Why Doesn't My Left Outer Join Return All Rows from the Left Table?

Left Outer Joins: Ensuring Complete Data Retrieval from the Left Table

A common misconception with left outer joins is that they always return all rows from the left table. This isn't always true; rows from the left table are omitted if no matching rows exist in the right table based on the join condition.

Let's illustrate this with an example. Imagine a query designed to count daily page views for a specific user (open_id):

SELECT day.days, COUNT(*) AS views
FROM day
LEFT OUTER JOIN tracking
ON day.days = DAY(FROM_UNIXTIME(open_date))
WHERE tracking.open_id = 10
GROUP BY day.days

This query intends to show all days and their associated view counts. However, if day contains entries for every day of the month, but tracking lacks entries for some days for open_id = 10, those days will be absent from the results.

The solution lies in incorporating the WHERE clause condition into the ON clause of the join:

SELECT day.days, COUNT(tracking.open_date) AS views
FROM day
LEFT OUTER JOIN tracking
ON day.days = DAY(FROM_UNIXTIME(open_date)) AND tracking.open_id = 10
GROUP BY day.days

By moving tracking.open_id = 10 into the ON clause, the condition becomes part of the join itself, not a post-join filter. This ensures all rows from the day table are included, with views showing 0 for days without matching entries in the tracking table. Note the use of COUNT(tracking.open_date) instead of COUNT(*) to accurately reflect the number of views; COUNT(*) would still count the days even if there were no matching rows in the right table.

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