数据表约束

非空约束

create table
  emp (
    id serial primary key,
    phone varchar(30) not null,
  );

唯一约束

create table
  emp (
    id serial primary key,
    phone varchar(30) unique,
  );

默认值约束

create table
  emp (
    id serial primary key,
    phone varchar(30) default 0
  );