Why Are My MySQL SUM() Results Incorrect When Joining Multiple Tables?

MySQL SUM() and Multi-Table Joins: Avoiding Incorrect Aggregations

Combining SUM() calculations with multiple table joins in MySQL requires careful consideration to prevent inaccurate results. A common pitfall arises from the Cartesian product effect during joins, leading to inflated sums.

The Problem: Inflated SUM() Results

A user attempted to consolidate two queries, each calculating sums from different tables, into a single joined query.

Query 1 (Mileage): Sums drive time (in minutes) per week, grouped by teacher.

<code class="language-sql">SELECT last_name, first_name, ..., SUM(drive_time) AS MINUTES
FROM bhds_mileage ...
WHERE mil_date BETWEEN ... AND ...
GROUP BY ...</code>

Query 2 (Timecard): Sums total hours per week, grouped by teacher.

<code class="language-sql">SELECT last_name, first_name, ..., SUM(tm_hours) AS total
FROM bhds_timecard ...
WHERE tm_date BETWEEN ... AND ...
GROUP BY ...</code>

Failed Join Attempt:

The user's attempt to combine these queries directly resulted in incorrect sums:

<code class="language-sql">SELECT last_name, first_name, ..., SUM(tm_hours) AS total, SUM(drive_time) AS MINUTES
FROM bhds_timecard ...
LEFT JOIN bhds_mileage ...
ON ...
WHERE ...
GROUP BY ...</code>

The issue? The SUM() functions were applied after the join, leading to the multiplication of sums due to the potential for multiple matching rows between the joined tables.

The Solution: Pre-aggregated Subqueries

The correct approach involves pre-aggregating the sums in subqueries before joining:

<code class="language-sql">SELECT last_name, first_name, ..., total, minutes
FROM bhds_teachers ...
LEFT JOIN (
    -- Subquery 1: Mileage SUM
    SELECT teacher_id, SUM(drive_time) AS minutes, ...
    FROM bhds_mileage ...
    WHERE mil_date BETWEEN ... AND ...
    GROUP BY teacher_id
) AS m ON ...
LEFT JOIN (
    -- Subquery 2: Timecard SUM
    SELECT teacher_id, SUM(tm_hours) AS total, ...
    FROM bhds_timecard ...
    WHERE tm_date BETWEEN ... AND ...
    GROUP BY teacher_id
) AS t ON ...</code>

By performing the SUM() operations within separate subqueries, we ensure that each table's data is correctly aggregated before the join occurs, preventing the multiplication of sums and producing accurate results. The teacher_id (or equivalent) is crucial for the correct join condition.

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