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

Understanding Unexpected Row Counts in LEFT OUTER JOINs

While LEFT OUTER JOINs are designed to return all rows from the left table, the result set can sometimes be larger than expected. This occurs due to a key characteristic of how LEFT OUTER JOINs function.

Unlike a simple assumption that unmatched rows from the left table will simply have NULL values in the right table's columns, a LEFT OUTER JOIN actually includes all matching rows from both tables. Therefore, if a single row in the left table matches multiple rows in the right table, the result will contain multiple copies of that left table row. This is similar to how an INNER JOIN behaves, resulting in a higher row count than anticipated.

Consider this SQL query:

<code class="language-sql">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</code>

Notice that only columns from SUSP.Susp_Visits are selected. However, because of the potential for multiple matches in DATA.Dim_Member, each row in SUSP.Susp_Visits might appear multiple times in the output. This explains the increased row count.

If you only need data from the left table and want to avoid duplicate rows, a LEFT OUTER JOIN is not necessary. A simple SELECT statement from the left table will provide the required data without the inflated row count. Understanding this behavior is crucial for effectively using LEFT OUTER JOINs and obtaining accurate results.

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