Question: This is just a question out of curiosity but I am looking at a database and pulling data from a table with a query on one of the columns. The column has four possible values `null`, `0`, `1`, `2`. When I run the query as:

```
SELECT * FROM STATUS WHERE STATE != '1' AND STATE != '2';

```

I get the same results as running:

```
SELECT * FROM STATUS WHERE STATE = '0';

```

I.e. rows with a null value in the top command in the queried column seem to be omitted from the results, does this always happen in SQL?

I'm running my commands through Oracle SQL Developer.

Answer: 
In several languages NULL is handled differently: Most people know about two-valued logic where `true` and `false` are the only comparable values in boolean expressions (even is false is defined as 0 and true as anything else).

In Standard SQL you have to think about [three-valued logic](http://en.wikipedia.org/wiki/Three-valued_logic). NULL is not treated as a real value, you could rather call it "unknown". So if the value is unknown it is not clear if in your case `state` is 0, 1, or anything else. So `NULL != 1` results to `NULL` again.

This concludes that whereever you filter something that may be NULL, you have to treat NULL values by yourself. Note that the syntax is different as well: NULL values can only be compare with `x IS NULL` instead of `x = NULL`. See Wikipedia for a truth table showing the results of logic operations.
I created script to [research Oracle](https://livesql.oracle.com/apex/livesql/file/index.html) behavior:

```
create table field_values
(
is_user_deletable VARCHAR2(1 CHAR),
resource_mark VARCHAR2(3 CHAR)
)

insert into field_values values (NULL, NULL);
insert into field_values values ('Y', NULL);
insert into field_values values ('N', NULL);

select * from field_values; -- 3 row

-- 1 row, bad
update field_values set resource_mark = 'D' where is_user_deletable = 'Y'; 
update field_values set resource_mark = 'D' where is_user_deletable <> 'N'; 
update field_values set resource_mark = 'D' where is_user_deletable != 'N'; 
update field_values set resource_mark = 'D' where is_user_deletable not in ('Y');

-- 2 rows, good, but needs more SQL and more comparisons. Does DB optimizes?
update field_values set resource_mark = 'D' where is_user_deletable = 'N' or is_user_deletable is null; 

-- it better to have ('Y' and NULL) or ('N' and NULL), but not 'Y' and 'N' and NULL (avoid quires with https://en.wikipedia.org/wiki/Three-valued_logic)
-- adding new column which is 'Y' or 'N' only into existing table may be very long locking operation on existing table (or running long DML script)

```