My professor is telling me to have 2 relationships between my employee and department entities, one for the employee relationship and the other for the manager relationship (a manager is also an employee). What attributes do I connect them on? Do I need another primary/foreign key?
I intend making manager id a primary key in the department table, but I'm not sure if that is correct.
It seems to me that the departmentId has to be the primary key so that you don't have duplicate departments. If that's true, then in the Employee table, I would have one relationship from Employee.Department_ID to Department.Department_ID to define which department the employee belongs to. Then I would have a second relationship from Employee.Employee_ID to Department.Manager_ID which defines which employee is the department manager.
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".
conceptual ERD of a recursive relationship
This relationship is employee mentor employees and employees mentored by employee.
since this is an Optional both side and one to many relationship, you just need to create a foreign key mentor(Fk) inside the employee table and remove the recursive line form the table.
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 need to model this problem :
One faculty may supervise several students or not. One student has at
least one, at most two supervisors.
I'm looking for actual Relational table design on how to do this.
The easiest (somewhat bold) way to model this is
a STUDENT table
a FACULTY table
Now give STUDENT columns FACULTY1 and FACULTY2. Both are foreign keys to FACULTY. Constrain FACULTY1 with NOT_NULL. This enforces that a student has at least one FACULTY. FACULTY2 can be null, but it still a foreign key.
FACULTY somehow does not know anything about STUDENT. When no STUDENT references a FACULTY it has no students and any number of Students can reference faculty.
This model has a number of drawbacks (I said it is bold):
A STUDENT being supervised by only one FACULTY must have this set in FACULTY1 (not FACULTY2). Likewise when a student who was supervised by 2 FACULTIES deletes one of them, you can only delete FACULTY2 or you must first swap the fields. You can circumvent this by a more clever constraint (FACULTY1 is not null or FFACULTY2 is not null)
If you ever want to change the design, so a STUDENT can have 3 FACULTIES you need to add columns to STUDENT. This however, is not as bad as it may sound.
On the pro side, there is nothing but referential integrity and NOT NULL involved in this design.
I am designing a database and my tables are as follows
A company can have many departments,and a department belong to one company
A company can have many Employees, and a employee belong to one company
A department can have many employees and a employee belong to one department
Company(
ID,
Name,
)
Department(
ID,
Title,
CompanyID (FK_Department_Company)
UNIQUE(Title,CompanyID)
)
Employee(
ID,
Fname,
LName,
CompanyID, (FK_Employee_Company)
DepartmentID (FK_Employee_Department)
)
So i found that there is possibility to insert wrong data
into the Employee table
here are the sample values
company (
25,Spar Pvt LTD, Sweden, 12345678.
26,Mr.Wheel Pvt Ltd, Germany, 123456789)
Department(
101, Manager,25
102, Front Desk,25
103, Host,26)
Employee( 81,25,103,....)
Here DepartmentID 103 belongs to CompanyID 26, but still I can insert,
So is there anyway to solve this problem (sqlserver),
or i have to change the design, please give me some suggestions and ideas.
Regards,
You don't need CompanyID in the Employee table
From 2 of your statements above
A company can have many departments,and a department belong to one company
A department can have many employees and a employee belong to one department
This implies
A company can have many Employees, and a employee belong to one company
Therefore, the Employee-Company relationship is implied by the intermediate Department table. A separate foreign key/column is not needed
tl;dr: Identified by this assertion: "...a employee belong to one company".