Why Does My SQL LEFT OUTER JOIN Return More Rows Than the Left Table?

Understanding SQL LEFT OUTER JOIN: Why More Rows Than the Left Table?

A common misconception about LEFT OUTER JOIN is that it always returns the exact number of rows from the left table. While it includes all rows from the left table, it can actually produce more rows if there are duplicate matches in the right table.

Let's illustrate this with an example:

Consider this query:

SELECT     
    SUSP.Susp_Visits.SuspReason, SUSP.Susp_Visits.SiteID
FROM
    SUSP.Susp_Visits 
LEFT OUTER JOIN
    DATA.Dim_Member ON SUSP.Susp_Visits.MemID = DATA.Dim_Member.MembershipNum

This query aims to join Susp_Visits (left table) with Dim_Member (right table). Intuitively, one might expect a maximum of 4935 rows (assuming that's the count of Susp_Visits). However, the actual result could be larger.

The reason is that a LEFT OUTER JOIN creates a row in the result set for every match between the left and right tables. If a single row in Susp_Visits matches multiple rows in Dim_Member, it will be duplicated in the output, resulting in more rows than initially expected.

Therefore, a larger-than-expected row count after a LEFT OUTER JOIN indicates that at least one row in the left table has multiple corresponding rows in the right table based on the join condition.

To address this:

• Only need left table data? Simply use a SELECT statement on the left table without any joins.
• Eliminate duplicates? Add a DISTINCT clause to your SELECT statement: SELECT DISTINCT SUSP.Susp_Visits.SuspReason, SUSP.Susp_Visits.SiteID ... This will remove duplicate rows from the result set.

This clarifies the behavior of LEFT OUTER JOIN and provides solutions for handling situations where the result set exceeds the number of rows in the left table.

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