MySQL查询父子

Question

I am working with mysql and php and have a Parent Child table created in mysql. Every parent has 3 Childs. Every Child becomes also a Parent and has also 3 Childs and so on.

Now I need to write a function that gives me the last parent in the tree (depending on the selected parent ID) that does not have 3 Childs. So the next Child subscription can be dedicated to that parent.

Can this be done with mysql query or function?

Thanks, M

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I use the following query:

SELECT t1.uid, t2.pid FROM cms_users AS t1 LEFT JOIN cms_users AS t2 ON t1.uid = t2.pid 

Which gives mr the following output:

t1.uid t2.pid
1      1 
1      1 
1      1 
2      2 
2      2 
2      2 
3      3 
4      NULL 
5      NULL

What I actualy need is a result like:

p1.uid p2.pid 
1      3 
2      3 
3      1 
4      0 
5      0 

This result also starts from the root Parent 1 I need to get the results starting from a selected uid somewhere in the three. Every parent has his own tree starting from his uid. Probably I need to write a stored procedure or something but this is all new to me and I don't know how to do this.

This is an example of how the tree looks like. http://www.musafreaks.com/images/tree.jpg User ID 1 has his own tree, even user 2, 3 and 4 have there own tree and so on.

Answer 1

This problem is not trivial to solve within MySQL. Multiple strategies to manage hierarchical data are described in the manual . The rest of my answer is based on this article.

Your setup is sort of a "Adjacency List Model", so you could adapt the LEFT JOIN solution for your dataset.

SELECT t1.name FROM
category AS t1 LEFT JOIN category as t2
ON t1.category_id = t2.parent
WHERE t2.category_id IS NULL;

This assumes that the parent field of the root node is null . Your possible new parent will be selected as t1 . Note that this query will return all nodes which have no children, if you want to "fill" up each node with three children you'll have to extend the query a bit.