Why are My MySQL LEFT JOIN Queries Returning Fewer Rows Than Expected?

Resolving the Issue with Null Rows in MySQL Left Join

In MySQL, when executing a left join query, it is common to encounter a scenario where not all rows from the left table are returned. This issue arises when the join condition is specified in the WHERE clause instead of the ON clause.

Consider the following query, which joins tables jos_hp_properties (pr) and jos_hp_properties2 (pr7):

As mentioned by the user, this query only returns one row from the first table despite there being 27 rows in the jos_hp_properties table. The reason for this is that the WHERE condition pr7.field=23 filters out all rows from jos_hp_properties that do not have a matching field value of 23 in the jos_hp_properties2 table.

To resolve this issue and retrieve all rows from the first table, including the room_price_high value from the second table (or NULL if there is no matching field), the WHERE condition should be moved to the ON clause of the left join:

By placing the join condition in the ON clause, the left join will retain all rows from the jos_hp_properties table and only filter the jos_hp_properties2 table based on the matching field value. This ensures that all rows from the first table are returned, even if some do not have a corresponding row in the second table.

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