Difference between these queries

Question

Hi,
I have two tables table1 and table2. Both have a common column say for ex. col1
I tried to find the rows in table1 where there is no corresponding value for col1.
My query is
Select * from table1 a where a.col1 not in (select b.col1 from table2 b);
This returned me nothing.
But when my co-worker executed a query like this
select a.col1 from table1 a
minus
select b.col1 from table2 b;
This gave us the results. How is both of them different and how to know where to use what?

Answer 1

Hi,
where   a.col1 not in (select b.col1 from table2 b);will never evaluate to TRUE if there are NULLs in table2.col1.
Remember that SQL uses three-valued logic.
Expressions such as x IN (y) can be TRUE, FALSE, or UNKNOWN.
Since x = NULL is UNKNOWN (even if x itself IS NULL), then x IN (y), where y is a list or a sub-query, will be either TRUE or UNKNOWN if y contains NULL.
So x NOT IN (y) will be either FALSE or UNKNOWN (if y contains NULL).
To be practical, NOT IN sub-queries usually need to guarantee that they will not return NULL rows, e.g.
WHERE   a.col1 NOT IN
        SELECT  col1
        FROM    table2
        WHERE   col1 IS NOT NULL
        )Repeat: this applies only to NOT IN sub-queries.
IN sub-queries behave the way most people find intuitive.