Postgres NOT IN does not work as expected

Question

I am trying to add the below condition in my query to filter data.

SELECT * 
FROM dump
WHERE letpos NOT IN ('0', '(!)','NA','N/A') ;

I need only records with id 1,2,3 and 6. But the query does not return ids 3 and 6. I get only 1,2.

TABLE:

id	name	letpos	num
1	AAA	A	60
2	BBB	B
3	CCC		50
4	DDD	0
5	EEE	(!)	70
6	FFF		70

I am not sure what is missing? Could anyone advise on how to resolve this?

-Thanks

Answer 1

In the row with id = 3 the value of letpos is (I suspect) NULL , so the boolean expression in the WHERE clause is:

WHERE NULL NOT IN ('0', '(!)','NA','N/A');

The comparison of NULL with operators like IN , NOT IN , = , > etc always returns NULL and is never TRUE .
So you don't get this row in the results.

Check for NULL also in the WHERE clause:

SELECT * 
FROM dump 
WHERE letpos IS NULL
   OR letpos NOT IN ('0', '(!)', 'NA', 'N/A');