Submitted by: @pavel-zotov

The following code sample allows to create CHECK-constraint for some table with previously entered data that violates a new requirement for them:

recreate table tmp(id int not null, val int);
insert into tmp(id,val) values (1, 123);
insert into tmp(id,val) values (2, null);
insert into tmp(id,val) values (3, 345);
alter table tmp add constraint ck_tmp_enough_val check (val > 1000);
commit;

Note that the new rule for data (val>1000) will be effectively checked only for NEW data of the column `VAL`.
Currently Firebird do NOT validate old values of this column against new rules.

It will be good if some opportunity to do this will be avaliable, i.e. force the engine to validate old values.
The clause may be like this:

alter table tmp add constraint ck_tmp_enough_store check(val > 1000) WITH CHECK