How to check if value of one column is in another column of another row

Question

I have the following table "myTable":

ID    | Name   | Parent | Memo
________________________________________
1     | Obj1   | 1      | server object
2     | Obj2   | 1      | local object
3     | Obj3   | 2      | server object
4     | Obj4   | 3      | local object

and I want all IDs of objects that are never used in the parent column and where memo = local object. So in this example it would be row 4, cause there isn't any row in this table with parent = 4 and its memo = local object.

What would be a good sql statement for that case?

Mine looks like this:

select ID from db.myTable t1 where t1.memo = 'local object' and not exists
(select * from db.myTable t2 where t2.parent = t1.ID)

Answer 1

I usually do a left outer join for these types of statements, combined with a filter, something like this;

select ID 
  from db.myTable t1 
  left join db.myTable t2
    on t2.parent = t1.ID
 where t1.memo = 'local object' 
   and t2.parent is null -- arbitrary none nullable column

Not sure if it's more efficient, but it looks nicer (IMHO).

Answer 2

Your query is fine:

select ID
from db.myTable t1
where t1.memo = 'local object' and
      not exists (select 1 from db.myTable t2 where t2.parent = t1.ID);

For performance, you want indexes on mytable(memo, id) and mytable(parent) .