Check constraints do not seem to be working!!!

On 11/11/2020 06:44, Jitendra Loyal wrote:

Consider this table definition:
 create table t ( i serial, b bool, c bool,
                      constraint b_c check ( (b = true and c is not
null ) or (b is distinct
from true and c is null) )
                      constraint b_c check ( (b = true and c is not
null ) or (b = false and c
is null) or (b is null and c is null) )
                      );
Despite the above two constraints, the following rows get into the table:
 insert into t (b , c) values (null, true), (null, false);

(b =TRUE AND c IS NOT NULL) evaluates to null when b is null

Cheers,
Chris Sterritt

st 11. 11. 2020 v 9:38 odesílatel Chris Sterritt <chris.sterritt@yobota.xyz>
napsal:

On 11/11/2020 06:44, Jitendra Loyal wrote:

Consider this table definition:
create table t ( i serial, b bool, c bool,
constraint b_c check ( (b = true and c is
not null ) or (b is distinct
from true and c is null) )
constraint b_c check ( (b = true and c is
not null ) or (b = false and c
is null) or (b is null and c is null) )
);
Despite the above two constraints, the following rows get into the table:
insert into t (b , c) values (null, true), (null, false);

(b = TRUE AND c IS NOT NULL) evaluates to null when b is null

yes, constraint is violated only when result is false, no when it is null.

Regards

Pavel

On Wed, Nov 11, 2020 at 12:26 AM Jitendra Loyal <jitendra.loyal@gmail.com>
wrote:

Despite the above two constraints, the following rows get into the table:
insert into t (b , c) values (null, true), (null, false);

This behavior is described in the docs
https://www.postgresql.org/docs/current/ddl-constraints.html#DDL-CONSTRAINTS-CHECK-CONSTRAINTS
:

It should be noted that a check constraint is satisfied if the check

expression evaluates to true or the null value. Since most expressions will
evaluate to the null value if any operand is null, they will not prevent
null values in the constrained columns. To ensure that a column does not
contain null values, the not-null constraint described in the next section
can be used.

Thanks Nikolay

I read that but is there a way to meet the above requirement. And I will
like to add that IS NULL and IS NOT NULL should evaluate to true/false.
These operators are made for this and should not be returning NULL.

Regards,
Jitendra

On Wed 11 Nov, 2020, 14:18 Nikolay Samokhvalov, <samokhvalov@gmail.com>
wrote:

On 11/11/20 10:06 AM, Jitendra Loyal wrote:

Thanks Nikolay

I read that but is there a way to meet the above requirement. And I will
like to add that IS NULL and IS NOT NULL should evaluate to true/false.
These operators are made for this and should not be returning NULL.

This has nothing to do with IS [NOT] NULL, it's the first part of the
expression (b = TRUE) causing trouble. Essentially, the constraint

(b = true) and (c is not null)

is evaluated in two steps. First we evaluate the two parts individually,
and for (null, true) the results would look like this:

(b = true) => null
(c is not null) => true

and then we combine those results using 'AND'

null AND true => null

which is considered as if the constraint matches. If you want to handle
NULL for the first expression, you may do this, for example:

(b it not null and b = true) and (c is not null)

Or something like that.

regards

--
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On 11 Nov 2020, at 11:15, Jitendra Loyal <jitendra.loyal@gmail.com> wrote:


Thanks Nikolay

I read that but is there a way to meet the above requirement. And I will like to add that IS NULL and IS NOT NULL should evaluate to true/false. These operators are made for this and should not be returning NULL.

That is exactly what they do. Your problem is with the equality operator and its behaviour with NULL values, which is described in the referenced document.

--
If you can't see the forest for the trees,
Cut the trees and you'll find there is no forest.

Thanks Tomas....

Understood... My bad.... Was just not looking at that aspect

Thanks once again,
Regards,
Jitendra

On Wed, 11 Nov 2020 at 16:17, Tomas Vondra <tomas.vondra@enterprisedb.com>
wrote:

On Wed, 2020-11-11 at 11:47 +0100, Tomas Vondra wrote:

you may do this, for example:

(b it not null and b = true) and (c is not null)

Or something like that.

My (equivalent) suggestion:

b IS TRUE AND c IS NOT NULL

Yours,
Laurenz Albe
--
Cybertec | https://www.cybertec-postgresql.com

Thanks Laurenz

This is interesting...b is True

Thanks and regards,
Jitendra

On Wed 11 Nov, 2020, 22:52 Laurenz Albe, <laurenz.albe@cybertec.at> wrote: