Define Outer join
Outer join - If there are any type of values in the one table that do not have corresponding value(s) in the other, in an equi-join which will not be selected. Such types of rows can be forcefully selected by using the outer join, The corresponding columns for that row will comprise NULLs. In fact there are three forms of the outer-join operation: left outer join (_X), right outer join (X_) and full outer join (_X_).