How to Perform Conditional Summation in MSSQL Group By Queries?

Conditional Summation with MSSQL's GROUP BY Clause

Microsoft SQL Server (MSSQL) offers flexible data aggregation using the GROUP BY clause. To perform conditional summation within a GROUP BY query, leverage the CASE statement to selectively include values in the sum based on specific criteria.

Consider an OrderDetails table with the following structure:

<code>----------------------------------------------------------------
|  OrderId  |  CopyCost  |  FullPrice  |  Price  |  PriceType  |
----------------------------------------------------------------
|  16       |  50        |  100        |  50     |  CopyCost   |
----------------------------------------------------------------
|  16       |  50        |  100        |  100    |  FullPrice  |
----------------------------------------------------------------
|  16       |  50        |  100        |  50     |  CopyCost   |
----------------------------------------------------------------
|  16       |  50        |  100        |  50     |  CopyCost   |
----------------------------------------------------------------</code>

To calculate the ItemCount, TotalCopyCost, and TotalFullPrice for each OrderId, use this query:

SELECT OrderId, 
       COUNT(*) AS ItemCount,
       SUM(CASE WHEN PriceType = 'CopyCost' THEN Price ELSE 0 END) AS TotalCopyCost,
       SUM(CASE WHEN PriceType = 'FullPrice' THEN Price ELSE 0 END) AS TotalFullPrice
FROM OrderDetails
GROUP BY OrderId;

The CASE statement within the SUM() function conditionally sums Price values. If PriceType is 'CopyCost', the Price is included; otherwise, 0 is added. This allows for separate totals for 'CopyCost' and 'FullPrice' per OrderId.

The output would be:

<code>----------------------------------------------------------------
|  OrderId  |  ItemCount  |  TotalCopyCost  |  TotalFullPrice  |
----------------------------------------------------------------
|  16       |  4          |  150            |  100             |
----------------------------------------------------------------</code>

This demonstrates how CASE statements enable powerful conditional aggregation within GROUP BY queries, providing a flexible method for data summarization and analysis in MSSQL.

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