There are three types of joins: Inner Join, Outer Join,
Cross Join

In some situations we may need to pull data from one or 
more related tables at a time. In such cases we use JOINs 
to extract data from more than one table at a time and 
display the information in a single result set.
There are several types of joins.(1)Inner Join 
                                                   (2)Outer 
Join------Right Outer Join,Left Outer Join, Full Outer Join
Inner join is used to link tables on a common column and 
return records that match in those columns.
Right Outer Join displays all records from the table on 
right most side of the JOIN clause whether or not there are 
matching records in the Left table.
Left Outer Join  displays all records from the table on the 
left most side of the JOIN clause whether or not there are 
matching records on the Right table.
Full Outer Join diplays all records from both right and 
left tables.
Example: If we have a HR database with employee information 
table, salary information table, sick days table etc, and 
if we want to know which employee
have used more than 15 sick days, we would need the 
information form sick days table and employee information 
table in a single result set.