–添加主键 alter table cities add PRIMARY KEY(name);

–添加外键 alter table weather add FOREIGN key(city) REFERENCES cities(name) on update CASCADE on DELETE CASCADE;

on update cascade: 被引用行更新时，引用行自动更新；

on update restrict: 被引用的行禁止更新；

on delete cascade: 被引用行删除时，引用行也一起删除；

on dellete restrict: 被引用的行禁止删除；

–删除外键

alter table orders drop constraint orders_goods_id_fkey;

–添加唯一约束

alter table goods add constraint unique_goods_sid unique(sid);

–删除默认值

alter table goods  alter column sid drop default;

–修改字段的数据类型

alter table goods alter column sid type character varying;

–重命名字段

alter table goods rename column sid to ssid;