Using Case When in Combination with Group By

Introduction to Conditional Statements and Window Functions

When working with data, it’s common to encounter situations where we need to perform multiple conditions on a dataset. In this case, we’re dealing with a scenario where we want to use the CASE WHEN statement in combination with grouping and aggregation.

In SQL, the CASE WHEN statement allows us to evaluate conditional expressions and return one value if the condition is true and another value if it’s false. Meanwhile, window functions like max() enable us to perform calculations on a set of rows that are related to the current row.

In this article, we’ll explore how to use these two concepts together to solve a specific problem.

Background: Understanding the Problem Statement

The original question presents a table with several columns, including PersoonID, AttributeID, StartDate, EndDate, and Verhouding. The goal is to identify rows where the Verhouding column has a value other than zero and flag those rows using a verhouding_flag column.

The original solution uses a CASE WHEN statement with an aggregate function (CASE WHEN Verhouding != 0 THEN 1 ELSE 0 END) to generate a binary flag for each row. However, this approach doesn’t consider cases where there are multiple rows with non-zero values in the same group (i.e., PersoonID, AttributeID, and StartDate).

Solution: Using max() Window Function

To address this issue, we can use the max() window function to find the maximum value of the binary flag for each group. This will ensure that if there are multiple rows with non-zero values in the same group, only one row is flagged.

The modified SQL query looks like this:

SELECT t.*,
       Max(case when Verhouding != 0 then 1 else 0 end) over(partition by PersoonID) verhouding_flag
FROM t;

Here’s a breakdown of what’s happening:

• case when Verhouding != 0 then 1 else 0 end: This expression generates a binary flag for each row based on the value in the Verhouding column. If the value is not zero, the flag is set to 1; otherwise, it’s set to 0.
• Max(...) over(partition by PersoonID): This applies the max() window function to the binary flag expression, partitioning the result by the PersoonID column. The partition by clause ensures that the calculation is performed separately for each group of rows with the same PersoonID.
• verhouding_flag: The resulting value from the Max() window function is assigned to the new verhouding_flag column.

Example Use Cases

Here’s an example table based on the original problem statement:

Running the modified SQL query will produce the following result:

As you can see, the verhouding_flag column now correctly identifies rows where the Verhouding value is non-zero and aggregates the result across groups.

Conclusion

In this article, we explored how to use the CASE WHEN statement in combination with the max() window function to solve a specific problem. By using these two SQL concepts together, we can efficiently identify rows where the specified condition is met while considering cases where there are multiple non-zero values in the same group.

This approach provides a clear and concise solution to the original problem and demonstrates how to effectively use conditional statements and window functions in SQL queries.

Last modified on 2023-10-09