Difference Between UNION and UNION ALL in SQL

Q: What is the difference between the UNION and UNION ALL operators in SQL?

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When working with SQL databases, data manipulation is a fundamental skill every developer should master. Among the various operations available, the UNION and UNION ALL operators are crucial for combining results from multiple queries. Understanding the distinction between these two operators is essential, especially for those preparing for technical interviews or data-related roles. Both UNION and UNION ALL are used to merge the results from two or more SELECT statements.

However, they differ significantly in terms of how they handle duplicate records. This fundamental difference can impact performance and the result set in various scenarios, making it critical to choose the appropriate operator based on the use case. In the realm of SQL, performance is often a key consideration. While UNION automatically removes duplicate rows from the result set, which can lead to a more manageable dataset, it requires additional processing time to sort and eliminate duplicates.

This may result in slower query performance, especially with large datasets. Conversely, UNION ALL preserves all duplicates, allowing for faster execution since it does not require the overhead of filtering out duplicates. Thus, understanding this difference is vital for SQL optimization, particularly when dealing with large volumes of data. Additionally, candidates preparing for interviews should familiarize themselves with scenarios that highlight the use of each operator.

Questions about performance trade-offs, real-world applications, and when to opt for one over the other can frequently arise in interviews. This knowledge not only enhances technical proficiency but also showcases an understanding of practical database management. Moreover, mastering these operators lays a solid foundation for more advanced SQL concepts and functions. As data manipulation becomes increasingly relevant in a data-driven world, knowing how to efficiently combine datasets will enhance both the quality of results and the overall performance of SQL queries.

Familiarity with UNION and UNION ALL is an excellent step toward becoming a competent SQL developer in today's competitive job market..

The difference between the UNION and UNION ALL operators in SQL lies in how they handle duplicate rows in the results.

The UNION operator combines the results of two or more SELECT queries and removes duplicate rows from the final output. This means that each row in the result set will be unique. For example:

```sql
SELECT column1 FROM table1
UNION
SELECT column1 FROM table2;
```

In this case, if `table1` has the value 'A' and `table2` also has 'A', the final result will only show 'A' once.

On the other hand, the UNION ALL operator also combines the results of two or more SELECT queries but includes all duplicates, meaning if the same row appears in both queries, it will be included multiple times in the final output. For instance:

```sql
SELECT column1 FROM table1
UNION ALL
SELECT column1 FROM table2;
```

In this scenario, if both tables contain the value 'A', the result will display 'A' twice, reflecting all occurrences.

To summarize, use UNION when you want to eliminate duplicates, and use UNION ALL when you want to retain all occurrences, including duplicates.