I have a problem with a many-to-many relation in my tables, which is between an employee and instructor who work in a training centre. I cannot find the link between them, and I don't know how to get it. The employee fields are:
employee no.
employee name
company name
department job title
business area
mobile number
ext
ranking
The Instructors fields are
instructor name
institute
mobile number
email address
fees
in a many-to-many relationship the relationships will be in a 3rd table, something like
table EmployeeInstructor
EmployeeID
InstructorID
to find all the employees for a specific instructor, you'd use a join against all three tables.
Or more likely there will be classes involved --
Employee takes Class
Instructor teaches Class
so you'll have and EmployeeClass table,
an InstructorClass table,
and join through them. And Class needs to be unique, or else you'll need
Class is taught in Quarter on ClassSchedule
and end up joining EmplyeeClassSchedule to InstructorClassSchedule.
This ends up being one of your more interesting relational designs pretty quickly. If you google for "Terry Halpin" and "Object Role Modeling", this is used as an illustrative situation in the tutorial.
First of all, you will need a unique key in both tables. The employee number may work for the employee table, but you will need another for the instructor table. Personally, I tend to use auto incrementing identity fields called ID in my tables. This is the primary key.
Second, create a new table, InstructorEmployee. This table has two columns, InstructorID and EmployeeID. Both fields should be indexed. Now you can create an association between any Employee and any Instructor by creating a record which contains the two IDs.
Related
We had a discussion today related to W3 lecture case study about how many entities we need for each situation. And I have some confusion as below:
Case 1) An employee is assigned to be a member of a team. A team with more than 5 members will have a team leader. The members of the team elect the team leader. List the entity(s) which you can identify in the above statement? In this cases, if we don't create 2 entities for above requirement, we need to add two more attributes for each employee which can lead to anomaly issues later. Therefore, we need to have 2 entities as below:
EMPLOYEE (PK is employeeId) (0-M)----------------(0-1) TEAM (PK teamId&employeeId) -> 2 entities
Case 2) The company also introduced a mentoring program, whereby a new employee will be paired with someone who has been in the company longer." How many entity/ies do you need to model the mentoring program?
The Answer from Lecturer is 1. With that, we have to add 2 more attributes for each Employee, mentorRole (Mentor or Mentee) and pairNo (to distinguish between different pairs and to know who mentors whom), doesn't it?
My question is why can't we create a new Entity named MENTORING which will be similar to TEAM in Q1? And why we can only do that if this is a many-many relationship?
EMPLOYEE (PK is employeeId) (0-M)----------------(0-1) TEAM (PK is pairNo&employeeId) -> 2 entities
Thank you in advance
First of all, about terminology: I use entity to mean an individual person, thing or event. You and I are two distinct entities, but since we're both members of StackOverflow, we're part of the same entity set. Entity sets are contrasted with value sets in the ER model, while the relational model has no such distinction.
While you're right about the number of entity sets, there's some issues with your implementation. TEAM's PK shouldn't be teamId, employeeId, it should be only teamId. The EMPLOYEE table should have a teamId foreign key (not part of the PK) to indicate team membership. The employeeId column in the TEAM table could be used to represent the team leader and is dependent on the teamId (since each team can have only one leader at most).
With only one entity set, we would probably represent team membership and leadership as:
EMPLOYEE(employeeId PK, team, leader)
where team is some team name or number which has to be the same for team members, and leader is a true/false column to indicate whether the employee in that row is the leader of his/her team. A problem with this model is that we can't ensure that a team has only one leader.
Again, there's some issues with the implementation. I don't see the need to identify pairs apart from the employees involved, and having a mentorRole (mentor or mentee) indicates that the association will be recorded for both mentor and mentee. This is redundant and creates an opportunity for inconsistency. If the goal was to represent a one-to-one relationship, there are better ways. I suggest a separate table MENTORING(menteeEmployeeId PK, mentorEmployeeId UQ) (or possibly a unique but nullable mentorEmployeeId in the EMPLOYEE table, depending on how your DBMS handles nulls in unique indexes).
The difference between the two cases is that teams can have any number of members and one leader, which is most effectively implemented by identifying teams separately from employees, whereas mentorship is a simpler association that is sufficiently identified by either of the two people involved (provided you consistently use the same role as identifier). You could create a separate entity set for mentoring, with relationships to the employees involved - it might look like my MENTORING table but with an additional surrogate key as PK, but there's no need for the extra identifier.
And why we can only do that if this is a many-many relationship?
What do you mean? Your examples don't contain a many-to-many relationship and we don't create additional entity sets for many-to-many relationships. If you're thinking of so-called "bridge" tables, you've got some concepts mixed up. Entity sets aren't tables. An entity set is a set of values, a table represents a relation over one or more sets of values. In Chen's original method, all relationships were represented in separate tables. It's just that we've gotten used to denormalizing simple one-to-one and one-to-many relationships into the same tables as entity attributes, but we can't do the same for many-to-many binary relationships or ternary and higher relationships in general.
In a new database model that I am designing, I have a company table that has a contacts table related to it using a foreign key companyID field.
I have a branch table with PK branchId and FK companyId.
The problem is: Sometimes contacts can move from the company or branch they are in to a different one. And I need to know all the former companies and branches the contact was in.
What is the best way to do this?
I can't figure out anything that makes sense. If I insert another record into contacts with the different details, There will be a duplicate record, and if I just change the details, how will I know what the former details were?
I would suggest a table that joins your contacts table to your company table. This new table would have a contactsid, a companyid, a date (or date range, depending on your needs) and any other info that you would need to describe the relationship between the contact and the company. The contacts table would then no longer need an FK companyID.
I have two tables in my database, Department and Academic_staff. the primary key in the Department table is depId and the primary key in the Academic_staff table is aNo.
Each department is managed by only one member of academic staff, so the relation between the two tables is one-to-one.
I need to record the date when someone of the academic staff starts managing a department, so the relation must have it's own attribute(mStartDate).
How can I implement this new attribute?
At first I was thinking to create a new table with three attributes (depId, aNo, mStartDate) and make two relations between the new table and the other two tables, but I then realized that it's not many-to-many relationship.
So how can I add the attribute mStartDate to the one-to-one relation between the two tables?
There's more than one relation between the two tables, and some of those relations are one-to-many (the department employs more than one academic staff), so I can't merge the two tables.
Your proposed new table (which I shall refer to as DepartmentManagement) could in principle record the history of managers for each department, in which case it would be a many-to-many (temporal) relationship between Department and Academic.
However, if you want to record only the current manager, it's reasonable to "absorb" DepartmentManager into the Department table, giving two columns there (Manager_aNo and Manager_StartDate). Conceptually the object "DepartmentManagement" still exists, but it's absorbed, it doesn't have its own table.
You could also absorb it in the other direction (into Academic) but that wouldn't allow an Academic ever to manage more than one department. You might not need that now, but in principle it's more likely than having a Department with two managers.
Departments
depId
fk_aNo (Unique )
*****- Primary key (depId, fk_aNo)*****
academicStuffs
aNo (PK)
newTable
depId --- Important "Both depId,aNo are from Departments, be sure"
aNo ---
mStartDate
- Primary key (depId, aNo)
Constraints--> a academicStuff can't start to manage a department more
than one time. If this structure proper for you and you want to enable
a aacademic stuff manage a department more than one time inform me.
Hello I have a Database like three tables:
Customer: Customer ID, Customer Name, Customer Surname
Product: Product ID, Product Name, Product Price
Now I have another table
CustomerProduct: Customer ID, Product ID
Here are all bought stuff standing, while taking the customer id and product id.
I just ask what kind of normalization of database structure this is?
Can you give me an explanation ?
Such a table, which handles many-to-many relationships by storing the two related primary keys (as foreign keys) is called by various names, but junction table seems to be the standard (I personally use association table).
Most relational databases do not handle many-to-many relationships natively - you need an extra table like this.
I want to create an entity relationship diagram for a three way relationship in the following contrived situation:
Each COMPANY has many DEPARTMENTS. An EMPLOYEE can work simultaneously for many DEPARTMENTs. However, an EMPLOYEE can work for only one DEPARTMENT of a COMPANY. In other words, the EMPLOYEE works for as many DEPARTMENTS as COMPANYs.
What kind of associations and relationships will such a diagram have? My purpose is not to create tables in a database but just as an aid to understanding.
Thanks in advance for your help!
COMPANY_EMPLOYEE_DEPARTMENT:
COMPANY (PK)
EMPLOYEE (PK)
DEPARTMENT
Each combination of company and employee, can only be associated with one department. That is, an employee can only work for one department in each company.
This should work as long as a department does not belong to more than one company, and that the COMPANY_EMPLOYEE_DEPARTMENT relation does not link a department of one company with another company.