What is UNION?

UNION is an operator in SQL that combines the results of two or more SELECT statements into a single result set. It eliminates duplicate records and presents only distinct values, making it a useful tool for data processing and analytics.

Functionality and Features

UNION draws together data from different tables that have the same column structure, enabling the creation of a comprehensive, non-redundant data set. Its main features include:

  • Combining rows from two or more tables based on corresponding columns
  • Filtering out duplicate data
  • Providing results sorted in ascending order by default

Benefits and Use Cases

The UNION operator is used primarily to amalgamate data from different tables into a single, insightful dataset. Its key benefits include:

  • Efficiency: It reduces the number of queries, improving query performance
  • Simplicity: It offers an easy way to combine data from multiple tables
  • Flexibility: It allows the combination of data from unrelated tables as long as they have a similar structure

Challenges and Limitations

Despite its features, UNION has several limitations such as:

  • Performance: It may be slow when dealing with large datasets
  • Restrictions: It requires perfectly matching data types in combined columns
  • Duplication: It might unnecessarily filter data perceived as duplicates

Integration with Data Lakehouse

In the context of a data lakehouse, the UNION operator plays a significant role in merging disparate data sources. This unique environment houses a blend of structured and unstructured data, and UNION can be instrumental in creating unified datasets for advanced and diverse analytics.

Security Aspects

Since UNION merges data from various tables, it's essential to ensure that it doesn't expose sensitive information. In this context, SQL database management systems handle security, enforcing permissions at table and row levels.


UNION simplifies data amalgamation and improves performance by reducing the number of queries processed. However, on large datasets, it can be slower due to its need to filter out duplicate data.


What is the difference between UNION and UNION ALL? UNION removes duplicate records, whereas UNION ALL doesn't, potentially providing faster results.

What types of data can UNION combine? UNION can combine any data types, as long as matched columns from different tables share the same data type.


SQL: A standard language for managing and manipulating databases.

Data Lakehouse: A hybrid data management model that combines the best aspects of data lakes and data warehouses.

SELECT Statement: A command used in SQL to select specific data from a database.

Result Set: It's the output of a database query, often presented in a structured table format.

Dremio and UNION

Dremio enhances UNION operations with its self-service data platform. Its powerful optimizer makes query translation more efficient, potentially offering a significant performance boost over traditional UNION operations, especially for large datasets within a data lakehouse environment.

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