Why Are My Two SQL LEFT JOINS Returning Incorrect Visit Counts?

Debugging SQL LEFT JOINs: Accurate Visit Counts

A common problem arises when using multiple LEFT JOINs in SQL to count visits across different tables. This example uses three tables: users, grocery, and fishmarket. A query designed to count each user's visits to both grocery and fishmarket stores produces inaccurate results.

The issue stems from the sequential nature of the joins. A naive approach joins users and grocery first, creating a row for each user with a non-null account_balance and their grocery visits. The subsequent join with fishmarket then incorrectly multiplies the grocery visit count by the fishmarket visit count for each user. This leads to inflated visit totals.

The solution involves restructuring the query to aggregate visit counts before joining them to the users table. This is efficiently accomplished using two correlated subqueries. Each subquery independently counts visits for either the grocery or fishmarket table, grouping by user_id. These aggregated counts are then joined to the users table, ensuring accurate visit totals.

Here's the corrected query using subqueries:

<code class="language-sql">SELECT u.id, u.account_balance, g.grocery_visits, f.fishmarket_visits
FROM users u
LEFT JOIN (
    SELECT user_id, COUNT(*) AS grocery_visits
    FROM grocery
    GROUP BY user_id
) g ON g.user_id = u.id
LEFT JOIN (
    SELECT user_id, COUNT(*) AS fishmarket_visits
    FROM fishmarket
    GROUP BY user_id
) f ON f.user_id = u.id
ORDER BY u.id;</code>

This revised query provides accurate, independent counts of grocery and fishmarket visits for each user, resolving the multiplication error of the original query.

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