I am trying to find best solution for this scenario and have researching but couldn't find a best way.
We are providing Service A to the clients, Service A has set of schema structure. Now we are in the process of building another/different application which will be Service B with separate schema structure. What is the best way that I can keep one consistent customer and their users in both schemas.
I have looked into FK relation with different database but it is not recommended
I am not sold on the replicating the table in the other database
Schema A: Customer table, user table, service A schema
Schema B: Customer table, user table, Service B Schema
Any recommendation or resources you can point me to?
I am using Microsoft Access 2016 and am attempting to use an Access SQL query to create a new table with a one-to-one relationship with an existing table.
I have already set-up the first table of the database - employees.
I have then used another query to create a second table (desks) and to link the EmployeeID field as a foreign key. However, it creates a one-to-many relationship, rather than a one-to-one relationship.
How do I need to alter the queries to create a one-to-one relationship?
Thanks for your help!
EmployeeID in table Employess linking to the unique attribute of table Desks guarantees a 1-to-1 relationship even if the relationship diagram says otherwise.
I would claim that the relationship diagram does not consider unique attributes and therefore shows it as a 1-to-many relationship.
We know that if there's an multivalued attribute in any entity - A different new table must be created. My ER Diagram has an entity named 'Restaurant' which has 2 multivaued attributes - 1. Location 2. Contact_no
Restaurants may have more than one locations and every location/branch has more than one Contact_no.
How to create relational database for this? What will the primary keys of the table?
How do I go about making an ERD in which the 3 tables I have are not linked in any way, so there is not a foreign key amongst the tables?
One table for registered users
One table for storing events (e.g event name, location, time/date etc.)
One table for image storing (for the gallery)
You have answer right into your question:
3 tables are not linked in any way
So you just need to add desired entities on your ERD diagram and no relations. Your Entity Relationship Diagram will be boring and looks like:
i am a student and i have a question about database schema , i already created the Entity Relationship Diagram [ERD] and in this step i should do the database schema , must all the entities on my database has a relation between them and the other entities ? i.e : each entity should have a foreign key for the entity before it , because i can create all the tables and only 2 tables can have a relation between them and i control the other tables using C# program i am going to create .
** in my ERD all the entities have a relation between each other .
...must all the entities on my database has a relation between them and
the other entities?
No. Entities of course can stand on their own. Although in practice if you're finding that you have very few relationships you're probably doing something wrong...
You're sort of right in saying that only two tables can have a relationship but I'd word it as a relationship can have only two participants; tables can certainly relate to more than just one other table.
Any time you create tables in the database if there is a relationship between the information in the tables then you should create a relationship to make sure that when the data is entered into the tables that the reference data will be there. This will enforce referential integrity. For example:
Employees Database:
EmployeeInfoTable:
pk EmployeeID
EmployeeName
EmployeeDateHired
fk SalaryID
EmployeeLoginTable
pk LoginID
fk EmployeeID
Username
Password
SalaryTable
pk SalaryID
SalaryRange
So what this would do is make sure that if you add a salary to an employee it has to exist in the Salary table and if you added a login then you would have to have an employee to reference. It's just an added layer of security to make sure that the data that is being input into the database is valid and can be used.
While you can technically control this with C# or whatever programing language you want it's easy to forget these little rule especially in a database that has like 200 tables or more. So getting in the habit of using relationships and maintaining referential integrity is a very good thing to do.
All entities need not have relationships with each other. A relationship will only exist between two entiries in the ERD if one entity somehow relates to the other.
The general rule of thumb for creating table is:
Each entity in the ERD will have a table for it
For a one-to-many relationship in the ERD, the many side table will contain a foreign key to the one side table
For a many-to-many relationsip, there will be a separate table for the relationship that will contain foreign keys to both the entity tables
If a relationship has an attribute, there will be a separate table for the relationship that will contain foreign keys to both the entity tables
the primary key should always be id due to convention, and your foreign keys could be called othertablename_id so if a table has a link to all the other tables it will need a key for each one of those.
for example: if dogs has more than one owners but owners can only have one dog, you could just have dog_id in the owner table. but then you would have to query the owners table to find all the owners of one dog.
if dogs have more than one owner and owner can have more than one dog, then you have to have a join table called dogs_owners with id and both foreign key dog_id and owner_id
of course you can name your tables and fields whatever you want but it's convention to use id and _id
yes an entity usually equates to just one table, apart from when you have many-to-many relationships between your entities.