Why Doesn't My LEFT OUTER JOIN Work with Date Filters in the WHERE Clause?

Debugging a Failing LEFT OUTER JOIN with Date Filtering

A common SQL query challenge involves using LEFT OUTER JOIN with date filters. The expectation is that data from the left table (e.g., Salesrep) is returned even if there's no matching data in the right tables (Prescriber and Prescriptions). However, adding date filters to the WHERE clause can unexpectedly filter out records from the left table.

Understanding the Issue:

The problem arises from the location of the date range conditions. Placing them in the WHERE clause incorrectly filters results after the join, effectively removing rows from the left table that lack matching entries within the specified date range.

The Correct Approach:

To maintain the integrity of the LEFT OUTER JOIN, the date range criteria should be incorporated into the ON clause of the join itself. This ensures the filtering occurs during the join, preserving all records from the left table, regardless of matching prescriptions within the date range.

Revised Query:

<code class="language-sql">LEFT OUTER JOIN prescriptions ON prescriber.dea_no = prescriptions.dea_no
                             AND prescriptions.filldate >= '09-01-12'
                             AND prescriptions.filldate <= '09-30-12'</code>

By relocating the date filters to the ON clause, the query accurately returns all data from the Salesrep table, including sales representatives without prescriptions within the specified timeframe. This correctly implements the behavior expected from a LEFT OUTER JOIN.

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