SQL Union Queries

Understanding UNION and UNION ALL

The UNION and UNION ALL operators in SQL are used to combine the results of two or more SELECT statements. Both operators have their distinct use cases:

• UNION combines the results and removes duplicates.
• UNION ALL combines all results including duplicates.

Key Differences Between UNION and UNION ALL

While both UNION and UNION ALL are used to combine results, the primary difference lies in how they handle duplicates:

• UNION is useful when you want a clean set of results without any repetition. It is generally slower because it involves a unique sort operation.
• UNION ALL is faster as it merely concatenates results, making it suitable for large datasets where duplicates are not a concern.

Working with INTERSECT

The INTERSECT operator returns the common records between two SELECT statements. This is useful for finding overlapping data.

Using EXCEPT for Data Comparison

The EXCEPT operator returns records from the first SELECT statement that are not found in the second SELECT statement. This can be useful for identifying unique records in one dataset compared to another.

Practical Examples and Use Cases

Let's consider a scenario where you have two tables: employees_2022 and employees_2023. You want to:

• Get a list of all unique employees across both years.
• Find employees present in both years.
• Identify employees who left after 2022.

Conclusion and Best Practices

Using UNION, UNION ALL, INTERSECT, and EXCEPT effectively allows you to perform complex data operations with ease. Always choose the operator that best fits the specific data requirement, and be mindful of performance differences, especially with large datasets.

In the next post, we will delve into Query Optimization to further enhance your SQL query skills.