mysql递归查询树形的叶子

SELECT
t.name,
t.unit_id,
t.parent_id
FROM
`scpi_unit_struct` t
WHERE NOT EXISTS(
select * from `scpi_unit_struct` t1,`scpi_unit_struct` t2   where
t1.unit_id=t2.parent_id AND t.unit_id=t1.unit_id
)