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.
Related
Perhaps, this could seem like the basics of database design.But,certainly i find these concepts a lil tricky in the sense that how they relate to each other.
Theory as per my understanding.
One to One :
When each particular entity has one record.
One to Many :
When each particular entity has many record
Many to Many :
Multiple entities have multiple records.
As per the above if i could relate an example as
ONE TO ONE
Each employee having a unique passport number.
Table Employee
Empid(pk)
empname
passpordid(fk to passport)
Table passport
passportid(pk)
passportno
ONE TO MANY
An organisation having multiple employees
TABLE ORGANISATION
ORGID (PK)
ORGNAME
EMPID (FK TO EMPLOYEE)
TABLE EMPLOYEE
EMPID (PK)
EMPNAME
SALARY
This is the part that i want to know more that is many to many. I mean if we see one to many here. As a whole it could be said as many to many as many organisations having many employees but does that mean the whole relationship is many to many not one to many or one to many is a subset of many to many in the above example.
I wanna know the difference mainly between one to many and many to many both theoritically and by implementation.
An example of a many-to-many relationship would be EMPLOYEES and SKILLS, where SKILLS are things like "SQL", "Javascript", "Management" etc. An employee may have many skills (e.g. may know SQL and Javascript), and a particular skill by be possessed by many employees (e.g. Jack and Jill both know SQL).
In a database like Oracle, you need a third table to express the many-to-many relationship between EMPLOYEES and SKILLS:
create table EMPLOYEE_SKILLS
( empid references EMPLOYEES
, skillid references SKILLS
, constraint EMPLOYEE_SKILLS_PK primary key (empid, skillid)
);
Note that this third table has a foreign key to both of the other tables.
The table can also hold further information about the relationship - for example:
create table EMPLOYEE_SKILLS
( empid references EMPLOYEES
, skillid references SKILLS
, rating number
, date_certified date
, constraint EMPLOYEE_SKILLS_PK primary key (empid, skillid)
);
This is more a theory question that a programming one, but why not.
You could imagine a "many to many" as 1 table being a list of employees, and another table being a list of products sold.
Each employee is likely to "handle" more than 1 product, and each product could very well be handled by multiple employees, and it's quite possible that products A and B can be handled by, at the same time, employees C and D.
You seem to have the basic idea of one-to-one and one-to many down, in theory.
One-to-one: each item A has a separate and unique item B
In a database, you could include the new data in the same table. You could put it in a separate table, and enforce the one-to-one constraint by making the id into the remote table "unique" (indexing constraint), forcing the id for item B to not be repeated in the table. In your example, this would be a constraint added to the passportid.
One to many: each item A has some number of item Bs
In a database, you put the key in the table on the "many" side. So many Bs would have the same foreign key into table A. In your example, put an orgid as a foireign key into the employee table. Many employees can then point to a single organization, and an employee can belong to one organization.
Many to many: some number of As can be linked to any number of Bs and vice-versa
In a database, this is usually implemented as a link table, having just the IDs from both A and B. Following you example, consider that an employee can be part of multiple groups (or projects), multitasking across those groups. You could have a group table:
GROUPID (PK)
GROUPNAME
And the link table would then look like
LINKID (PK)
EMPID (FK TO EMPLOYEE)
GROUPID (FK TO GROUP)
This lets any employee be part of any number of groups, and groups to have any number of employees, AKA many-to-many.
A side comment:
Technically, you don't HAVE to have a LINKID in the link table. But it makes it easier to manage the data in that table, and is generally considered "best practice".
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.
I have to create two tables in SQL one with employees and one with their projects. The problem is: one employee can work to one or more projects and on a project can work one or more employees and I don't know how to create these 2 tables and how to put the foreign keys. I guess the primary keys are the IDs.
This is a many-to-many relationship. You need three tables: Employee, Project, and EmployeeProject. EmployeeProject would have two columns EmployeeId and ProjectId. Its sole purpose would be to represent the relationship between Employee and Project.
I think you should create third table which is link employee to projects
like below:
Project Table:
ID ProjName Date ...
Employee Table:
ID EmpName ....
Link Table:
ID EmpID ProjID
Just curious if I can have the same primary key in 3 different tables? I am going to create an Employee, FullTime and PartTime tables. I would like to make an EmployeeID the primary key for all 3. Any thoughts?
You can have the primary key EmployeeId in a table called Employees. This would have common information, such as date of hire and so on.
Then, each of your subtables can have an EmployeeId that is both a primary key in the table and a foreign key reference to Employees.EmployeeId. This is one way to implement a one-of relationship using relational tables.
Unfortunately, unless you use triggers, this mechanism doesn't prevent one employee from being in the two other tables, but that is not part of your question.
It sounds like your design is wrong.
The entity is the employee
An attribute of an employee is their [current^] employment status.
Therefore, in its simplest form, you need a single employee table, with a column that indicates their status.
To improve this further, the employee status column should have a foreign key relationship with another table that stores the possible employee statuses.
^ current status is a 1:1 relationship. If you wanted the history of changes, this is a 1:M and needs modelling differently.
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.