Why Does My LEFT OUTER JOIN Fail to Return Expected Results When Using Date Filters?

Solving the problem of inconsistent LEFT OUTER JOIN query results

In SQL, the LEFT OUTER JOIN operation is designed to retain the data in the left table even if the corresponding data does not exist in the right table. However, it can be confusing when a LEFT OUTER JOIN does not produce the expected results.

Suppose you need to retrieve data from three tables using LEFT OUTER JOIN. You want to get information from the Salesrep table even if no matching records exist in the Prescriber and Prescriptions tables. When executing this query with a date parameter in the WHERE clause, you encounter an unexpected result: no rows are returned for sales representatives that are not associated with any data.

To solve this problem, the key is to move the constraints of Prescriptions.filldate from the WHERE clause to the ON condition of JOIN. The modified query should look similar to the following:

<code class="language-sql">LEFT OUTER JOIN prescriptions ON prescriber.dea_no = prescriptions.dea_no AND
                             prescriptions.filldate >= '09-01-12'
                             AND prescriptions.filldate < '10-01-12'</code>

Initially, records with NULL values ​​in Prescriptions.filldate were incorrectly filtered out when the date parameter was in the WHERE clause. By including them in the ON condition, the sales representative will still be included in the result set even if there is no associated prescription details, and NULL values ​​will be displayed appropriately.

Now, when executing a query, you will successfully obtain the Salesrep table information regardless of the corresponding data in the Prescriber and Prescriptions tables.

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