maria-discuss team mailing list archive

[Pantelis Theodosiou <ypercube@xxxxxxxxx>]

Would this be good to be added in the documentation of VIRTUAL columns?

Or as a separate page, as a way to enforce/emulate arbitrary CHECK
constraints?

It can be slightly simplified (IF is not needed) and the BOOLEAN could be
BIT (not sure if that adds any complication):

CREATE TABLE truth (t BIT PRIMARY KEY) ;
INSERT INTO truth (t) VALUES (TRUE) ;
-- and remove all write permissions to the table

CREATE TABLE checker (
    i float,
    i_must_be_between_7_and_12 BIT
         AS (i BETWEEN 7 AND 12)                   -- whatever CHECK
constraint we want here
         PERSISTENT,
    CONSTRAINT check_i_must_be_between_7_and_12
    FOREIGN KEY (i_must_be_between_7_and_12)
      REFERENCES truth (t)
    );

On Wed, Apr 6, 2016 at 6:46 PM, Pantelis Theodosiou <ypercube@xxxxxxxxx>
wrote:

>
>
> On Mon, Apr 4, 2016 at 2:10 PM, Peter Laursen <peter_laursen@xxxxxxxxxx>
> wrote:
>
>> As described in this Blog http://mablomy.blogspot.
>> dk/2016/04/check-constraint-for-mysql-not-null-on.html. A very nice
>> hack/trick IMO.
>>
>> However it is not working with MariaDB as VC's cannot be declared NOT
>> NULL.  What prevents that?
>>
>>
> (Peter, sorry fro the previous private reply, not sure how I got the reply
> buttons wrong.)
>
> I can't answer that, but there's another workaround for (some) CHECK
> constraints, described here: http://dba.stackexchange.com/
> questions/9662/check-constraint-does-not-work/22019#22019
>
> Unfortunately, it works only for smallish (int or date) ranges. We can't
> use for floats or decimals (as it would require a very big reference table).
>
> But it could be combined with the hack you link, using something like:
>
>
> CREATE TABLE truth (t BOOLEAN PRIMARY KEY) ;
> INSERT INTO truth (t) VALUES (TRUE) ;
> -- and remove all write permissions to the table
>
> CREATE TABLE checker (
>     i int,
>     i_must_be_between_7_and_12 BOOLEAN
>          AS (IF(i BETWEEN 7 AND 12, TRUE, FALSE))
>          PERSISTENT,
>     CONSTRAINT check_i_must_be_between_7_and_12
>     FOREIGN KEY (i_must_be_between_7_and_12)
>       REFERENCES truth (t)
>     );
>
>
> Haven't tested it but should work for more complex constraints as well.
>
> Pantelis
>
>