DR+SQL Tip: Using UNION to Best Effect

May 17, 2022


Welcome to OptWizard SEO's expert guide on utilizing the UNION operator in SQL to enhance your data retrieval and analysis capabilities. In this article, we will dive into the benefits and best practices of using UNION to combine queries, remove duplicates, and improve efficiency in your SQL queries.

Understanding UNION

UNION is a powerful SQL operator that allows you to combine the result sets of multiple SELECT statements into a single result set. It is commonly used when you need to merge similar data from different tables or retrieve data from multiple tables with similar structures.

Benefits of Using UNION

By using UNION, you can:

  • Combine data from multiple tables into a single result set
  • Remove duplicate rows from the result set
  • Perform calculations or transformations on the combined data
  • Create customized reports by selecting specific columns from different tables

Best Practices for Using UNION

1. Ensure Data Compatibility

Before using UNION, make sure that the SELECT statements being combined have compatible data types and column names. The column order must also match in all SELECT statements.

2. Use UNION ALL When Possible

If you don't require duplicate removal, use the UNION ALL operator instead of UNION. UNION ALL is faster because it does not perform the additional step of removing duplicates from the result set.

3. Handle NULL Values

When working with NULL values, be aware that UNION treats them as distinct values. If you want to remove NULL values from the result set, you can use the IS NOT NULL condition in your SELECT statements.

4. Optimize Performance

To optimize the performance of your UNION queries, ensure that proper indexing is in place on columns used for JOINs, WHERE clauses, and ORDER BY statements. This will help speed up data retrieval and improve overall query execution time.

5. Test and Debug

Always thoroughly test and debug your UNION queries before deploying them to production environments. Pay attention to result set order, column names, and potential data inconsistencies to avoid unexpected issues.


In summary, understanding how to effectively use the UNION operator in SQL can greatly enhance your data retrieval and analysis tasks. By following the best practices outlined in this article, you can harness the power of UNION to combine and manipulate data from multiple tables, remove duplicates, and create insightful reports.

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