Why Doesn't My MySQL LEFT JOIN Return All Rows?

Left Join Not Returning All Rows: MySQL Query Issue

In MySQL, a LEFT JOIN operation is commonly used to combine rows from two tables based on a common key. However, in certain scenarios like the one described in this question, the query may not return all expected rows.

Query Structure and Problem

The issue arises when the WHERE clause is applied directly to the join condition, as seen in the original query:

SELECT pr.*, pr7.value AS `room_price_high`
FROM `jos_hp_properties` pr
LEFT OUTER JOIN `jos_hp_properties2` pr7 ON pr7.property=pr.id
WHERE pr7.field=23

In this case, the WHERE clause restricts the join to only return rows where pr7.field = 23. As a result, it excludes any rows in the jos_hp_properties table that do not have a matching pr7.field entry with the value 23. This leads to the query returning fewer rows than expected.

Solution: Moving the WHERE Condition

To resolve the issue and ensure that all rows from the first table are returned, the WHERE condition should be moved to the ON condition of the JOIN, as follows:

SELECT pr.*, pr7.value AS `room_price_high`
FROM `jos_hp_properties` pr
LEFT JOIN `jos_hp_properties2` pr7 ON pr7.property=pr.id
AND pr7.field=23

By moving the condition to the JOIN, the query first establishes the left join between the tables, and then filters the joined rows based on the specified criteria. This ensures that all rows from the jos_hp_properties table are included in the result set, regardless of whether they have a matching entry in the jos_hp_properties2 table with pr7.field = 23.

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