I have four tables in my data model viz.,
Student,
Elective,
Elective Value, and
Student Elective.
The Student table contains the list of students with the student id as primary key.
The Elective table contains the list of elective the college offers with elective id as primary key.
The Elective Value table contains the list of the possible values for each elective. The primary key for this table is combination of the elective id and value id(Composite primary key), so for next elective the value starts from 1.
The student elective table contains the student id, elective id and value id as primary key. But I have been asked to do the following changes,
Have a id for the elective value table instead of composite primary key and use the primary key(id) in the student elective table. So the link between the elective table and the student elective breaks. May I know what is the best approach the former or the latter?
The second way is generally considered the 'better' way since it is normalized and allows the elective value to change in the future with far less disruption and allows the db engine to optimize queries on the fly. And if the value is a string you may actually save some space in the db. Here's the model as I think you described it:
Student
StudentID
Elective
ElectiveID
ElectiveValue
ElectiveValueID
FK_ElectiveID
Value
StudentElective
StudentElectiveValueID
FK_StudentID
FK_ElectiveValueID
Related
Say if a real life operation like loaning a library book is to be performed on two entities Teacher and Student. And the operation details are present in a table like Loans or Transaction. How do I define a foreign key in the Loans table for both Teacher and Student which have different schemes for primary keys? (student has 6digit key but teacher has 4 digit)
A similar question arises when a hospital database system Appointment table attempts to link to both an Employee table and EmployeeRelatives table using a foreign key, where both Employees of the hospital and their relatives are to be given free treatment. How does one specify format of foreign key in Appointments table?
A foreign key can reference only one table.
My solution in these cases is to introduce an additional table (People or whatever name you prefer) which contains all the IDs of Teacher and Student. Teacher and Student can also have the People table key as foreign key.
You can find other possible solutions here:
Foreign Key to multiple tables
I am making a Django web app and need help designing the a table within the DB.
I am to insert into the table an employee with a specific employee ID. Lets say there are three employees with the IDs (15039, 98443, 29234). Would the employee ID be the primary key or do I have to make some arbitrary column starting from 1 the primary id with employee id as a standalone column?
In a sense what I am i asking is if the 15039, 98443, and 29234 employees were inserted into the table with empl ID being primary key which order would the DMBS order them?
You did not specify which database you will use, but most likely the primary key will be the clustered index, in which case the database will order the rows by that id.
Many argue you should always create an auto-increment artifical primary key, and that usually saves you a lot of pain in the long run.
However, if you know the value will always be unique and you won't ever need to change the value, you can opt to use it as the PK for the table.
This matter confuses me,
I have a College Information system the junction table between students table and subjects(curriculum) table, the primary key is composite key (StudentID, SubjectID) and both of them are Foreign keys but the student may be fail in exam and repeat the subject so we will have duplicate PK and we need to record all data. I have two ways to solve this matter but i don't know the best way?
Add new column as primary Key instead of composite key.
Join to the composite key Season Column and year column and the composite key will be(StudentID, SubjectID, Season, Year). I have to mention that i don't need this composite key as foreign key.
Which way is better for performance and DB integrity?
Subject and exam are separate (if related) concepts, so you should not try to represent them within the same table. Also, the fact that an exam has been held for the given subject is separate from the fact that any particular student took that exam. Split all these concepts into their own tables, and the model becomes more natural, for example:
Representing a student that took the same exam several times is just a matter of adding multiple rows to the STUDENT_EXAM table.
NOTE: STUDENT_SUBJECT just records the fact that the student has enrolled in the subject, but not when (which year/semester). Keeping semester-specific information may require additional tables and more complicated relationships within the model.
NOTE: There is a diamond-shaped dependency in this model. Since SUBJECT_ID was passed from the "top" (SUBJECT), down both "sides" (STUDENT_SUBJECT, EXAM) and then merged at the "bottom" (STUDENT_EXAM) of the diamond, a student cannot take an exam on a subject (s)he has not enrolled in.
How can we make a constraint on the database, so that only one entity fits in a desired position?
For example, we want to have a president for the Country database.
How can we define the "is-president" attributes so that only one entry can have the "true" value.
You can do it when inserting the data to your table. If you want to add a president, first check whether there is a president or not.
You're better off modelling this in a different way - for example having a Countries table, a People table, and then a Presidents table which has a foreign key to the Countries table, and a foreign key to the People table.
You can then add a unique constraint on the composite value of both foreign keys to ensure only 1 person exists per country within the Presidents table.
EDIT: my revised Entity relationship diagram
A Student can have many contact times but this does not relate to what course they are on. So the courseID in tblContact was unnecessary, so I used to Primary keys in tblStudent relating to the grade for a particular tutor marked assignment and the course a particular student is on with that TMA.
Phew
http://i.imgur.com/cf3td.png
/Edit
My Old ERD
note that StudID and CourseID are a merged compound primary key
My question: Should I have studID and courseID in tblContact? or should I just have StudID, because I'm using a compound primary key I thought I should have both values in tblContact and tblStudentTMA?
Is this right?
The answer depends on whether the contact is related to a course or not.
If it relates to a course then you need some way of identifying the Course from the contact, but you can link to the tblCourse table from tblContact.
My preference for a many-many table is to use a separate primary key in your example StudentCourseID, which is a Identity column, this removes the need to store multiple foreign keys in a related table.
The primary key in tblContact has to have at least two columns. One of them has to be StudID.
It has to have at least two columns, because you need to store more than one contact per student. One of the columns has to be StudID to guarantee the contact row refers to an actual student. The second column probably needs to be DateOfContact.
A primary key {StudID, DateOfContact} allows one contact per student per day. If you use {StudID, TimeOfContact} instead--use a timestamp instead of a date--you can have more than one contact per student per day.
In addition to that, if every row in tblContact must refer to both a student and to one of that student's courses, then you should probably include CourseID in the primary key. You also need a foreign key reference from tblContact (StudID, CourseID) to tblStudentCourse (StudID, CourseID).
If it's not necessary for every row in tblContact to refer to a course, then tblContact.CourseID should be nullable, and it shouldn't be part of the primary key. But you should still have a foreign key reference from tblContact (StudID, CourseID) to tblStudentCourse (StudID, CourseID).