Question

I have a table in my database which stores a tree structure. Here are the relevant fields:

mytree (id, parentid, otherfields...) 

I want to find all the leaf nodes (that is, any record whose id is not another record’s parentid)

I’ve tried this:

SELECT * FROM mytree WHERE `id` NOT IN (SELECT DISTINCT `parentid` FROM `mytree`) 

But that returned an empty set. Strangely, removing the ‘NOT’ returns the set of all the non-leaf nodes.

Can anyone see where I’m going wrong?

Update: Thanks for the answers folks, they all have been correct and worked for me. I’ve accepted Daniel’s since it also explains why my query didn’t work (the NULL thing).

Answer 1

Your query didn’t work because the sub-query includes NULL. The following slight modification works for me:

SELECT * FROM `mytree` WHERE `id` NOT IN (     SELECT DISTINCT `parentid` FROM `mytree` WHERE `parentid` IS NOT NULL)