<P> Relational databases are usually normalized to eliminate duplication of information such as when entity types have one - to - many relationships . For example, a Department may be associated with a number of Employees . Joining separate tables for Department and Employee effectively creates another table which combines the information from both tables . </P> <P> All subsequent explanations on join types in this article make use of the following two tables . The rows in these tables serve to illustrate the effect of different types of joins and join - predicates . In the following tables the DepartmentID column of the Department table (which can be designated as Department. DepartmentID) is the primary key, while Employee. DepartmentID is a foreign key . </P> <Table> Employee table <Tr> <Th> LastName </Th> <Th> DepartmentID </Th> </Tr> <Tr> <Td> Rafferty </Td> <Td> 31 </Td> </Tr> <Tr> <Td> Jones </Td> <Td> 33 </Td> </Tr> <Tr> <Td> Heisenberg </Td> <Td> 33 </Td> </Tr> <Tr> <Td> Robinson </Td> <Td> 34 </Td> </Tr> <Tr> <Td> Smith </Td> <Td> 34 </Td> </Tr> <Tr> <Td> Williams </Td> <Td> NULL </Td> </Tr> </Table> <Tr> <Th> LastName </Th> <Th> DepartmentID </Th> </Tr>

What is join and types of join in dbms