To answer this question, you need to understand the concept of JOINs in SQL.
A FULL OUTER JOIN returns all rows from both tables, including unmatched rows from both tables. It combines the results of a LEFT OUTER JOIN and a RIGHT OUTER JOIN.
Let's go through each option to understand why it is correct or incorrect:
Option A) Both tables have NULL values - This option is not a specific condition for using a FULL OUTER JOIN. NULL values in both tables can exist regardless of the type of JOIN used.
Option B) You want all matched data from both tables - This option is not correct because a FULL OUTER JOIN returns all data from both tables, including unmatched data.
Option C) You want all unmatched data from both tables - This option is correct because a FULL OUTER JOIN returns all rows from both tables, including unmatched rows from both tables.
Option D) You want all unmatched data from one table - This option is not correct because a FULL OUTER JOIN returns all unmatched data from both tables, not just one.
The correct answer is C. A FULL OUTER JOIN is used when you want to retrieve all unmatched data from both tables.