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.
Related
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.
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".
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 not sure how best to phrase the question, but essentially I have a table of contacts, and instead of doing the typical -- a contact has a reference to a table with spouse information, and a table with children, I want each of those people to be a contact, but then define a relationship between those contacts (brother, sister, child, spouse, etc.). So the contacts would exist in a single table, but I'm having trouble determining how best to define the relationship based upon their contact id and the relationship type. Any advice would be appreciated.
CONTACTS table
contact_id, pk
CONTACT_RELATIONSHIP_TYPE_CODE table
contact_relationship_type_code, pk
description
CONTACTS_RELATIONS table
parent_contact_id, pk, foreign key to CONTACTS table
child_contact_id, pk, foreign key to CONTACTS table
contact_relationship_type_code, foreign key to CONTACT_RELATIONSHIP_TYPE_CODE table
If you see the need to support multiple relationship types to a pair of people, add the CONTACTS_RELATIONS.contact_relationship_type_code column to the composite primary key
This is called a self join, it is pretty common and fairly easy to provide the functionallity you mention above. Take a look at this article.
Just implement an intersect table with four columns - key, contactid #1, contact id#2, and relationship.
Why do it this way? Because a contact can have several relationships.
I read here some reasons to use constraints instead of triggers. But I have a doubt. How can be assure (using only constraints), the coherence between SUPERCLASS tables and SUBCLASSES tables?
Whit a trigger is only a matter of check when INS.. UPD...
Is there a way to define that kinda relation by using only constraints (I'm newbie at this), thanks!
You can use constraints to ensure that every ContractEmployees row has a corresponding Employees row, and likewise for SalariedExployees. I don't know of a way to use constraints to enforce the opposite: making sure that for every Employees row, there is a row either in ContractEmployees or SalariedEmployees.
Backing up a bit... there are three main ways to model OO inheritance in a relational DB. The terminology is from Martin Fowler's Patterns of Enterprise Application Architecture:
Single table inheritance: everything is just in one big table, with lots of optional columns that apply only to certain subclasses. Easy to do but not very elegant.
Concrete table inheritance: one table for each concrete type. So if all employees are either salaried or contract, you'd have two tables: SalariedEmployees and ContractEmployees. I don't like this approach either, since it makes it harder to query all employees regardless of type.
Class table inheritance: one table for the base class and one per subclass. So three tables: Employees, SalariedEmployeees, and ContractEmployees.
Here is an example of class table inheritance with constraints (code for MS SQL Server):
CREATE TABLE Employees
(
ID INT IDENTITY(1,1) NOT NULL PRIMARY KEY,
FirstName VARCHAR(100) NOT NULL DEFAULT '',
LastName VARCHAR(100) NOT NULL DEFAULT ''
);
CREATE TABLE SalariedEmployees
(
ID INT NOT NULL PRIMARY KEY REFERENCES Employees(ID),
Salary DECIMAL(12,2) NOT NULL
);
CREATE TABLE ContractEmployees
(
ID INT NOT NULL PRIMARY KEY REFERENCES Employees(ID),
HourlyRate DECIMAL(12,2) NOT NULL
);
The "REFERENCES Employees(ID)" part on the two subclass tables defines a foreign key constraint. This ensures that there must be a row in Employees for every row in SalariedEmployees or ContractEmployees.
The ID column is what links everything together. In the subclass tables, the ID is both a primary key for that table, and a foreign key pointing at the base class table.
Here's how I'd model a contract vs salary employee setup:
EMPLOYEE_TYPE_CODE table
EMPLOYEE_TYPE_CODE, pk
DESCRIPTION
Examples:
EMPLOYEE_TYPE_CODE DESCRIPTION
-----------------------------------
CONTRACT Contractor
SALARY Salaried
WAGE_SLAVE I can't be fired - slaves are sold
EMPLOYEES table
EMPLOYEE_ID, pk
EMPLOYEE_TYPE_CODE, foreign key to the EMPLOYEE_TYPE_CODE table
firstname, lastname, etc..
If you're wanting to store a hierarchical relationship, say between employee and manager (who by definition is also an employee):
EMPLOYEES table
EMPLOYEE_ID, pk
EMPLOYEE_TYPE_CODE, foreign key to the EMPLOYEE_TYPE_CODE table
MANAGER_ID
The MANAGER_ID would be filled with the employee_id of the employee who is their manager. This setup assumes that an employee could only have one manager. If you worked in a place like what you see in the movie "Office Space", you need a different setup to allow for an employee record to associate with 2+ managers:
MANAGE_EMPLOYEES_XREF table
MANAGER_EMPLOYEE_ID, pk, fk to EMPLOYEES table
EMPLOYEE_ID, pk, fk to EMPLOYEES table
Databases are relational and constraints enforce relational dependencies pretty well, been doing so for some 30 years now. What is this super and sub class you talk about?
Update
Introducing the OO inheritance relationships in databases is actually quite problematic. To take your example, contract-employee and fulltime-employee. You can model this as 1) a single table with a discriminator field, as 2) two unrelated tables, or as 3) three tables (one with the common parts, one with contract specific info, one with fulltime specific info).
However if you approach the very same problem from a traditional normal form point of view, you may end up with a structure similar to 1) or 3), but never as 2). More often than not, you'll end up with something that looks like nothing you'd recommend from your OO design board.
The problem is that when this collision of requirements happens, today almost invariably the OO design will prevail. Often times, the relational model will not even be be on the table. Why I see this as a 'problem' is that most times databases far outlive their original application. All too often I see some design that can be traced back to a OO domain driven design session from an application long forgotten, and one can see in the database schema the places where, over time, the OO design was 'hammered' into place to fit what the relational engine underneath could support, scale and deliver. The tell sign for me is tables organized on a clustered index around a identity ID when no one ever is interrogating those tables for a specific ID.