MySQL左外连接，带有连接表中的Count，显示所有记录

Question

i am trying to create a query where there is a count of related records from another table. I'd like the "parent" records whether there are related records (a count) or not.

SELECT r.region_name, r.region_id, r.abbreviation, r.map_order,
    (IFNULL( COUNT( p.property_id ) , 0 )) AS propertyCount
FROM  `rmp_region` r
LEFT OUTER JOIN  `rmp_property` p 
    ON p.path LIKE CONCAT(  '%/', r.region_id,  '/%' ) 
WHERE p.active =1
AND r.parent_region_id =1
GROUP BY r.region_name, r.region_id, r.abbreviation, r.map_order
ORDER BY r.map_order ASC 

I've tried different variations of this.. but i cannot get the parent records to display if the count is zero/no related records.

thanks for any help!

Answer 1

You need to move "p.active = 1" from the WHERE clause into the OUTER JOIN criteria.

Here's your example with the change:

SELECT r.region_name, r.region_id, r.abbreviation, r.map_order,
    (IFNULL( COUNT( p.property_id ) , 0 )) AS propertyCount
FROM  `rmp_region` r
LEFT OUTER JOIN  `rmp_property` p 
    ON p.path LIKE CONCAT(  '%/', r.region_id,  '/%' ) and p.active =1
WHERE r.parent_region_id =1
GROUP BY r.region_name, r.region_id, r.abbreviation, r.map_order
ORDER BY r.map_order ASC 

Answer 2

You asked about optimizing it, hoping that a index on the "path" column might help. Unfortunately you are searching for a value of path that matches LIKE CONCAT( '%/', r.region_id, '/%' ) , and no index in the world is smart enough to work with that.