Bugs item #2679434, was opened at 2009-03-10 16:58
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Category: SQL/Core
Group: SQL CVS Head
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Jens Oehlschlägel (jens_oehl)
Assigned to: Niels Nes (nielsnes)
Summary: tri-boolean problems with NULL
Initial Comment:
Dear all,
I found a couple of inconsistencies (guess bugs) around tri-boolean evaluations
(involving NULL, or more precisely UNKNOWN).
1) NULL arithmetically compared should always give NULL, and in bi-boolean
decisions like WHERE or HAVING conditions NULL should always be mapped to
FALSE, i.e. dropping the rows. At least in combination with subquerying this is
not the case.
2) NULL logically compared should be propagated using tri-boolean logic. It is
not.
-- A greek seer says: if an operator is given a null operand, the result is
always null. The only operator that does not follow this rule is concatenation
(||).
-- A big blue Gaul says: if any operand determines the result (like TRUE OR
something is TRUE) then the result is NOT NULL
-- SQL92 defines tri-booleans
<truth value> ::=
TRUE
| FALSE
| UNKNOWN
-- and says
"A null value is an implementation-dependent special value that is distinct
from all non-null values of the associated data type. There is effectively only
one null value and that value is a member of every SQL data type. There is no
<literal> for a null value, although the keyword NULL is used in some places to
indicate that a null value is desired."
-- The truth tables in the SQL92 standard are hardly readable, but the
intention is clearly aiming on tri-boolean: (TRUE OR UNKNOWN) = TRUE
Cheers
Jens
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