SQL check constraint

Question

I'm trying to create a constraint to check that a project can have only one employee whose role is project leader but at the same time can have other employees with different roles.

My table definition:

CREATE TABLE employee
( employee_id INT NOT NULL PRIMARY KEY
,employee_role VARCHAR(15) NOT NULL
, CHECK (employee_role in ('project_leader', 'administrator', 'member')) 
)


CREATE TABLE project
( project_id INT NOT NULL PRIMARY KEY
, name VARCHAR(50)
, employee_id INT NOT NULL
, employee_role VARCHAR(15) NOT NULL
, CONSTRAINT employee_project_FK
    FOREIGN KEY (employee_id, employee_role)
    REFERENCES employee (employee_id, employee_role)
, CONSTRAINT only_one_project_leader
    CHECK (employee_role = 'project_leader')
) ;

It's unclear to me how this can be expressed in a constraint and what I need to change. If anyone would inform me what I'm doing wrong, I'd appreciate it.

Answer 1

You are missing a table. Your data structure wants three tables:

• Employee, which lists information about employees
• Project, which lists information about projects
• ProjectEmployee, which is an association table between the two

If you want a constraint that a project has only one leader, then you can simply add a column to Project called ProjectLeader. This will enforce the constraint, because there is only one slot per project for the leader. If you have to have a leader, then add in a check constraint to be sure this is not NULL.

A sign that something is wrong with the data model is that project_id is a primary key in the project table. This implies that for a given project_id, there is only one employee. I don't think that is what you intend.

EDIT:

The tables would look something like:

CREATE TABLE project
( project_id INT NOT NULL PRIMARY KEY,
  name VARCHAR(50),
  project_leader int references employee(employee_id)
) ;

CREATE TABLE projectemplyee
( projectemployee_id INT NOT NULL PRIMARY KEY,
  project_id int references project(project_id),
  employee_id int references employee(employee_id),
  employee_role VARCHAR(15) NOT NULL
) ;

There is only one slot for a leader in each project. You do not need a constraint to enforce the one-ness.