How to Retrieve a Row Based on the Sum of Previous Values Using MySQL\'s SUM() Function?

Using MySQL SUM() Function in WHERE Clause for Row Retrieval

MySQL allows users to perform aggregate calculations within subqueries and use them as criteria for row selection in the WHERE clause. However, it is important to understand the limitations of using aggregate functions in this context.

Consider the following scenario: you have a table with the columns id and cash, and you want to retrieve the first row where the sum of all previous cash values exceeds a specific threshold.

Example Table:

Desired Result:

For an input threshold of 500, the expected result is row 3 because the sum of cash values for rows 1 and 2 (200 301 = 501) exceeds the threshold.

Incorrect Approach:

Attempting to use WHERE SUM(cash) > 500 will not yield the correct result because aggregates cannot be directly compared in a WHERE clause.

Solution:

To overcome this limitation, you can use the HAVING clause in conjunction with a subquery to calculate the running sum of cash for each row.

<code class="sql">SELECT y.id, y.cash
FROM (
    SELECT t.id,
           t.cash,
           (SELECT SUM(x.cash)
              FROM TABLE x
             WHERE x.id <= t.id) AS running_total
     FROM TABLE t
     ORDER BY t.id
) y
WHERE y.running_total > 500
ORDER BY y.id
LIMIT 1;</code>

Explanation:

1. The subquery calculates the running total of cash for each row using a nested SELECT statement.
2. The result of the subquery is aliased as y.
3. The WHERE clause checks if the running total for a row exceeds the desired threshold (500 in this case).
4. Finally, the LIMIT clause retrieves only the first qualifying row.

This approach ensures that the first row where the sum of all previous cash values exceeds the threshold is accurately identified.

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