Final answer:
A Full outer join is used to return all rows from both tables, filling with null values in places where there is no matching record in the other table.
Step-by-step explanation:
A Full outer join is necessary when you need rows returned from either table that do not have a matching record in the other table. When you perform a Full outer join in a database, the result set includes all the records from both tables. Where there are no matches, the result set will contain null values. For instance, if you have two tables, Table A and Table B, and they are joined using a Full outer join, the result will include all rows from Table A and all rows from Table B. Any rows that do not have corresponding matches in the opposite table will be filled with null values to maintain the integrity of the data set.