MySQL：从自引用（？）SELECT查询生成的列

Question

I'm having a hard time wrapping my head around how the column product_count works. It seems to be somehow referencing itself with aliases see and e , which are both pointing to catalog_category_entity . Specifically,

WHERE (see.entity_id = e.entity_id) OR (see.path LIKE CONCAT(e.path, '/%'))

Both see and e are aliases for table catalog_category_entity . What is this doing?

Here's the entire query:

SELECT `e`.*, `d_name`.`value` AS `name`, IF(s_name.value_id > 0, s_name.value, d_name.value) AS `name`, `d_is_active`.`value` AS `is_active`, IF(s_is_active.value_id > 0, s_is_active.value, d_is_active.value) AS `is_active`, `d_is_anchor`.`value` AS `is_anchor`, IF(s_is_anchor.value_id > 0, s_is_anchor.value, d_is_anchor.value) AS `is_anchor`,
    (
        SELECT COUNT(DISTINCT scp.product_id)
        FROM `catalog_category_entity` AS `see`
            LEFT JOIN `catalog_category_product` AS `scp`
                ON see.entity_id=scp.category_id
        WHERE (see.entity_id = e.entity_id) OR (see.path LIKE CONCAT(e.path, '/%'))
    ) AS `product_count`,
    (
        SELECT COUNT(cp.product_id)
        FROM `catalog_category_product` AS `cp`
        WHERE (cp.category_id = e.entity_id)
    ) AS `self_product_count`
FROM `catalog_category_entity` AS `e`
    LEFT JOIN `catalog_category_entity_varchar` AS `d_name` ON d_name.entity_id=e.entity_id AND d_name.attribute_id=41 AND d_name.entity_type_id=e.entity_type_id AND d_name.store_id=0
    LEFT JOIN `catalog_category_entity_varchar` AS `s_name` ON s_name.entity_id=e.entity_id AND s_name.attribute_id=41 AND s_name.entity_type_id=e.entity_type_id AND s_name.store_id=0
    LEFT JOIN `catalog_category_entity_int` AS `d_is_active` ON d_is_active.entity_id=e.entity_id AND d_is_active.attribute_id=42 AND d_is_active.entity_type_id=e.entity_type_id AND d_is_active.store_id=0
    LEFT JOIN `catalog_category_entity_int` AS `s_is_active` ON s_is_active.entity_id=e.entity_id AND s_is_active.attribute_id=42 AND s_is_active.entity_type_id=e.entity_type_id AND s_is_active.store_id=0
    LEFT JOIN `catalog_category_entity_int` AS `d_is_anchor` ON d_is_anchor.entity_id=e.entity_id AND d_is_anchor.attribute_id=51 AND d_is_anchor.entity_type_id=e.entity_type_id AND d_is_anchor.store_id=0
    LEFT JOIN `catalog_category_entity_int` AS `s_is_anchor` ON s_is_anchor.entity_id=e.entity_id AND s_is_anchor.attribute_id=51 AND s_is_anchor.entity_type_id=e.entity_type_id AND s_is_anchor.store_id=0
WHERE (`e`.`entity_type_id` = '3') AND (e.entity_id IN('24', '533')) ORDER BY LENGTH(e.path) ASC

Answer 1

This is an example of a correlated query, here the query owner is checking a condition from outside of the sub-query with an OR clause.

In the below sub-query there is a distinct count of production_id which should exist in both tables: catalog_category_entity and catalog_category_product (even here there is no requirement of left join and an inner join may work better as you get count from the right side table) with condition that either entitiy_id should exist in catalog_category_entity as main table OR sub-query path field should match with the main table left part of path field means the main table may contain an extra string in right side but the left part should be same.

SELECT COUNT(DISTINCT scp.product_id)
    FROM `catalog_category_entity` AS `see`
    LEFT JOIN `catalog_category_product` AS `scp`
        ON see.entity_id=scp.category_id
    WHERE (see.entity_id = e.entity_id)
    OR (see.path LIKE CONCAT(e.path, '/%'))

You can simplify your query if the requirement is clear as you are joining table catalog_category_entity_int 4 times with left join, while you can use as below only a single time:

LEFT JOIN catalog_category_entity_int` AS `d_is_anchor`
ON d_is_anchor.entity_id=e.entity_id
    AND d_is_anchor.attribute_id IN (42,51)
    AND d_is_anchor.entity_type_id=e.entity_type_id
    AND d_is_anchor.store_id=0