How to Fetch Uncommon Data from Two Different Tables in Oracle SQL: A Step-by-Step Guide

Are you tired of struggling to fetch uncommon data from two different tables in Oracle SQL? Do you find yourself stuck in a query-writing rut, unable to extract the data you need? Worry no more! In this article, we’ll guide you through the process of fetching uncommon data from two different tables using Oracle SQL. By the end of this tutorial, you’ll be a master of query writing, and your data woes will be a thing of the past.

Understanding the Problem

Before we dive into the solution, let’s take a step back and understand the problem. Suppose we have two tables, Table A and Table B, each containing different columns and data. Our task is to fetch the uncommon data from both tables, i.e., the data that exists in one table but not the other.

Table A
+----+--------+
| ID | Name   |
+----+--------+
| 1  | John   |
| 2  | Mary   |
| 3  | David  |
| 4  | Emma   |
+----+--------+

Table B
+----+--------+
| ID | Name   |
+----+--------+
| 1  | John   |
| 2  | Mary   |
| 5  | Michael|
| 6  | Sarah  |
+----+--------+

In the above example, the uncommon data would be David and Emma from Table A, and Michael and Sarah from Table B.

The Solution: Using MINUS Operator

The Oracle SQL MINUS operator is used to return all rows from the first SELECT statement that are not returned by the second SELECT statement. This makes it the perfect solution for fetching uncommon data from two different tables.

SELECT * FROM Table_A
MINUS
SELECT * FROM Table_B;

This query will return all rows from Table A that do not exist in Table B.

Fetching Uncommon Data from Both Tables

To fetch uncommon data from both tables, we can use the MINUS operator twice, once for each table.

SELECT * FROM Table_A
MINUS
SELECT * FROM Table_B

UNION ALL

SELECT * FROM Table_B
MINUS
SELECT * FROM Table_A;

This query will return all uncommon data from both tables, i.e., the data that exists in one table but not the other.

The Solution: Using LEFT JOIN and IS NULL

Another way to fetch uncommon data from two different tables is by using a LEFT JOIN with an IS NULL condition.

SELECT A.*
FROM Table_A A
LEFT JOIN Table_B B
ON A.ID = B.ID
WHERE B.ID IS NULL

UNION ALL

SELECT B.*
FROM Table_B B
LEFT JOIN Table_A A
ON B.ID = A.ID
WHERE A.ID IS NULL;

This query uses a LEFT JOIN to combine both tables, and then filters out the rows where the join condition is NULL, effectively returning the uncommon data from both tables.

Performance Considerations

When working with large tables, performance considerations become crucial. The two solutions provided above have different performance characteristics, and the choice of solution depends on the specific use case.

• MINUS operator: This operator can be slower than the LEFT JOIN method, especially for large tables. This is because the MINUS operator requires the database to sort the result sets before returning the uncommon data.
• LEFT JOIN method: This method can be faster than the MINUS operator, especially for large tables. This is because the LEFT JOIN method uses an index-based join, which can be more efficient.

Best Practices

When fetching uncommon data from two different tables, it’s essential to follow best practices to ensure optimal performance and maintainability.

1. Use meaningful table aliases: Use meaningful table aliases to improve readability and maintainability.
2. Optimize your joins: Optimize your joins by using the most efficient join type (e.g., LEFT JOIN, INNER JOIN) and ensuring that your join columns are indexed.
3. Filter out unnecessary data: Filter out unnecessary data using the WHERE clause to reduce the result set and improve performance.
4. Use subqueries judiciously: Use subqueries judiciously, as they can impact performance. Consider rewriting subqueries as joins where possible.

Conclusion

In this article, we’ve shown you two different methods for fetching uncommon data from two different tables in Oracle SQL. By following the best practices outlined above and choosing the most efficient solution for your specific use case, you’ll be able to extract the data you need with ease.

Remember, practice makes perfect. Try out the examples provided in this article and experiment with different scenarios to become a master of query writing. Happy querying!

Method	Description
MINUS Operator	Returns all rows from the first SELECT statement that are not returned by the second SELECT statement.
LEFT JOIN and IS NULL	Returns all uncommon data from both tables using a LEFT JOIN with an IS NULL condition.

Note: The above methods assume that the tables have a common column (e.g., ID) for joining. If the tables do not have a common column, you may need to use other joining methods (e.g., FULL OUTER JOIN) or modify the query accordingly.

Frequently Asked Question

Get ready to dive into the world of Oracle SQL and uncover the secrets of fetching uncommon data from two different tables!