Скачать или смотреть How to Effectively Use UNION in Oracle SQL to Merge Query Results

Learn how to properly combine results from two queries in Oracle SQL using `UNION`, while filtering out specific rows based on conditions.
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Mastering UNION in Oracle SQL: A Simple Guide to Merging Query Results

In the world of SQL, combining results from multiple queries can often pose a challenge, especially when you want to filter out specific rows. One common requirement is the need to UNION the results of two separate queries. This guide will guide you through the process of achieving this in Oracle SQL, focusing on how to exclude certain rows from the final result set.

Understanding the Problem

Let’s break down a scenario to demonstrate the need for using the UNION operator effectively. Imagine you have two queries that provide different kinds of information, and you want to merge their outputs. Here’s what each query looks like:

Query 1 produces a list of products with their corresponding IDs:

[[See Video to Reveal this Text or Code Snippet]]

Query 2 gives us segments corresponding to some IDs:

[[See Video to Reveal this Text or Code Snippet]]

The challenge is to combine the results from these two queries but to exclude the row with ID 999 from Query 2. Let’s figure out how to do this efficiently.

The Solution: Using WITH to Create Derived Tables

The good news is that your initial approach is almost right! We can leverage the Common Table Expressions (CTE) feature in Oracle SQL using the WITH clause. This allows us to define temporary result sets that can be referenced within the main query. Here’s how you can implement the solution:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Code

Common Table Expressions (CTE):

q1: This CTE fetches records from tab1 where the ID is less than or equal to 4, thereby giving us the desired output from the first query.

q2: This CTE selects from tab1, but it only includes IDs that are present in q1, effectively filtering out the unwanted ID 999.

Combining Results:

The SELECT * FROM q1 retrieves all the products.

The UNION ALL SELECT * FROM q2 combines both sets of results into a single output.

Important Notes

If you need the final result in a specific order, remember that UNION does not guarantee any order. You might want to use ORDER BY at the end of your final selection to sort the results appropriately.

Sample Output

When you run the above SQL code, the final result should look like this:

[[See Video to Reveal this Text or Code Snippet]]

This output combines both queries while excluding ID 999, as required.

Conclusion

Using the UNION operator in Oracle SQL is straightforward once you understand how to set up your queries effectively. By implementing Common Table Expressions with the WITH clause, you can filter and combine your data seamlessly. This technique is not only helpful but essential for data manipulation and reporting tasks in SQL databases.

Mastering these query-building techniques will greatly enhance your SQL skills and improve your ability to write efficient queries. Happy coding!