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.
I'm creating a db schema that involves users that can have multiple users.
I want to register different companies to use the web services.
For example:
user A or B (etc) can signup and create a company account
user A can create multiple accounts of other users with their types, similarly user B
If user A or B create different accounts, how would I know this particular user is belong to User A or B company ? I think user table have many to many relationship with itself (like basic friendship design).
Please suggest the best design .
Ex.
User 3,4 belongs to User A
User 5,6 belongs to User B
In general, I would recommend starting by identifying all the entities you are trying to persist. It sounds like you have two distinct entities in your question. One being "user," which represents a single person. Your second entity is "company." A "user" can belong to a company.
An example of a database design would be one table for users, and one table for companies. In the "users" table, you would want to have a foreign key column that references the primary key (unique id) of the company the user belongs to. If each user can only belong to one company, this becomes a simple one to many relationship.
In short, I would highly recommend treating company accounts separately from user accounts, since they are fundamentally different entities.
in my database i have admins , merchants and drivers .
All of them share many attributes like name/email/phone/credentials..etc
So , the way i designed my database is a tabled called "users" and table for each role [admin,merchant,driver] where shared attributes are stored in users table.
My questions is do i have to maintain an ID for each role for example [ driver_id , merchant_id ] and link it to the user_id or depending only on the user_id and storing the type of the user in users table ?
Depending on the type of queries you'll run on your database, you could have a few different options. If you're going to query the user table without needing information from the role-specific tables, then just store the type of user i.e. role in the users table.
However, if you are going to be including role-specific information that is in the admin, merchant, or driver tables, then it may be beneficial to include a unique merchant_id, admin_id or driver_id in the user table to designate a role and simultaneously act as ID into the appropriately associated table.
Diagramming this in database modeling software can help to identify the relationships between different data pieces within your application to better design your tables and attributes.
I have a database with a "users" table containing data about my users. Each user is to be linked to a company or a college. I wish to have two separate tables "college" and "company" each with a field "ID". So how do I link each record in the users table to either a company or a college?
The basic thing is that I wish to establish an "OR" relationship in the database.
You can use subtype/super-type relationship. Keep all common fields in the organization table. College and company tables contain only fields specific to those entities.
You could use an 'institution' or 'organisation' lookup table, with a structure something like
InstitutionId[PK], InstitutionType, LookupKey
where LookupKey is the PK to either Company or College.
Or,
InstitutionId[PK], CompanyId[FK], CollegeId[FK]
In both cases you link from user to institution, then onto Company and/or College.
I personally prefer the second option, because it allows you to easily validate the FK relationship and also allows (if applicable) for a user to be a member of a company and/or a college.
I would create relationtables. UserCollege and UserCompany. This way you are even able to have users that are linked to both if needed in the future. If not you simply just create a relationrecord on one of the both
I have several entities which respresent different types of users who need to be able to log in to a particular system. Additionally, they have different types of information associated with them.
For example: a "general user", which has an e-mail address and "admin user", which has a workstation number (note that this a hypothetical case). Both entities also share common properties like first name, surname, address and telephone number. Finally, they naturally need to have a (unique) user name and a password to log in.
In the application, the user just has to fill in his user name and password, and the functionality of the application changes slightly according to the type of the user. You can imagine that the username needs to be unique for this work.
How should I model this effectively?
I can't just create two tables, because then I can't force a unique constaint on the user name.
I also can't put them all in just one table, because they have different types of specific information associated to them.
I think I might need 3 seperate tables, one for "users" (with user name and password), one for the "general users" and another one for the "admin users", but how would the relations between these work? Or is there another solution?
(By the way, the target DBMS is MySQL, so I don't think generalization is supported in the database system itself).
Your 3 tables approach seems Ok.
In users table have only ID, username, password,usertype.
In general users table have ID, UserID (from users table), other fields.
Same thing for admin users.
Usertype field will tell you from what table to search for additional info
if(usertype==admin)
select * from admins where userid=:id;
else
select * from general where userid=:id;
Two tables. USERS with user names, first, last, etc. ROLES with roles, and a link back to the user name (or user id or whatever). Put a unique constraint on the user name. Put workstation nbr, email, phone, whatever else you need, in the user table. Put 2 columns in the ROLES table -- USERID and ROLE.
You should decide how much specific information is being stored (or likely to be stored in the future) and make the decision based on that. If there are only a handful of fields for each user type then using a single table is alright.
USERS table (name, type, email, password, genfield1, genfield2, adminfield1, adminfield2)
Make sure to include the type (don't assume because some of the fields particular to that user are filled in that the user is of that type) field. Any queries will just need to include the "AND usertype = " clause.
If there are many fields or rules associated with each type then your idea of three tables is the best.
USERS table (ID, type, name, password)
GENUSERS (ID, genfield1, genfield2)
ADMINUSERS(ID, adminfield1, adminfield2)
The constraints between IDs on the table are all you need (and the main USERS table keeps the IDs unique). Works very well in most situations but reports that include both types of users with their specific fields have to be done in two parts (unioned SQL or subqueries or multiple left joins).
You can solve it with one 'general' users table containing the information thats available for all users and 1 table for every specific user type. In your example you will then need 3 tables.
Users: This table holds only information shared between all usertypes, ie. UserId, Name, Address, etc.
GeneralUsers: This table 'extends' the Users table by providing a foreing key UserId that references the Users table. In addition, information specific to general users are held here, fx. EmailAddress, etc.
AdminUsers: As with GeneralUsers, this table also 'extends' the Users table by providing a foreign key UserId referencing the Users table. In addition information specific to admin users are held here, fx. WorkstationId, etc.
With this approach you can add additional 'specializations' if the need arises by simply adding new tables that 'extends' the Users table using a foreign key reference. You can also create several levels of specialization. If for example admin users are general users as well as admin users then AdminUsers could 'extend' GeneralUsers instead of Users simply by using a foreing key to GeneralUsers instead of Users.
When you need to retreive data from this model you need to which type of user to query. If for example you need to query a GeneralUser you will need something similar to:
SELECT * FROM GeneralUsers
LEFT JOIN Users ON GeneralUsers.UserId = Users.UserId
Or if querying an admin user
SELECT * FROM AdminUsers
LEFT JOIN Users ON AdminUsers.UserId = Users.UserId
If you have additional levels of specialization, for example by having admin users also being general users you just join your way back.
SELECT * FROM AdminUsers
LEFT JOIN GeneralUsers ON AdminUsers.UserId = GeneralUsers.UserId
LEFT JOIN Users ON GeneralUsers.UsersId = Users.UserId
I most definitely would not do a model where you have separate tables as in GeneralUser, AdminUser and ReadOnlyUser.
In database design, a good rule of thumb is "Down beats across". Instead of multiple tables (one for each type), I would create a SystemUsers table, and a Roles table and define a join table to put SystemUsers in Roles. Also, I would define individual roles.
This way, a user can be added to and removed from multiple roles.
A role can have multiple permissions, which can be modified at any time.
Joins to other places do not need a GeneralUserId, AdminUserId and ReadOnlyUserId column - just a SystemUserId column.
This is very similar to the ASP.Net role based security model.
alt text http://img52.imageshack.us/img52/2861/rolebasedsecurity.jpg