Обсуждение: Check constraints do not seem to be working!!!

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

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



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.

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

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.

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