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Alex Rivera
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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).
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