Why Does My Left Outer Join Return More Records Than the Left Table?

Left Outer Joins and Unexpected Row Counts: A Closer Look

In SQL databases, a left outer join combines data from two tables, guaranteeing the inclusion of all rows from the left table in the result set. However, a common misconception is that the output will always have the same number of rows as the left table. This isn't necessarily true.

The behavior of a left outer join is to match each row from the left table with corresponding rows in the right table. If a match is found, the data from both tables is combined into a single row. Crucially, if no match is found in the right table for a given row in the left table, that left table row is still included in the result, but with NULL values for the columns from the right table.

The discrepancy arises when a single row in the left table matches multiple rows in the right table. In this case, the left outer join will generate multiple rows in the output, one for each match on the right side, leading to a row count exceeding that of the original left table.

Let's illustrate with an example SQL 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 intends to retrieve data solely from SUSP.Susp_Visits. However, if a MemID in SUSP.Susp_Visits corresponds to multiple entries in DATA.Dim_Member, the result will contain multiple rows for each such SUSP.Susp_Visits row.

To avoid this inflated row count and retrieve only the data from the left table, a simpler query is preferable:

SELECT     
    SuspReason, SiteID
FROM
    SUSP.Susp_Visits

This direct query efficiently retrieves the desired data without the potential for row count discrepancies inherent in the left outer join's behavior when dealing with one-to-many relationships between the tables.

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