Support the operation of dropping multi-foreign-keys in one statement...

[zimulala]

Description

Support dropping multi-foreign-keys in one statement, the syntax as follows:

ALTER TABLE tbl_name
    [alter_specification [, alter_specification] ...]

alter_specification:
DROP FOREIGN KEY fk_symbol

e.g.

create table t1 (a int, b int, key(a), key(b));
create table t2 (c int, CONSTRAINT a foreign key (c) references t1(a), CONSTRAINT b foreign key (c) references t1(b));
alter table t2 drop foreign key a, drop foreign key b;