Full outer join

Definition

A full outer join is a type of SQL join that returns all records from both tables being joined, including those that do not have matching records in the other table. This means it combines the results of both left outer and right outer joins, providing a complete view of the data by including all entries, even when there are no matches.

5 Must Know Facts For Your Next Test

1. Full outer joins can produce a larger result set than inner joins since they include non-matching records from both tables.
2. In SQL, a full outer join is typically written as `SELECT * FROM table1 FULL OUTER JOIN table2 ON condition`.
3. When using full outer joins, NULL values will be present in the result set where there are no corresponding matches in one of the tables.
4. Full outer joins are particularly useful for combining datasets where you want to analyze all available information, regardless of whether every record has a match.
5. Performance can be impacted with full outer joins on large datasets due to the increased size of the result set and additional processing required.

Review Questions

• How does a full outer join differ from an inner join and what are its practical applications?
  • A full outer join differs from an inner join by including all records from both tables regardless of matching criteria, while an inner join only includes rows where there is a match in both tables. This means that a full outer join can provide a more comprehensive view of data, making it useful in scenarios where understanding all aspects of two datasets is important, such as when combining customer data with order data where some customers may not have placed orders.
• What impact does using a full outer join have on the resulting dataset's size compared to other types of joins?
  • Using a full outer join typically results in a larger dataset compared to inner or even left/right outer joins because it includes all records from both joined tables, even those without matching counterparts. As a result, while inner joins provide only matching records, and left or right joins give priority to one table over another, full outer joins ensure that no information is lost, which can be crucial for comprehensive analysis but may also lead to performance considerations due to increased data volume.
• Evaluate the implications of including NULL values in the results of a full outer join and how this affects data interpretation.
  • Including NULL values in the results of a full outer join has significant implications for data interpretation as it indicates where matches were not found between the two datasets. These NULLs can affect analysis by signaling gaps in data or areas that require further investigation. Analysts must be careful when interpreting these results, as they need to consider whether these gaps reflect real-world scenarios or data quality issues. Thus, understanding how to handle NULLs becomes essential for accurate reporting and decision-making based on joined datasets.

Related terms

inner join:

An inner join is a type of join that returns only the rows with matching values in both tables involved in the join.

left outer join: A left outer join returns all records from the left table and the matched records from the right table; if there is no match, NULL values are returned for columns from the right table.

right outer join: A right outer join returns all records from the right table and the matched records from the left table; if there is no match, NULL values are returned for columns from the left table.