Is there in SQL a way to enforce unicity of undirected edge?

问题

create table Location (
id integer primary key(1, 1),
latitude decimal(8,6),
longitude decimal(9,6),
address varchar(100),
name varchar(60) unique
);

create table Journey (
id integer primary key(1,1),
id_from integer foreign key references Location(id),
id_to integer foreign key references Location(id),
name varchar(100) unique,
unique(id_from, id_to)
);

With this schema, you could create 2 different journeys for a pair of locations, one for the way in and one for the way back. What I want is to enforce a single journey for each pair of locations. What are the options to do that ?

回答1:

The simplest method is to enforce the "direction" and then use a unique constraint:

create table Journey (
    id integer primary key,
    id_from integer foreign key references Location(id),
    id_to integer foreign key references Location(id),
    name varchar(100) unique,
    unique(id_from, id_to),
    check (id_from < id_to)
);

However you have to remember to insert the values in order to use a trigger to ensure that they are in order.

Otherwise, you can use computed columns for the smallest and biggest values and then use a unique constraint on that.

回答2:

You can enforce unicity of undirected edge using sum and product computed columns:

create table Location (
  id integer primary key(1, 1),
  latitude decimal(8,6),
  longitude decimal(9,6),
  address varchar(100),
  name varchar(60) unique
);
create table Journey (
  id integer primary key identity(1,1),
  id_from integer foreign key references Location(id),
  id_to integer foreign key references Location(id),
  s as id_from + id_to persisted,
  p as id_from * id_to persisted,
  unique(s, p),
  name varchar(100) unique,
);

Is a correct method to enforce a single journey (either way in or way back) for each pair of locations. A quadratic equation has maximum two solutions. It has at least id_from and id_to. So equation xx - sx + p=0 always has exactly 2 solutions which are id_from and id_to. You can see mathematical explanations there https://math.stackexchange.com/questions/171407/finding-two-numbers-given-their-sum-and-their-product

来源：https://stackoverflow.com/questions/65552097/is-there-in-sql-a-way-to-enforce-unicity-of-undirected-edge

标签

sql

sql-server

unique-constraint

undirected-graph