看自己需要了
外键建立的时候有on delete cascade和on delete set null两种,
on delete cascade是说删除主键的表数据同时删除外键的表数据,
on delete set null是说删除主键数据以后,外键的字段值设为null.
外键建立的时候有on delete cascade和on delete set null两种,
on delete cascade是说删除主键的表数据同时删除外键的表数据,
on delete set null是说删除主键数据以后,外键的字段值设为null.
drop table A;
alter table B enable constraint fk_....;