6/10/2023 0 Comments Mysql limit![]() ![]() ON TRUE - this is redundant: ON i.id = si.parent_id ORDER BY s.id - assuming we don't want arbitrary results This is the LATERAL join we want to mimic: SELECT i.*, si.child, si.parent_id You can read more about the idea at for example: mysql-row_numberĪnother method that works even in ancient versions (eg 5.1) is to mimic the LATERAL join with what I call "poor man's" LATERAL JOIN / OUTER APPLY. The idea is to order the sub-query by parent_id, increment row_num by 1 for each row, and as soon as parent_id changes, reset row_num to 1. Window functions are however only supported in MySQL 8, but you can mimic them using variables: SELECT i.*, si.child, si.parent_id, si.rn Other ways to do something similar is to use a window function such as: row_number() over (partition by. What version of MySQL are you using? MySQL 8 supports LATERAL which you need to be able to refer to i from the derived table: SELECT i.*, si.child, si.parent_idĬoncider adding an ORDER BY to your sub-select. Then what is alternative approach to get result described above with 2 (or more) limit rows for each left join subquery? LEFT JOIN (SELECT * FROM sub_items WHERE parent_id = i.id LIMIT 2) as si If we try to use query bellow to achieve desired outcome MySQL will throw error: SELECT i.*, si.child, si.parent_id FROM items as i Now the challenge is to limit join table rows (in this example limit is 2 rows): '1', 'Main item1', 'Child Item11', '1' The result will be: '1', 'Main item1', 'Child Item11', '1' Now let's create query something like this: SELECT i.*, si.child, si.parent_id FROM items as i INSERT INTO `sub_items` (`child`, `parent_id`) VALUES ('Child Item31', '3') ![]() INSERT INTO `sub_items` (`child`, `parent_id`) VALUES ('Child Item24', '2') INSERT INTO `sub_items` (`child`, `parent_id`) VALUES ('Child Item23', '2') INSERT INTO `sub_items` (`child`, `parent_id`) VALUES ('Child Item22', '2') INSERT INTO `sub_items` (`child`, `parent_id`) VALUES ('Child Item21', '2') INSERT INTO `sub_items` (`child`, `parent_id`) VALUES ('Child Item13', '1') INSERT INTO `sub_items` (`child`, `parent_id`) VALUES ('Child Item12', '1') INSERT INTO `sub_items` (`child`, `parent_id`) VALUES ('Child Item11', '1') INSERT INTO `items` (`parent`) VALUES ('Main item3') INSERT INTO `items` (`parent`) VALUES ('Main item2') With the following content: INSERT INTO `items` (`parent`) VALUES ('Main item1') `child` VARCHAR(45) CHARACTER SET 'utf8' COLLATE 'utf8_unicode_ci' NOT NULL, `parent` VARCHAR(45) CHARACTER SET 'utf8' COLLATE 'utf8_unicode_ci' NOT NULL, Note that, if you only specify one argument out of the two, MySQL assumes that you have specified the count argument.Since MySql doesn't support main alias reference in subquery is a bit challenging to create query with multiple rows where you have to limit rows of left join tables.Īssuming there two tables: CREATE TABLE `items` ( For instance, if you want to return the first 3 rows, then you set the count to 3.
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