Hugo Kornelis wrote:

That rule is oversimplified. Really, it should be "any expression
whose value _depends_ on a NULL input results in NULL", i.e. could
replacing the NULL with different non-NULL values lead to different
values of the expression? NULL + 2 qualifies; NULL = 'ABC' qualifies;
but NULL AND FALSE does not, and neither does TRUE OR NULL.

(As usual, IS NULL and IS NOT NULL remain special cases.)

and | T N F or | T N F not |
----+------ ---+------ ----+--
T | T N F T | T T T T | F
N | N N F N | T N N N | N
F | F F F F | T N F F | T

Am I overlooking anything? <<

The NULL propagation rule.

and | T N F or | T N F not |
----+------ ---+------ ----+--
T | T N F T | T N T T | F
N | N N N N | N N N N | N
F | F N F F | T N F F | T

This means that TRUE OR NULL = NULL, etc. and you can now prove that
TRUE = FALSE. The UNKNOWN logical value does not have this behavior
and that is why we have it.

--CELKO-- wrote:

NULL represents the concept "unknown" in all other contexts; it should
represent it in the context of the Boolean data type as well.

IINM, while SQL doesn't have a mandatory Boolean *type*, it already
follows TRUE OR NULL = TRUE and FALSE AND NULL = FALSE in Boolean
*expressions*. Example:

CREATE TABLE Table1 (Column1 varchar(10), Column2 varchar(10))

INSERT INTO Table1 (Column1, Column2) values ('A' , null)
INSERT INTO Table1 (Column1, Column2) values (null, 'B' )

SELECT * FROM Table1 WHERE Column1 = 'A' OR Column2 = 'B'
-- returns 2 rows

SELECT * FROM Table1 WHERE Column1 = 'A' AND Column2 = 'B'
-- returns 0 rows