在mysql中，如何查找未在表中的其他行中引用其ID的行？

Question

My table has three columns

ID   parent_ID   item

As you can understand, it's for a nested menu. Now I need to find those elements which don't have any child elements. In other words, whose id isn't being used as parent_ID for any other rows( and order them by item ). Can I do this with a mysql query?

Answer 1

SELECT * FROM mytable m WHERE NOT EXISTS (SELECT 1 FROM mytable m2 WHERE m2.parent_ID = m.ID)

Answer 2

I would use a LEFT JOIN instead of a subquery:

SELECT Parents.* FROM mytable Parents
LEFT JOIN mytable Childs ON Parents.ID = Childs.parent_ID
WHERE Childs.ID IS NULL