Why are Rows Missing from My Left Outer Join Result?

Understanding Missing Rows in Left Outer Joins

A user attempting to calculate daily page views encountered a common left outer join pitfall: missing rows from the left table. The query aimed to combine data from day and tracking tables using a left outer join:

<code class="language-sql">SELECT day.days, COUNT(*) as opens
FROM day
LEFT OUTER JOIN tracking ON day.days = DAY(FROM_UNIXTIME(open_date))
WHERE tracking.open_id = 10
GROUP BY day.days</code>

The day table contained data for days 1-30, but the result only showed days 1 and 9. This discrepancy arises from the placement of the WHERE clause.

The Problem: Incorrect WHERE Clause Placement

The WHERE tracking.open_id = 10 clause filters after the join. This means it removes rows where tracking.open_id is not 10, even if those rows originated from the left (day) table. A left outer join is designed to include all rows from the left table, regardless of matches in the right table. The WHERE clause effectively negates this behavior.

The Solution: Integrating the Filter into the JOIN Condition

To correctly retrieve all days from the day table, the filtering condition must be integrated into the JOIN condition itself:

<code class="language-sql">SELECT day.days, COUNT(*) as opens
FROM day
LEFT OUTER JOIN tracking ON day.days = DAY(FROM_UNIXTIME(open_date)) AND tracking.open_id = 10
GROUP BY day.days</code>

By moving tracking.open_id = 10 into the ON clause, the filtering occurs before the rows are combined. This ensures that only matching rows from the tracking table (where open_id = 10) are considered during the join, while still retaining all rows from the day table in the final result. This accurately reflects the intent of a left outer join.

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