Answer: Joins are used to extract data from multiples tables on specified some conditions.
Joins are INNER JOIN, LEFT OUTER JOIN, RIGHT OUTER JOIN, SELF JOIN.
INNER JOIN: extract only matched data from two tables.
LEFT OUTER JOIN: extract all data from left table and matched data from right table, having null against unmatched records.
RIGHT OUTER JOIN: extract all data from right table and matched data from left table, having null against unmatched records.
SELF JOIN: exctract data from one table by creating two alias of a table on specified some condition.
Even there are more joins like cross join, non-equi joins but they are in less use.
For beginners link:
http://www.w3schools.com/sql/sql_join.asp
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