Prevent columns from having same value in record I'm working with PostgreSQL 10. I'd like to have a table that allows for the same value in a column, but not in the row. The table I created:

CREATE TABLE teams
(
  team_id SERIAL PRIMARY KEY
  team_lead character varying(250) NOT NULL,  
  member1 character varying(250) NOT NULL,
  member2 character varying(250),
  member3 character varying(250)
  )


This would be allowed:

| team_lead     |  member1     | member2       |
------------------------------------------------
| Jane Doe      | Bill Smith   | Shirley Green |
| Jane Doe      | Carol Lewis  | John  Doe     |


This would NOT:

| team_lead  | member1     | member2       |
--------------------------------------------
| Jane Doe   | Jane Doe    | Shirley Green |
| Bill Smith | Carol Lewis | Bill Smith    |


Does anyone know how I would accomplish this? Thank you in advance.
This is called a "many-to-many" relationship. One person can be a member of multiple teams and a team has multiple members. One way to model this, could be:

create table person
(
   person_id integer primary key, 
   name text not null
);

create table team
(
  team_id integer primary key, 
  name text not null
);

create table team_member
(
  team_id integer not null references team, 
  person_id integer not null references person,
  person_role text not null check (team_role in ('lead', 'member')), 

   -- each person can only be assigned to a team once
  primary key (team_id, person_id)
);