Why is My MySQL Query Returning Fewer Results Than Expected?

Distinguishing Between MySQL JOIN and LEFT JOIN

In the context of database queries, understanding the nuances between different join types is crucial. When encountering errors like the one described, it's important to delve into the differences between a regular JOIN and a LEFT JOIN.

Default Join Type

Contrary to your assumption, the default join type in MySQL is INNER JOIN, not LEFT JOIN. This means that if you omit specifying a join type, it defaults to INNER JOIN.

Understanding the Difference

An INNER JOIN, as depicted in the visual illustration, matches rows with common values from the participating tables. In contrast, a LEFT JOIN includes all the rows from the left table (in your case, DM_Server.Jobs) and only the matching rows from the right table. If there are no matches, the result includes null values for the missing data.

Solution

To obtain the desired behavior of returning all rows from the left table, you can explicitly use a LEFT JOIN:

SELECT
            `DM_Server`.`Jobs`.*,
            `DM_Server`.servers.Description AS server,
            digital_inventory.params,
            products.products_id,
            products.products_pdfupload,
            customers.customers_firstname,
            customers.customers_lastname
        FROM `DM_Server`.`Jobs`
        LEFT JOIN `DM_Server`.servers ON servers.ServerID = Jobs.Jobs_ServerID
        LEFT JOIN `cpod_live`.`digital_inventory` ON digital_inventory.jobname = Jobs.Jobs_Name
        LEFT JOIN `cpod_live`.`products` ON products.products_pdfupload = CONCAT(digital_inventory.jobname, ".pdf")
        LEFT JOIN `cpod_live`.`customers` ON customers.customers_id = products.cID
        ORDER BY `DM_Server`.`Jobs`.Jobs_StartTime DESC LIMIT 50

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