What is UNION?

UNION is a fundamental concept in database management systems that allows you to combine rows from multiple tables or queries into a single result set. It is a set operation that treats the result sets of multiple SELECT statements as sets and performs a union operation to combine them.

How UNION works

UNION works by concatenating the result sets of multiple SELECT statements into a single result set. The result set consists of all the unique rows from the individual SELECT statements. The columns of the result set must have compatible data types and be in the same order.

Why UNION is important

UNION is important because it enables you to combine and merge data from different sources or queries. It allows for efficient data processing, as you can perform complex transformations and aggregations on the combined result set. UNION is particularly useful when dealing with data sets that have similar structures and need to be consolidated or analyzed together.

The most important UNION use cases

UNION has several use cases in data processing and analytics:

  1. Data Integration: UNION can be used to combine data from multiple sources or tables, enabling you to create a unified view of your data.
  2. Data Migration: When migrating data from one database to another, UNION can help you merge the data from the source and target databases.
  3. Data Transformation: UNION allows you to perform data transformations, such as filtering, sorting, and aggregating, on the combined result set.
  4. Data Analysis: By combining multiple queries using UNION, you can perform comprehensive data analysis across different data sets.

Other terms related to UNION

There are several technologies and terms closely related to UNION:

  • UNION ALL: Unlike UNION, which returns only unique rows, UNION ALL returns all the rows from the individual SELECT statements, including duplicates.
  • INTERSECT: INTERSECT is a set operation that returns the common rows between the result sets of two SELECT statements.
  • EXCEPT: EXCEPT is a set operation that returns the rows from the first SELECT statement that are not present in the result set of the second SELECT statement.

Why Dremio users would be interested in UNION

Dremio users would be interested in UNION because it allows them to combine and analyze data from different sources efficiently. With Dremio's data lakehouse architecture, which integrates data lakes and data warehouses, users can leverage UNION to perform complex data transformations and create unified views of their data. UNION in Dremio enables users to optimize their data processing and analytics workflows, saving time and improving overall data quality and insights.

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