Why are Multiple LEFT JOINs Producing Incorrect Visit Counts?

Understanding Inaccurate Visit Counts from Multiple LEFT JOINs

The provided query generates incorrect visit counts because the sequential nature of multiple LEFT JOIN operations leads to multiplicative, rather than additive, results.

Initially, joining with the grocery table correctly counts grocery visits per user. However, the subsequent LEFT JOIN with fishmarket creates multiple rows for each user, effectively multiplying the grocery visit count by the number of fishmarket visits for each user. This explains the erroneous "12" count for users visiting both stores.

The Solution: Subqueries for Accurate Aggregation

The problem is solved using subqueries to independently aggregate visit counts for each store:

SELECT u.id,
       u.account_balance,
       (SELECT COUNT(*) FROM grocery WHERE user_id = u.id) AS "# of grocery visits",
       (SELECT COUNT(*) FROM fishmarket WHERE user_id = u.id) AS "# of fishmarket visits"
FROM users u
ORDER BY u.id;

This revised query accurately counts and displays grocery and fishmarket visits for each user, providing the correct aggregated data.

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