Why Does My Left Outer Join Fail When Adding Date Parameters to the WHERE Clause?

Troubleshooting a LEFT OUTER JOIN with Date Filters

This SQL query uses a LEFT OUTER JOIN across three tables: salesrep, prescriber, and prescriptions. The goal is to retrieve all data from salesrep, even if there are no matching entries in prescriber or prescriptions. The problem arises when date parameters are added to the WHERE clause. Instead of returning all salesrep data as expected, the query only returns results where matches exist in all three tables.

The issue stems from filtering on prescriptions.filldate within the WHERE clause. This condition effectively transforms the LEFT OUTER JOIN into an INNER JOIN for those salesrep entries lacking corresponding prescriptions data, as the WHERE clause eliminates rows with NULL values for prescriptions.filldate.

The Solution: Relocate Date Filters to the JOIN Condition

The correct approach is to move the date constraints from the WHERE clause into the ON clause of the LEFT OUTER JOIN between prescriber and prescriptions. This ensures that the date filtering only affects the join between these two tables, while still preserving the intended behavior of the LEFT OUTER JOIN with respect to salesrep.

The corrected join should look like this:

LEFT OUTER JOIN prescriptions ON prescriber.dea_no = prescriptions.dea_no
                             AND prescriptions.filldate >= '09-01-12'
                             AND prescriptions.filldate <= '09-17-12'

By placing the date restrictions within the ON clause, rows from salesrep will be included in the results even if there are no matching entries in prescriptions that satisfy the date criteria. These rows will simply have NULL values for the columns from prescriber and prescriptions. This accurately reflects the desired behavior of a LEFT OUTER JOIN.

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