Understanding the Difference between MySQL JOIN vs LEFT JOIN
MySQL JOIN is a fundamental operation that combines rows from two or more tables based on a common column. By default, MySQL performs INNER JOINs, which return only the rows that match in all the joined tables.
However, a common misunderstanding arises when users assume that the absence of an explicit join type implies a LEFT JOIN. This is not the case; the default join type remains INNER JOIN.
Example of LEFT JOIN vs INNER JOIN
Consider the following query:
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` INNER JOIN `DM_Server`.servers ON servers.ServerID = Jobs.Jobs_ServerID JOIN `cpod_live`.`digital_inventory` ON digital_inventory.jobname = Jobs.Jobs_Name JOIN `cpod_live`.`products` ON products.products_pdfupload = CONCAT(digital_inventory.jobname, ".pdf") JOIN `cpod_live`.`customers` ON customers.customers_id = products.cID ORDER BY `DM_Server`.`Jobs`.Jobs_StartTime DESC LIMIT 50
This query performs an INNER JOIN on all the tables, returning only the rows that have matching values in all the joined tables.
Now, let's modify the query to use LEFT JOINs:
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
The LEFT JOINs in this query return all the rows from the left table ("Jobs") and only the matching rows from the right tables. This means that even if there is no matching row in the right tables, the row from "Jobs" will still be included in the result.
Visual Representation of Joins
Below are visual representations of INNER JOIN and LEFT JOIN:
INNER JOIN:
TABLE 1 TABLE 2 +-----------+------+ +-----------+------+ | Column 1 | KEY | | Column 3 | KEY | +-----------+------+ +-----------+------+ | ROW 1.1 | 1 | | ROW 2.1 | 1 | | ROW 1.2 | 2 | | | | | | | | ROW 2.2 | 2 | +-----------+------+ +-----------+------+ Result: | Column 1 | Column 3 | +-----------+-----------+ | ROW 1.1 | ROW 2.1 | | ROW 1.2 | ROW 2.2 |
LEFT JOIN:
TABLE 1 TABLE 2 +-----------+------+ +-----------+------+ | Column 1 | KEY | | Column 3 | KEY | +-----------+------+ +-----------+------+ | ROW 1.1 | 1 | | ROW 2.1 | 1 | | ROW 1.2 | 2 | | | | | ROW 1.3 | 3 | | | | +-----------+------+ +-----------+------+ Result: | Column 1 | Column 3 | +-----------+-----------+ | ROW 1.1 | ROW 2.1 | | ROW 1.2 | NULL | | ROW 1.3 | NULL |
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