u010410697的博客

null排在有值的行前面还是后面通过语法来指定--null值在前select * from tablename order by id nulls first; --null值在后select * from tablename order by id nulls last; --null在前配合desc使用select * from tablename order by id desc nulls first; --null在后配合desc使用select * from t

#1. 删除主键约束ALTER TABLE "table_name" DROP CONSTRAINT "table_name_pkey";#2. 增加主键约束ALTER TABLE "table_name" add primary key (xx,xx,xx);

1. 删除非空约束alter table xxx alter xxx drop not null;2. 添加非空约束alter table xxx alter xxx set not null;