SQL：对于每一行，请检查涉及该行元素的条件

Question

I am looking to do something along the lines of:

For each row in a table, check if there are children (this is done by selecting all elements with a parentid equal to the current element's id). If nothing shows up, then there are no children and the parent needs to get deleted. This parentid needs to come from the id of the current element.

I was thinking something along the lines of (in pseudo-code-ish):

DELETE FROM table WHERE ((SELECT * FROM table WHERE parentid = id) IS NULL)

Ideally, I would do a for each loop, and create a variable (note I'm writing this in C#) for the current id to use for the parentid search.

I think I may be overlooking an easier approach... Is there a easier way to implement this? If not, is the approach I have correct, and how could I translate this to SQL?

Either way any help is appreciated.

Answer 1

Not sure this would be the right syntax, but what you are looking for is a list of all parentid s - this can be queried from table by fetching all parentid values from it. Once you have this list, you want to delete all items that are not in it and do not have a parentid of their own:

DELETE FROM table
WHERE id NOT IN
(
  SELECT parentid FROM table
)
AND parentid IS NULL -- assumption: parents do not have a parent id

Answer 2

Selecting rows that have no children with not exists() :

--delete t
select *
from tbl t
where not exists (
  select 1
  from tbl i
  where t.id = i.parentid
  )