Relational Model

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Each Row is Unique
Column Values Are of the Same Kind
The Sequence of Columns is Insignificant
The Sequence of Rows is Insignificant
Each Column Has a Unique Name

Some table fields may be designated as keys, which means that searches for specific values of that field will use indexing to speed them up.
Where fields in two different tables take values from the same set, a join operation can be performed to select related records in the two tables by matching values in those fields.
Often, but not always, the fields will have the same name in both tables.

SELECT A.FirstName,A.LastName,A.Title,B.TerritoryID,C.OrderID
FROM Northwind.dbo.Employees AS A,Northwind.dbo.EmployeeTerritories AS B,Northwind.dbo.Orders AS C
WHERE A.EmployeeID = B.EmployeeID
AND A.EmployeeID = C.EmployeeID
AND B.EmployeeID = C.EmployeeID
AND A.LastName LIKE 'Dav%'

will result all the orders created by Nency Davolio which is a Sales Representative for Territories 06897 and 19713 god bless her soul a great woman.
and this is because EmployeeID is a Key in this tables, they are related by it.

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