How to Convert a Multiple Select Query into a Case Statement in Oracle

Learn how to effectively convert multiple select queries into a single case statement in Oracle SQL, ensuring efficient counting based on specified conditions.
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How to Convert a Multiple Select Query into a Case Statement in Oracle

When working with Oracle SQL, you may find yourself needing to aggregate data based on multiple conditions. In this post, we'll explore how to convert multiple select queries into a single, more efficient case statement. This technique not only simplifies your SQL code but also improves performance. Let's dive in.

The Problem

You may have a scenario where you want to count records from a table based on different status identifiers while also restricting the results based on a recent update time. For instance, consider the following select queries:

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

The aim here is to obtain counts for these different identifiers in one query instead of multiple ones. While you created an effective query using joins, you wanted to know how to do this using case statements.

The Solution

Using a single query with a case statement allows us to streamline our counting process. Here's a revised approach using a CASE statement that manages to do all the necessary counting in one go:

Steps to Make the Transition

Understand Conditional Aggregation: We will use conditional aggregation to summarize data based on the person_status_id.

Use of WHERE Clause: Implement the common condition only once in the WHERE clause to avoid redundancy.

Counting with CASE: Utilize the CASE statement to count different person_status_id values.

Final Query

By implementing the above strategies, we can refactor the multiple select queries into a single query as follows:

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

Explanation of the Query

COUNT with CASE: For each distinct status identifier, we use the CASE statement to count records that match the criteria:

Completed: Counts records where person_status_id is 8.

Pending: Counts records where person_status_id is one of 5, 7, 13.

Started: Counts records where person_status_id is within 3, 4, 6.

Single WHERE Clause: The restriction on updated_on is applied only once which enhances performance by minimizing the workload on the database.

Conclusion

Converting multiple select queries into a single case statement not only helps in writing cleaner SQL code but also allows for more efficient data retrieval. By following the structure outlined above, you can easily implement this solution to count records based on varied status identifiers in Oracle SQL.

This approach is beneficial not only for performance but also for maintainability of your SQL code as your queries grow in complexity.

With the solutions provided in this guide, you'll be able to tackle similar SQL challenges with confidence.