How to use UNION in SQL?

Understanding how SQL can manipulate and combine datasets efficiently is crucial for data management and analysis. One of the key capabilities in SQL for data combination is the use of the UNION operator. This tutorial will cover the essentials of using UNION in SQL, comparing it with UNION ALL, and offering insight into their practical applications.

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UNION in SQL

The UNION operator is used to combine the result-set of two or more SELECT queries. It is particularly useful because it automatically removes duplicate records from the combined data set. As such, it is an excellent tool when you want to merge data and keep it clean.

SELECT column_name(s) FROM table1
UNION
SELECT column_name(s) FROM table2;

Each SELECT statement within the UNION must have the same number of columns in the result sets with similar data types. If not, SQL will throw an error because it needs a predictable structure to align columns accurately.

Benefits of Using UNION in SQL

• Data Combination Simplified: UNION allows you to combine data from multiple tables while filtering out duplicates. This is helpful when working with large datasets.
• Complex Queries Made Simple: Instead of using multiple joins to gather data from different tables, UNION offers a way to manage complex queries more simply.
• Performance Enhancement: By reducing duplicates, UNION can lower the data volume that needs to be processed, enhancing performance.

UNION without duplicates

For example, we might want to combine unique records from two tables: sales_2023 and sales_2024.

Here, the UNION operation removed the duplicate product 'A', retaining only one entry per product.

For some surprising use cases of UNION, check out this link.

UNION ALL with duplicates

To combine all records (including duplicates) from sales_2023 and sales_2024:

Unlike UNION, UNION ALL includes all duplicates. Here, both entries for product 'A' are kept.

For insights on performance differences between UNION and UNION ALL, have a look here.

Difference Between UNION and UNION ALL

• Duplicate Handling: UNION removes duplicate records from the result set, whereas UNION ALL includes duplicates.
• Performance: UNION must scan for duplicates, which might add overhead, while UNION ALL is faster as it doesn't check for duplicates.
• Use Case: Use UNION for cleaner datasets and UNION ALL when every entry is needed, including duplicates.

Common Use Cases for UNION in SQL

• Merging Tables: Ideal for merging similarly structured tables in data warehousing where each period is a separate table.
• Data Integrity Checks: Useful in consolidating information from various transactional databases to ensure data consistency.
• Periodical Reporting: Perfect for reports covering different timeframes, like merging sales_2023 and sales_2024.
• ETL Processes: Common in ETL processes where synthesized views from multiple data sources are a norm.

Wrapping Up

The UNION operator is a versatile tool for combining result sets efficiently, handling duplicates or including them with UNION ALL to suit different analytical needs. It's an indispensable part of SQL that aids in effective data management.

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