found.