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

LEFT OUTER JOIN Unexpected Behavior: More Rows Than the Left Table

A common misconception about LEFT OUTER JOINs is that the result set will always have the same or fewer rows than the left table. However, this isn't always true. The number of rows can exceed the left table's row count under specific conditions.

This issue was observed in a query using a LEFT OUTER JOIN between "SUSP.Susp_Visits" (left table) and "DATA.Dim_Member" (right table). The joined result had more rows than the 4935 rows in "SUSP.Susp_Visits".

Understanding LEFT OUTER JOINs

A LEFT OUTER JOIN functions as follows:

• Includes all rows from the left table.
• For each left table row, it searches for matching rows in the right table based on the join condition.
• If a match is found, the corresponding right table row is included.
• If no match is found, NULL values are returned for the right table's columns.

Why Extra Rows Appear

The extra rows aren't created by duplicating left table rows, but rather by the right table's structure. The problem arises when multiple rows in the right table ("DATA.Dim_Member") match a single row in the left table ("SUSP.Susp_Visits").

For instance, if a "MemID" in "SUSP.Susp_Visits" corresponds to multiple "MembershipNum" values in "DATA.Dim_Member", the JOIN will produce multiple output rows for that single left table row, thus increasing the overall row count.

Solutions

To avoid this row inflation in a LEFT OUTER JOIN:

• Restrict Selection: Select only columns from the left table. This guarantees the output row count won't exceed the left table's row count.
• Use INNER JOIN: If you only need rows where a match exists between both tables, an INNER JOIN is more appropriate. This will eliminate the extra rows caused by multiple matches on the right side.

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