Re: "NOT NULL"

You MUST distinguish between NULL (that means "unknown" or "nothing")
and ZERO (0).


To prevent a field of a table being set to NULL (unknown/nothing) you
can add the NOT NULL contraint:

# CREATE TABLE test (some_field int NOT NULL);
CREATE TABLE

# INSERT INTO test (some_field) VALUES (NULL);
ERROR:  null value in column "some_field" violates not-null constraint

# INSERT INTO test (some_field) VALUES (0);
INSERT 141314 1


If you want to make ZERO (0) values impossible, you can use a constraint
by using the CHECK keyword:

# CREATE TABLE test (some_field int NOT NULL, CHECK (some_field <> 0));
CREATE TABLE

# INSERT INTO test (some_field) VALUES (NULL);
ERROR:  null value in column "some_field" violates not-null constraint

# INSERT INTO test (some_field) VALUES (0);
ERROR:  new row for relation "test" violates check constraint
"test_some_field_check"


You may want to have a look for "table constraints" at the SQL reference
of CREATE TABLE and ALTER TABLE:
http://www.postgresql.org/docs/8.0/interactive/sql-createtable.html
http://www.postgresql.org/docs/8.0/interactive/sql-altertable.html


Jan Behrens


tövis wrote:
> OK
> How to prevent insert/update rows with NULL = 0 values?
> How to modify table or server change NULL = 0 values to fields DEFAULT
> values, if is it given?
> Regards
>    Tövis
>
> ----- Original Message ----- From: "Jaime Casanova"
> <systemguards@gmail.com>
> To: "tövis" <tovises@freemail.hu>
> Cc: "pgsql novice" <pgsql-novice@postgresql.org>
> Sent: Tuesday, May 24, 2005 7:28 AM
> Subject: Re: [NOVICE] "NOT NULL"
>
>
> On 5/23/05, tövis <tovises@freemail.hu> wrote:
>
>> Thanks Jaime!
>> Using pgAdmin III selected all rows:
>> SELECT aid,num,name30 FROM nod ORDER BY num;
>>
>> First rows from result in a CSV file:
>>
>> aid;num;name30
>> "374";"0";"2181                          "
>> "371";"0";"2178                          "
>> ...
>>
>> Is this a NULL or not? May be the method is not precise enough?
>>
>>
> The better way you can prove that is:
> SELECT aid,num,name30 FROM nod
> WHERE num IS NULL ORDER BY num;
>
> like Andreas said NULL is not 0, NULL is "unknown".
>
> BTW, do the reply in this thread, that will do searches in
> archives.postgresql.org easier.
>