父子级的MySQL顺序

Question

I want to display a result of query sorted by parent and then the child. Here's my code

SELECT * FROM 
mst_category AS parent 
LEFT JOIN mst_category AS child 
ON child.category_parent = parent.category_id
GROUP BY parent.category_id
ORDER BY parent.category_group, COALESCE(parent.category_parent, parent.category_id), parent.category_parent != 0, child.category_id

The result of the query :

As you can see some of the children are not sorted by parent id. My question is how to sort it ?

Answer 1

Simplify your order by clause to just what you want the sort order to be:

SELECT 
    parent.category_id as `parent_id`,
    parent.category_group as `parent_group`,
    child.category_id as `child_id`
    child.category_group as `child_group`
FROM mst_category AS parent 
LEFT JOIN mst_category AS child 
    ON child.category_parent = parent.category_id
GROUP BY parent.category_id
ORDER BY parent.category_group, child.category_group

For 3 levels:

SELECT 
    parent.category_id as `parent_id`,
    parent.category_group as `parent_group`,
    child.category_id as `child_id`
    child.category_group as `child_group`,
    child2.category_id as `child2_id`
    child2.category_group as `child2_group`
FROM mst_category AS parent 
LEFT JOIN mst_category AS child 
    ON child.category_parent = parent.category_id
LEFT JOIN mst_category AS child2
    ON child2.category_parent = child.category_id
WHERE parent.category_id = 0
GROUP BY parent.category_id, child.category_id, child2_category_id
ORDER BY parent.category_group, child.category_group, child2.category_group