How to Count Rows with a CASE WHEN Statement in SQL

Details

Description

Learn how to accurately count rows in SQL with a CASE WHEN statement. This guide provides tips and examples to help you solve the common issue of miscounting values in your queries.
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This video is based on the question https://stackoverflow.com/q/72469881/ asked by the user 'Lawrence Lau' ( https://stackoverflow.com/u/19224227/ ) and on the answer https://stackoverflow.com/a/72469893/ provided by the user 'Max' ( https://stackoverflow.com/u/332468/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding SQL Row Counting with CASE WHEN

In the realm of SQL, counting rows based on specific conditions can often present challenges for many users, especially when employing the CASE WHEN statement. This guide will explore a common issue where querying a database for row counts by location can lead to unexpected results. We'll go over an example, identify the problem, and provide a clear solution.

The Problem: Incorrect Row Counts

Consider the following scenario presented by a user: they want to gather a total count of students by their respective locations (UK and Canada). Below is the structure of the student's data table:

StudentMarksLocationDateKenn66UK09-01-2022Kenn89UK09-01-2022Kenn77Canada09-01-2022The user attempted to execute the following SQL query:

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

The Output Confusion

What did they receive? An output with both counts equal to 3, which was not the expected outcome. They anticipated to see:

2 counts for the UK

1 count for Canada

Here’s the output they actually received:

StudentUKCanadaKenn33Clearly, something was amiss. But what?

The Solution: Switching to SUM

The key to resolving this issue lies in the aggregation function used. Instead of using COUNT, which counts non-null values, we should use SUM in conjunction with a CASE WHEN statement. This method allows you to directly count occurrences where your condition evaluates to true.

Revised SQL Query

Here's the corrected SQL query:

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

Why This Works

SUM with CASE: Using SUM lets us tally occurrences of the specified conditions. By returning 1 when a match is found (e.g., the location is UK) and 0 otherwise, we can directly accumulate the results accurately.

Conditional Logic: Each condition is evaluated independently, allowing for the counting of values across different categories effectively.

Expected Output

After implementing the revised query, the output should now correctly represent the counts:

StudentUKCanadaKenn21Conclusion

Counting rows effectively in SQL, particularly with a CASE WHEN statement, is crucial for obtaining accurate data insights. Transitioning from using COUNT to SUM for conditional tallying helps generate the correct outcome. By understanding this distinction, you can ensure that your SQL queries yield the results you intend to see.

This guide aims to clarify how to manage SQL row counting properly and enhance your database querying skills. If you ever find discrepancies in your row counts, remember to revisit the aggregation methods you're using!