How Can Multiple Left Joins Lead to Incorrect Count Results in SQL, and How Can This Be Fixed?

Traps and solutions for SQL multiple left joins that lead to incorrect counting results

When using SQL, if multiple left joins are not handled correctly, unexpected results may occur. Consider the following query:

SELECT
    t1."id" AS "User ID",
    t1.account_balance AS "Account Balance",
    count(t2.user_id) AS "# of grocery visits",
    count(t3.user_id) AS "# of fishmarket visits"
FROM users t1
LEFT OUTER JOIN grocery t2 ON (t2.user_id=t1."id") 
LEFT OUTER JOIN fishmarket t3 ON (t3.user_id=t1."id") 
GROUP BY t1.account_balance,t1.id
ORDER BY t1.id

This query attempts to count the number of visits to two related tables users and grocery for each user in the fishmarket table. However, due to the nature of left joins, it produces incorrect results.

Misunderstandings about left joins

In SQL, joins are performed from left to right. In this query, a left join between users and grocery is first performed. This results in each user record being connected to its corresponding grocery purchase record. Next, a second left join is performed between the first join result and fishmarket, which means that each user record with a grocery purchase record will be further joined with its corresponding fish market purchase record.

Multiplication instead of addition

An unintended consequence of this sequential processing is that the access counts from grocery and fishmarket are multiplied rather than added. For example, if a user had 3 grocery visits and 4 fish market visits, the query would yield 12 visits instead of the expected 7.

Solution: Use subqueries for aggregation

To correct this, we need to make sure we perform an aggregation (count) of the number of visits before joining the tables. This can be achieved by using a subquery:

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;

This modified query aggregates the visits in a subquery and then joins the aggregated results with the users table, ensuring that the visits are not accidentally multiplied.

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