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    <p>First, your example tables are doing too much. There is a design principle that states that a single table should model an entity or a relationship between entities but not both. The relationships between departments, directors and employees (I'm assuming that directors are not employees; I'm also omitting divisions for the moment).</p> <p>Second, a table can have more than one key, known as candidate keys. Further, you can create a <code>UNIQUE</code> constraint by 'appending' a non-unique column to a key. For example, employees' names do not make for a good key, hence the reason for having an employee ID (I don't think the same can be said for departments i.e. department name in itself is a good enough key). If <code>employee_ID</code> is unique then it follows that <code>(employee_name, employee_ID)</code> will also be unique.</p> <p>Third, a table can be referenced by any <code>UNIQUE</code> constraint, it doesn't have to be the table's 'primary' key (which partly explains why 'primary key' is a bit of a nonsense).</p> <p>The great thing about the above is that one can model the required constraints using <code>FOREIGN KEY</code> and row-level <code>CHECK</code> constraints. SQL optimizers and programmers prefer declarative solutions to procedural code (triggers, stored procs, etc). This vanilla SQL DDL will port to most SQL products.</p> <p>So, the department name can be combined with both the director key and the employee key respectively and these compound keys can be referencesd in a simple two-tier org chart table: because both the employee's department and their director's department will appear in the same table, a simple row-level <code>CHECK</code> constraint can be used to test that they are the same e.g.</p> <p>Entity tables: </p> <pre><code>CREATE TABLE Departments ( department_name VARCHAR(30) NOT NULL UNIQUE ); CREATE TABLE Employees ( employee_ID INTEGER NOT NULL UNIQUE, employee_name VARCHAR(100) NOT NULL ); CREATE TABLE Directors ( director_ID INTEGER NOT NULL UNIQUE, director_name VARCHAR(100) NOT NULL ); </code></pre> <p>Relationship tables:</p> <pre><code>CREATE TABLE EmployeeDepartments ( employee_ID INTEGER NOT NULL UNIQUE REFERENCES Employees (employee_ID), employee_department_name VARCHAR(30) NOT NULL REFERENCES Departments (department_name), UNIQUE (employee_department_name, employee_ID) ); CREATE TABLE DirectorDepartments ( director_ID INTEGER NOT NULL UNIQUE REFERENCES Directors (director_ID), director_department_name VARCHAR(30) NOT NULL REFERENCES Departments (department_name), UNIQUE (director_department_name, director_ID) ); CREATE TABLE OrgChart ( employee_ID INTEGER NOT NULL UNIQUE, employee_department_name VARCHAR(30) NOT NULL, FOREIGN KEY (employee_department_name, employee_ID) REFERENCES EmployeeDepartments (employee_department_name, employee_ID), director_ID INTEGER NOT NULL, director_department_name VARCHAR(30) NOT NULL, FOREIGN KEY (director_department_name, director_ID) REFERENCES DirectorDepartments (director_department_name, director_ID), CHECK (employee_department_name = director_department_name) ); </code></pre> <p>Now a slightly more interesting scenario would be when a director is assigned a division, rather than a specific department, and you had to test that the employee's department was in the same division as her director:</p> <p>Entity tables:</p> <pre><code>CREATE TABLE Divisions ( division_name VARCHAR(20) NOT NULL UNIQUE ); CREATE TABLE Departments ( department_name VARCHAR(30) NOT NULL UNIQUE, division_name VARCHAR(20) NOT NULL REFERENCES Divisions (division_name), UNIQUE (division_name, department_name) ); CREATE TABLE Employees ( employee_ID INTEGER NOT NULL UNIQUE, employee_name VARCHAR(100) NOT NULL ); CREATE TABLE Directors ( director_ID INTEGER NOT NULL UNIQUE, director_name VARCHAR(100) NOT NULL ); </code></pre> <p>Relationship tables:</p> <pre><code>CREATE TABLE EmployeeDepartments ( employee_ID INTEGER NOT NULL UNIQUE REFERENCES Employees (employee_ID), employee_department_name VARCHAR(30) NOT NULL REFERENCES Departments (department_name), UNIQUE (employee_department_name, employee_ID) ); CREATE TABLE DirectorDivisions ( director_ID INTEGER NOT NULL UNIQUE REFERENCES directors (director_ID), director_division_name VARCHAR(20) NOT NULL REFERENCES divisions (division_name), UNIQUE (director_division_name, director_ID) ); CREATE TABLE OrgChart ( employee_ID INTEGER NOT NULL UNIQUE, employee_department_name VARCHAR(30) NOT NULL, FOREIGN KEY (employee_department_name, employee_ID) REFERENCES EmployeeDepartments (employee_department_name, employee_ID), employee_division_name VARCHAR(20) NOT NULL REFERENCES divisions (division_name), FOREIGN KEY (employee_division_name, employee_department_name) REFERENCES Departments (division_name, department_name), director_ID INTEGER NOT NULL, director_division_name VARCHAR(20) NOT NULL, FOREIGN KEY (director_division_name, director_ID) REFERENCES DirectorDivisions (director_division_name, director_ID), CHECK (employee_division_name = director_division_name) ); </code></pre>
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