What is the difference between outer join and inner join ?

You use INNER JOIN to return all rows from both tables where there is a match. i.e. in the resulting table all the rows and columns will have values.

In OUTER JOIN the resulting table may have empty columns. Outer join may be either LEFT or RIGHT

LEFT OUTER JOIN returns all the rows from the first table, even if there are no matches in the second table.

RIGHT OUTER JOIN returns all the rows from the second table, even if there are no matches in the first table.