Using ELSE in CASE Expression in SQL Server

Details

Description

Learn how to properly use the ELSE statement in CASE expressions with SQL Server to return hardcoded values effectively.
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Using ELSE in CASE Expression in SQL Server

When working with SQL Server, one of the powerful features is the CASE expression, which allows you to incorporate conditional logic into your queries. A common challenge developers face is effectively using the ELSE statement within a CASE expression while ensuring that it adheres to SQL's syntax rules. In this guide, we will explore how to implement a CASE expression that not only checks specific conditions but also returns hardcoded values as required.

The Problem Statement

Imagine you have a database and you want to retrieve user data along with a modified output based on certain conditions from the Event table. Given some specific entries, such as 2.1 and ABC001, you expect the output to replace these values with a hardcoded string "ABC Hospital". You also want to make sure that any other value continues to display correctly as it is. Here's the initial query you worked with:

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Current Output

The query as is returned output that was not what you expected:

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Desired Output

Your goal was to transform the output to something like this:

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The Solution

After reviewing the requirements, the ideal solution is to nest the CASE statement in a subquery. This way, you can first derive the initial DHB values and then apply the necessary changes in an outer query. Here’s how you can structure your SQL query to achieve the desired result effectively:

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Explanation of the Query

Subquery (Inner Query):

The inner query fetches all the users and derives the DHB value based on the logic checking for the character -.

It does all necessary transformations but does not yet replace any values with hardcoded strings.

Outer Query:

The outer query applies the CASE logic to the results of the subquery.

It checks if the DHB is either 2.1 or ABC001. If it is, it returns "ABC Hospital". Otherwise, it returns the original DHB.

Conclusion

Using nested queries is a powerful way to enhance the functionality of your SQL statements, particularly when you need to perform conditional replacements. In this case, by utilizing a subquery, we can effectively rewrite the output of our DHB values in a clear, structured manner, achieving our required results without violating SQL syntax. Implement these selective transformations into your SQL Server queries and see how they can clarify and enhance your data output!

If you have further questions or run into other issues with SQL queries, feel free to ask for assistance!