I'm having a little bit of trouble with the following relationship between two tables:
Say, there's two tables, one being Student, the other being Module. 1 student can have 1 or many modules, however, 1 module can technically have 0 or more students. How would I implement this in a database? Thanks.
This is an N:N relationship. The answer is that you have to create a middle table that will create a link between your Student table and your Module table.
The middle table, which you can name StudentByModule for example, will hold a key identifying the student and another key identifying the module. This assumes that you have created a proper key in both tables.
http://en.tekstenuitleg.net/articles/software/database-design-tutorial/many-to-many.html
As for the case where a module is not assigned to any student this will be modelled by the absence of rows in the middle table linking the module to a student.
Also note that in order to associate a student to a module, you will first have to make sure that both the student and the modules are created first.
It's actually a many-to-many relationship. You implement it using a third table to link your tables
Student table
--
StudentId PK
...
Module table
---
ModuleId PK
...
StudentModule table
---
StudentId FK
ModuleId FK
The last table has a record only if there's a link between a student and a module. You insert primary keys of both Student and Module tables to make them linked.
Related
Let me try to make it simple with an example.
I am creating a database with 2 tables, School and Students
School table with 2 columns SchoolID(PK) and Location
Student table with 3 columns StudentID(PK), SchoolID(FK) and Grade
The Student table will have students from all the schools which can be identified by the foreign key SchoolID. The StudentID column will be unique to each student across all schools. Well and good so far.
I need another column in Student table which will be unique only with respect to the foreign key value. Let's call this ID. I need this column to be unique only to each SchoolID. So, If I filter out the students belonging to a particular SchoolID, I shouldn't get 2 students with same ID.
I'm not sure if that's a valid scenario.
Edit:
This is good
This is not
I think there is something wrong with the datamodel. Since you have StudentID as primary key in Student-table, this column will always be unique in this table. But it seems like you are trying to create a Student-School table where you can have the same student connected to multiple schools (but no the same school multiple times for one single student). I think you need at least 3 tables:
Student (Pk StudentID)
School (Pk SchoolId)
StudentSchool
The StudentSchool table will have two FK-columns: StudentID and SchoolID. To protect from the same student beeing mapped to the same school multiple times you could either have the PK include StudentId and SchoolId or create a Unique constraint.
I have two tables in a database Faculty and Project. In project table there's a supervisor id and co-supervisor id which acts as a foreign key in project table and both are referenced to Faculty Id in Faculty table. Simply I have two one-to many relations in my database between these two tables. My question is that Is there any specific name we used to call this type of relationships?
Faculty has an ”one to many” relationship with Project.
i have a database for university so the students take many courses but each student have id as primary key also each course have an id as primary key its not allowed to have duplicate id at same table what should i use to make a table that same id but different courses id
In a normalized relational database, when you have a situation where items from one table can be associated with many items in a second table, and likewise those related items can be associated with many items in the first table, you should use a third table to store those relationships. In your case, that table might be called Enrollments.
You can use JOIN operations on that third table to relate, group, and aggregate across that relationship. You can also store any metadata associated with that relationship in the JOIN- table, for example you might store the enrollment date there.
Like vicatcu said, create a third table
Course_id | Student_id
The ID's of the courses and the students are uniq in their own tables. The relation between course < - > student is stored in that third table.
Here is the problem I'm trying to solve:
Entity A : a_id
Entity B : b_id
One A can use Many B's. However, not all Bs are used by all As.
Here is the best example I can think of :
One teacher has many students.
Some students are taught by more than one teacher.
What is a relationship so I can add/remove students being taught by one teacher, but not affect the teachers already teaching said students?
you need a third table known as mapping between this two tables.create this table so:
Table Student_Teacher_Mapping
Id (Int)
TeacherId(Int) // foreign key for teacher table
StudentId(Int) // foreign key for student table
i think this is what you want.
You need a StudentTeacher entity that will relate the two together. It would have an a_id column and a b_id column.
I have two tables
Department
Professor
in which Department has an attribute called HeadID referencing Professor
and Professor has an attribute called DeptID referencing Department
They form a circular relationship.
But the problem is that, how to insert a row to any of these tables?
Oracle complained "parent key not found" after I tried insert a row.
You can define one of the foreign key constraints as DEFERRABLE and defer constraint checking until the end of your transaction (instead of checking at the end of statement which ends with "parent key not found"). Read here
The other solutions described here are simpler.
But if you really want the DB to describe your buisiness (which is not necessarily the best approach) then you can have another table, lets say DEPT_HEAD_POSITIONS. the Department table will have the FK (HeadID) refer to this table, and the Professor table will have another nullable field as a FK to this new table.
Now, what you have is:
departments head positions
departments (that must have a head position)
professors (which must belong to a department and may be head of the department)
It is possible for a foreign key consisting of multiple columns to allow one of the columns to contain a value for which there is no matching value in the referenced columns, per the SQL-92 standard. To avoid this situation, create NOT NULL constraints on all of the foreign key's columns
for reference
so I think you can insert data in one of the row without giving value in foreign key column and then insert row into second row referring value of primary key in the first table and then you can proceed ...
If you have the authority to redesign the schema you should. If not I think the simplest and best approach is described in deathApril's comment.
In the use case where you want to add a new department and a new professor who heads it, you're best of:
Adding the Professor under a different department
Adding the Department with the Professor from Step 1 as head
Updating the Professor record from Step 1 to refer to his new Department created in Step 2
Oracle and SQL Server do not allow circular referencing because there is always a problem when deleting a row from a table having dependencies to another row from another table (foreign key) which refers to the row being deleted.....
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