Returning rows in a table

Question

Hi,
I am looking for a query which only should return the rows exists in table1 not in table2. My query is as follows:
slect t1.id from table1 t1 where t1.id not in (select t1_id from table2);
thanks,
Sri

Answer 1

There are at least 4 ways find records in one table that are not in another. Using NOT IN or NOT EXISTS as already noted, as well as:
SELECT id
FROM t1
MINUS
SELECT id
FROM t2
or
SELECT t1.id
FROM t1, t2
WHERE t1.id = t2.id(+) and
      t2.id IS NULLAll four are perfectly valid, and, depending on many factors, any one of them could be the fastest. Be aware that the NOT IN and NOT EXISTS versions do not neccessarily return the same results. If t2.id can be NULL, then the NOT IN version may return no rows.
If t2 is very large, or if the sub-query takes a long time to run, then NOT EXISTS can be very slow because the sub-query is run once for every row in t1. Where the other three versions will, at worst, full scan each table once.
HTH
John