I have two tables. One is Employee and another is Role. The Employee table contains EmployeeId and EmployeeName. Similarly, the Role table will contain RoleId and RoleName column. An employee will belong to one of several different roles.
I am new to database design and having problem figuring out if it's better to have a RoleId column in the Employee table or to create another table called EmployeeRoleMapping which will contain rows indicating this employee is mapped to this role. What are the pros and cons of both approach?
the most important thing is that you must sure of the relationship between Employee and Role in your application.
In my point of view, an Employee may have one or many roles and vise versa one role may belong to one or many Employees. So that, we have to create a new table called EmployeeRoles and this table has employeed_id and Role_id.
Related
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.
A company is hired by another company for helping in a certain field.
So I created the following tables:
Companies: id, company name, company address
Administrators: (in relation with companies) id, company_id, username, email, password, fullname
Then, each company has some workers in it, I store data about workers.
Hence, workers has a profession, Agreement Type signed and some other common things.
Now, the parent tables and data in it for workers (Agreement Types, Professions, Other Common Things) are going to be the same for each company.
Should I create 1 new database for each company? Or store All data into the same database?
Thanks.
Since "Agreement Types", "Professions" are going to be same for each company, I would suggest to have a lookup table like "AgreementTypes" with columns such as "ID", "Type" and refer "ID" column in "Workers" table. I don't think new database is required, relational databases are used to eliminate data redundancy and create appropriate relationships between entities.
By imagining having one database for one company, it ends up with having one record in "Company" table in each database. "Administrators" & "Workers" are associated with that single record. And other common entities such as "AgreementTypes" will be in other tables.
So, if there is any addition/modification to agreement type, it is difficult to do it in all databases. Similarly, if there is any new entity to be linked to "Company" entity, again all databases needs to be revisited based on assumption that these entities belong to ONE application.
You should have one single database, with a structure something like this (this is somewhat over-simplified, but you get the idea):
Companies
CompanyID PK
CompanyName
CompanyAddress
OtherCompanySpecificData
Workers
WorkerID PK
CompanyID FK
LastName
FirstName
DOB
AgreementTypeID FK
ProfessionID FK
UserID FK - A worker may need more than one user account
Other UserSpecificData
Professions
ProfessionID PK
Profession
OtherProfessionStuff
AgreementType
AgreementTypeID PK
AgreementTypeName
Description
OtherAgreementStuff
Users
UserID PK -- A Worker may need more than 1 user account
WorkerID FK
UserName
Password
AccountStatus
Groups
GroupID PK
GroupName
OtherGroupSpecificData
UserGroups --Composite Key with UserID and GroupID
UserID PK
GroupID PK
Obviously, things will grow a little more complex, and I don't know your requirements or business model. For example, if companies can have different departments, you may wish to create a CompanyDepartment table, and then be able to assign workers to various departments.
And so on.
The more atomic you can make your data structures, the more flexible your database will be as it grows. Google the term Database Normalization, and specifically the Third Normal Form (3NF) for a database (Considered the minimum for efficient database design).
Hope that helps. Feel free to elaborate if you are stuck - there is a lot of great help here on SO.
By ref table I'm referring to a kind of entity 'type' table,
and by storage table I'm referring to a table that stores a lot of changing information.
For example:
I have a 'user' table named as such, which is a storage table since it can hold an indeterminate amount of users.
Then i have 'roles' table, which holds role information, it is a type table, as there are many users for each role.
I then have a 'profiles' table, which hold a one to one relationship with the 'user' table.
Now, I've tried this:
user
userrole
userprofile
However, this convention to me implies that the roles and profile tables each have a one to one relationship with user whereas I know the role table does not.
How to people usually name tables for semantic purposes for the example I described?
The Oracle convention works well here.
plural for normal tables (Users for a table of Users. The table itself is a store of many users, so I name it as I would describe the data within it)
User_Roles would be a distinct list of roles.
User_Role_Assignments a list of roles for users. I would imagine you would want a many to many here if a user can have any role and a role can be assigned to any user.
User_Profiles would be a distinct list of profiles.
User_Profile_Assignments for a table which was a many to many relationship between users and profiles.
If you have a one to one relationship, then one of the tables should have a key to the other. Users should have a profile_id if there is a one to one relationship.
In my database I got a list of companies with columns CompanyId and Name.
Further on there is users and those users can add products.
Tables: user and user_products
There is no product table defining products, they are unique per user therefore "user_product"
When a user adds a product he types a name of a company. If that company name exists in the company table I want to make a connection to the company table instead of saving only the name on the user_product. So far so good.. I just store CompanyId in the user_product table.
The problem is when the user enters a name that doesn't exists in the company table. Instead of saving the name as varchar, I want to create a new record in a table called user_company.
The table got columns: UserCompanyId (PK), UserId, Name. If the combination Name and UserId already exists i will of course not create a new row, just reference to this id.
What should I do to maintain a good database design here.. Should i add this record and also a new column in user_product called UserCompanyId. So that either CompanyId or UserCompanyId is always set when adding a new row. It feels like this could be done in a better way. Anyone got any ideas?
I could of course only have one table "company" and have a column UserId which is null when it's a global company added by the system, or the UserId is actually set when a user has added a company name that didn't existed globally. This doesn't feel good either...
Actually, I think you nailed it in your last paragraph. A company is either defined by a user or isn't, so the userId makes sense as a nullable column. This would also allow you to have a unique key on the company name, which allows you to use the database to enforce the fact that a company name can't be duplicated.
Your company table exists to define companies--which user (or whether a user) created the company is just information ABOUT a company.
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.