Question

I am working a problem from the Stanford DB class and have a question. I have 2 tables. The first table has 2 fields, so we have T1(ID1,ID2). The second table has some more columns.

If I do

select T1.ID1, T1.ID2
from T1, T2 as X, T2 as Y, T2 as Z
where <condition>
except
select T1.ID2, T1.ID1
from T1, T2 as X, T2 as Y, T2 as Z
where <different condition>;

then I get 2 tuples

A1 | B1
A2 | B2

as a result. This is just what I was expecting.

The issue is how do I delete the rows from T1 using these tuples?

I tried

delete
from T1
where ID1=(<select from above using only the first variable in the select>)
  and ID2=(<select from above using only the second variable in the select>);

This didn’t work and I don’t see a way to do it.
Any clues?
Thanks

Answer 1

Not sure if this is possible in SQLite, but you could give it a try:

DELETE FROM T1
WHERE EXISTS (
  SELECT *
  FROM (
    select T1.ID1, T1.ID2
    from T1, T2 as X, T2 as Y, T2 as Z
    where <condition>
    except
    select T1.ID2, T1.ID1
    from T1, T2 as X, T2 as Y, T2 as Z
    where <different condition>
  ) s
  WHERE s.ID1 = T1.ID1
    AND s.ID2 = T1.ID2
)

That might be sub-optimal and maybe you could instead split your EXCEPT query like this:

DELETE FROM T1
WHERE EXISTS (
  /* slightly modified left part of the EXCEPT query */
  select T1.ID1, T1.ID2
  from T1 AS T1_, T2 as X, T2 as Y, T2 as Z
  where <condition>
    AND T1_.ID1 = T1.ID1
    AND T1_.ID2 = T1.ID2
)
AND NOT EXISTS (
  /* slightly modified right part of the EXCEPT query */
  select T1.ID2, T1.ID1
  from T1 AS T1_, T2 as X, T2 as Y, T2 as Z
  where <different condition>
    AND T1_.ID2 = T1.ID1
    AND T1_.ID1 = T1.ID2
)