Fetching Data from Multiple Tables in Oracle Without Joins or Subqueries: Alternative Methods

Finding ways to fetch data from multiple tables in Oracle without using joins or subqueries can be a challenge. Fortunately, there are alternative methods available. This article explores how to use UNION and UNION ALL, as well as other set operators, to combine data from different tables. We will also examine the use of views as an alternative approach. The goal is to provide a comprehensive guide to help you achieve efficient and effective data fetching in Oracle.

Using UNION and UNION ALL

When dealing with multiple tables in Oracle, the UNION and UNION ALL operators can be used to combine their result sets. However, there are important considerations to ensure the process is both correct and efficient.

UNION Operator

The UNION operator combines the result sets of two or more SELECT statements and removes any duplicate rows. Here's how you can use it effectively:

Both SELECT statements must return the same number of columns. The data types of the corresponding columns must be compatible. The order of the columns in the SELECT statements must match.

For example, if you have two tables, employees and contractors, and you want to fetch the names from both tables, you can write:

SELECT first_name, last_name FROM employees
UNION
SELECT first_name, last_name FROM contractors

UNION ALL Operator

The UNION ALL operator combines the result sets of two or more SELECT statements but does not remove any duplicate rows. It is useful when you need to include all records, including duplicates:

SELECT first_name, last_name FROM employees
UNION ALL
SELECT first_name, last_name FROM contractors

Considerations and Performance Issues

While using UNION and UNION ALL can be convenient, it is generally not as efficient as using joins or subqueries, especially when dealing with related data. These methods can lead to performance issues and slower query execution times, especially for large datasets.

Using Set Operators

Other set operators like INTERSECTION can be used depending on specific requirements. For instance, the INTERSECTION operator returns only the rows that are common to both queries:

SELECT column1, column2 FROM table1
INTERSECT
SELECT column1, column2 FROM table2

Using Views for Alternative Approach

An alternative method to fetching data from multiple tables without joins or subqueries is to use views. A view is essentially a virtual table derived from the result of a SELECT statement. Although views can be created using complex joins and subqueries, they can still be queried efficiently.

For example, to fetch data from tables table1, table2, and table3 with specific conditions, you can create a view:

CREATE VIEW combined_view AS
SELECT column1, column2 FROM table1
UNION
SELECT column1, column2 FROM table2
UNION
SELECT column1, column2 FROM table3

Then, you can query the view instead of using the original tables:

SELECT * FROM combined_view

Conclusion

While it is possible to fetch data from multiple tables in Oracle without using joins or subqueries, it is generally recommended to use these constructs for better performance and readability. However, if you prefer to avoid joins and subqueries, you can use UNION, UNION ALL, INTERSECTION, and views to achieve your goals. Understanding the differences and implications will help you choose the most appropriate method for your specific situation.

FAQs

Q: Are UNION and UNION ALL reliable for large datasets?

No, UNION and UNION ALL may not be as reliable for large datasets due to potential performance issues. Consider using joins or optimizing your queries for better efficiency.

Q: Can I use INTERSECTION without joins or subqueries?

Yes, you can use INTERSECTION to find common rows between tables without joins or subqueries:

SELECT column1, column2 FROM table1
INTERSECT
SELECT column1, column2 FROM table2

Q: What are the advantages of using views?

Using views can simplify complex queries, improve performance, and encapsulate query logic. Views can be created using joins and subqueries, providing a flexible way to retrieve and manipulate data.