How Can I Avoid Multiplied SUM Results When Joining Tables in MySQL?

Avoiding Multiplied SUM Results in MySQL Joins

MySQL joins, when combined with SUM aggregations, often produce inaccurate, inflated results due to the Cartesian product effect. Multiple joins can lead to row duplication, inflating the SUM values. The solution is to pre-aggregate data using subqueries.

Effective Strategy: Subqueries for Accurate Aggregation

To prevent inflated SUMs, perform the aggregations before joining tables. This is achieved through subqueries that group and sum data independently, then join the aggregated results.

Query Example and Explanation:

This query demonstrates how to correctly join tables while using SUM aggregations:

SELECT 
    i.last_name, 
    i.first_name, 
    DATE_FORMAT(LEAST(m.mil_date, t.tm_date), '%m/%d/%y') AS dates, 
    t.total, 
    m.minutes
FROM 
    bhds_teachers AS i
LEFT JOIN (
    SELECT 
        ds_id, 
        YEARWEEK(mil_date) AS week, 
        MIN(mil_date) AS mil_date, 
        SUM(drive_time) AS minutes
    FROM 
        bhds_mileage
    WHERE 
        mil_date BETWEEN '2016-04-11' AND '2016-04-30'
        AND ds_id = 5
    GROUP BY 
        ds_id, week
) AS m ON m.ds_id = i.ds_id
LEFT JOIN (
    SELECT 
        ds_id, 
        YEARWEEK(tm_date) AS week, 
        MIN(tm_date) AS tm_date, 
        SUM(tm_hours) AS total
    FROM 
        bhds_timecard
    WHERE 
        tm_date BETWEEN '2016-04-11' AND '2016-04-30'
        AND ds_id = 5
    GROUP BY 
        ds_id, week
) AS t ON t.ds_id = i.ds_id AND t.week = m.week;

The subqueries (m and t) independently calculate the sums for drive_time and tm_hours, respectively, grouping by ds_id and week. These aggregated results are then joined with bhds_teachers (aliased as i) using the ds_id and matching week numbers. LEAST() selects the earliest date from the two aggregated tables. This method avoids the multiplication problem inherent in joining before summing.

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