For employees you don't have birthdates for, you could use NULL in SQL. However, as relationally one shouldn't use NULL, you would do the following:
CREATE TABLE employees ( emp_id SERIAL NOT NULL UNIQUE , emp_name TEXT NOT NULL );
CREATE TABLE employees_birthdates ( emp_id INTEGER NOT NULL REFERENCES employees (emp_id) , birthdate DATE NOT NULL );
In any case, one would never use NULL. Either the domain includes a value for all possible values (including N/A) or you set up the db schema appropriately.
Hm, that can be painful. What if I have ten optional attributes; separate them to ten different tables?
-- dave
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