Understanding the Issue with SQL Group By and Aggregation Functions

As a technical blogger, I’ve come across many questions and issues on Stack Overflow that highlight common pitfalls in SQL programming. In this article, we’ll explore one such issue related to the GROUP BY clause and aggregation functions.

Background and Context

The original question posted on Stack Overflow is about a SQL query that’s intended to group data by specific columns and calculate various aggregations. However, the query is producing unexpected results due to incorrect grouping and aggregation strategies.

To understand this issue, let’s break down the key concepts involved:

GROUP BY: This clause is used to group rows in a result set based on one or more columns.

**Aggregation functions**: These are functions that perform calculations on groups of rows, such as `SUM`, `AVG`, `MAX`, and `MIN`.

The Problem with the Original Query

The original query uses a Common Table Expression (CTE) to transform the data before grouping it. However, there’s an issue with how the GROUP BY clause is applied.

WITH ParkeonCTE AS (
    SELECT 
        OccDate = CONVERT(DATE, OC.LocalStartTime),
        TotalOccSessions = COUNT(OC.SessionId),
        AuthorityId,
        TotalOccDuration = ISNULL(SUM(OC.DurationMinutes),0),
        TotalNumberOfOverstay = SUM(CAST(OC.IsOverstay AS INT)),
        TotalMinOfOverstays = ISNULL(SUM(OC.OverStayDurationMinutes),0),
        (CASE 
            WHEN OC.OspId IS NULL THEN 'OffStreet' ELSE 'OnStreet'
        END) AS ParkingContextType,
        SUM(CASE 
            WHEN CAST(OC.LocalStartTime AS TIME) >= '08:00:00' 
            AND CAST(OC.LocalStartTime AS TIME) <= '18:00:00'
                THEN 1
                ELSE 0
         END) AS TotalRestrictedSessions
    FROM Analytics.OccupancySessions AS OC
    WHERE OC.AuthorityId IS NOT NULL
    GROUP BY  CONVERT(DATE,OC.LocalStartTime), OC.AuthorityId,OC.OspId
)
SELECT 
    OC.OccDate,
    OC.ParkingContextType,
    OC.AuthorityId,
    SUM(OC.TotalRestrictedSessions),
    SUM(OC.TotalOccSessions) AS TotalOccSessions,
    AVG(OC.TotalOccDuration) AS AvgOccMinutesDuration, 
    SUM(OC.TotalOccDuration) AS TotalOccDuration,
    SUM(OC.TotalNumberOfOverstay) AS TotalNumberOfOverstays,
    SUM(OC.TotalMinOfOverstays) AS TotalMinOfOverstays,
    CAST(AVG(OC.TotalMinOfOverstays) AS decimal(10,2)) AS AvgMinOfOverstays
FROM ParkeonCTE AS OC
GROUP BY OC.OccDate, OC.AuthorityId, OC.ParkingContextType
ORDER BY OC.OccDate DESC;

The Issue with the Corrected Query

The corrected query removes TotalRestrictedSessions from the GROUP BY clause. However, it still uses this column in the SELECT statement.

WITH ParkeonCTE AS (
    SELECT 
        OccDate = CONVERT(DATE, OC.LocalStartTime),
        TotalOccSessions = COUNT(OC.SessionId),
        AuthorityId,
        TotalOccDuration = ISNULL(SUM(OC.DurationMinutes),0),
        TotalNumberOfOverstay = SUM(CAST(OC.IsOverstay AS INT)),
        TotalMinOfOverstays = ISNULL(SUM(OC.OverStayDurationMinutes),0),
        (CASE 
            WHEN OC.OspId IS NULL THEN 'OffStreet' ELSE 'OnStreet'
        END) AS ParkingContextType,
        SUM(CASE 
            WHEN CAST(OC.LocalStartTime AS TIME) >= '08:00:00' 
            AND CAST(OC.LocalStartTime AS TIME) <= '18:00:00'
                THEN 1
                ELSE 0
         END) AS TotalRestrictedSessions
    FROM Analytics.OccupancySessions AS OC
    WHERE OC.AuthorityId IS NOT NULL
    GROUP BY  CONVERT(DATE,OC.LocalStartTime), OC.AuthorityId,OC.OspId
)
SELECT 
    OC.OccDate,
    OC.ParkingContextType,
    OC.AuthorityId,
    SUM(OC.TotalRestrictedSessions),
    SUM(OC.TotalOccSessions) AS TotalOccSessions,
    AVG(OC.TotalOccDuration) AS AvgOccMinutesDuration, 
    SUM(OC.TotalOccDuration) AS TotalOccDuration,
    SUM(OC.TotalNumberOfOverstay) AS TotalNumberOfOverstays,
    SUM(OC.TotalMinOfOverstays) AS TotalMinOfOverstays,
    CAST(AVG(OC.TotalMinOfOverstays) AS decimal(10,2)) AS AvgMinOfOverstays
FROM ParkeonCTE AS OC
GROUP BY OC.OccDate, OC.AuthorityId, OC.ParkingContextType
ORDER BY OC.OccDate DESC;

The Corrected Query

The corrected query removes TotalRestrictedSessions from the SELECT statement and keeps it in the GROUP BY clause.

WITH ParkeonCTE AS (
    SELECT 
        OccDate = CONVERT(DATE, OC.LocalStartTime),
        TotalOccSessions = COUNT(OC.SessionId),
        AuthorityId,
        TotalOccDuration = ISNULL(SUM(OC.DurationMinutes),0),
        TotalNumberOfOverstay = SUM(CAST(OC.IsOverstay AS INT)),
        TotalMinOfOverstays = ISNULL(SUM(OC.OverStayDurationMinutes),0),
        (CASE 
            WHEN OC.OspId IS NULL THEN 'OffStreet' ELSE 'OnStreet'
        END) AS ParkingContextType,
        SUM(CASE 
            WHEN CAST(OC.LocalStartTime AS TIME) >= '08:00:00' 
            AND CAST(OC.LocalStartTime AS TIME) <= '18:00:00'
                THEN 1
                ELSE 0
         END) AS TotalRestrictedSessions
    FROM Analytics.OccupancySessions AS OC
    WHERE OC.AuthorityId IS NOT NULL
    GROUP BY  CONVERT(DATE,OC.LocalStartTime), OC.AuthorityId,OC.OspId
)
SELECT 
    OccDate = OC.OccDate,
    ParkingContextType = OC.ParkingContextType,
    AuthorityId = OC.AuthorityId,
    SUM(OC.TotalRestrictedSessions) AS TotalRestrictedSessions,
    SUM(OC.TotalOccSessions) AS TotalOccSessions,
    AVG(OC.TotalOccDuration) AS AvgOccMinutesDuration, 
    SUM(OC.TotalOccDuration) AS TotalOccDuration,
    SUM(OC.TotalNumberOfOverstay) AS TotalNumberOfOverstays,
    SUM(OC.TotalMinOfOverstays) AS TotalMinOfOverstays,
    CAST(AVG(OC.TotalMinOfOverstays) AS decimal(10,2)) AS AvgMinOfOverstays
FROM ParkeonCTE AS OC
GROUP BY OC.OccDate, OC.AuthorityId, OC.ParkingContextType
ORDER BY OC.OccDate DESC;

Conclusion

In this article, we’ve explored the issue with SQL GROUP BY and aggregation functions. We’ve seen how incorrect grouping and aggregation strategies can lead to unexpected results.

To avoid similar issues in the future, it’s essential to carefully consider the columns used in the GROUP BY clause and ensure that they match the columns used in the SELECT statement.

Additionally, when using Common Table Expressions (CTEs), make sure to properly define the columns used in the CTE and use them consistently throughout the query.

Last modified on 2025-03-10