Null values in IN clause - Teradata

Question

Here is my Table1

personid
1
?
2
3
4
?
6

---

Here is my query

select * 
from table2
where personid not in 
(
select personid
from table1
)

The result is nothing

---

Here is my second query

select * 
from table2
where personid not in 
(
select personid
from table1         
where personid is not null
)

The result is ok

---

Question : why the first query did not work ? I can't see any logical problem . Do nulls skrew up teradata ?

Answer 1

It's not specific to Teradata, it's the same (or at least should be the same) in all RDBMSes. Any comparison to a NULL results in UNKNOWN and NOT IN is an ANDed condition:

personid <> 1st_value_in_list AND personid <> 2nd_value_in_list AND ... AND personid <> NULL

This has been discussed multiple times, e.g. NOT IN clause and NULL values

Answer 2

Yes. If one of the value in the subquery is NULL, the IN clause does not return anything.