Difference between relation and relational schema?

Answer 1

In database design a RELATION is the relationship that you build between two or more tables.

As an example lets say you have a Customer table. In this table you have lots of fields

FirstName, LastName, Address, City, State, PostalCode, StatusID; etc.......

Now StatusID is going to be a numeric value. Lets say that the value is 1

Now you will have a Status Table with multiple fields (ex)

StatusID, StatusName, Active, etc.....

Now when you display customer information you will not want to show a StatusID as nobody will know what this is. You will want to show the NAME associated with the status. To do this you will create a releationship between StatusID in the Customer table and StatusID in the Status table. In doing so you will provide for faster lookup results, a constraint that does not permit a numeric value in the StatusID field in the customer table that is NOT in the StatusID field of the Status table.

THEN... To show correct Results you would write a query something like:

SELECT FirstName, LastName, Address, City, State, PostalCode, StatusName

FROM Customers as c

JOIN Status as s on s.StatusID = c.StatusID

A relational Schema is simply a representation of your database highlighting the relationships that you have created throughout.