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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
On Mon, Apr 4, 2016 at 2:10 PM, Peter Laursen
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