Question

I’m in training period and my trainer gave me some database exercises.

I have to make Project, Employee, Roles and Employee_Projects_Role tables. Last table has the details of the employees working on a project with a specified role.

The conditions he gave to me are:
1. One employee can work in multiple projects.
2. One employee can not have different roles in same projects.

I made those tables and inserted the data. In roles, I had roles Leader, Manager, Developer. I made those tables and entered the data.

He checked my tables and said that a project have more than 1 leader and 1 manager so I restricted the columns like there wont be same roles in one project but now I can not enter 2 developer to that mapping table in one project.

But a project can obviously has 2 developers. So, If you understand the whole scenario then I want to ask that is that possible to restrict a particular column that it would be having only one leader and manager in one project?

I am not really into database but before assigning me a project work he wants to just brush up my basics. So, I am really confused in that.

Schema for those tables is as following:

1. Project with following fields :
   project_pid – primary key,
   project_name

2. Employee with following fields:
   employee_pid – primary key,
   employee_name,
   gender – M/F,
   salary,
   date_of_joining,
   is_active – Y/N

3. Roles with following fields:
   role_pid – primary key,
   role_name – (team member or leader)

4. Employee_Project_Role with following fields:
   project_xid – foreign key (references table Projects),
   employee_xid – foreign key (references table Employee),
   role_xid – foreign key (references table Roles)

Answer 1

To implement such logic, you need to use procedural language. The easiest way in my opinion is defining a trigger:

CREATE TRIGGER role_restrict
ON employee_project_role
FOR INSERT, UPDATE
AS
  BEGIN
      DECLARE @role VARCHAR(100)
      DECLARE @ldr_xid VARCHAR(100)

      SELECT @role_xid = (SELECT role_xid
                          FROM   inserted)

      SELECT @ldr_xid = (SELECT role_xid
                         FROM   roles
                         WHERE  role_name = "leader")

      IF @role = @ldr_xid
         AND (SELECT Count(*) AS cnt
              FROM   employee_project_role
              WHERE  project_xid = (SELECT project_xid
                                    FROM   inserted)
                     AND role_xid = @ldr_xid) > 1
        /* Cancel the insert, there is already a leader in a project.*/
        BEGIN
            ROLLBACK TRANSACTION
        END
  /* Otherwise we insert the record */
  END