how to design a schema where the columns of a table are not fixed

Question

I am trying to design a schema where the columns of a table are not fixed. Ex: I have an Employee table where the columns of the table are not fixed and vary (attributes of

Answer 1

I would use a combination of 1 and 2. If you are adding attributes frequently, I don't think you have a handle on the data requirements.

I supect some of the attributes being added belong in a another table. If you keep adding attribututes like java certified, asp certified, ..., then you need a certification table. This can be relationship to a certifications code table listing available certifications.

Attributes like manager may be either an attribute or relationship table. If you have multiple relationships between employees, then consider a relationship table with a releation type. Organizations with a matrix management structure will require a releationship table.

Addresses and phone numbers often go in separate tables. An address key like employee_id, address_type would be appropriate. If history is desired add a start_date column to the key.

If you are keeping history I recommend using start_date and end_date columns on the appropriate columns. I try to use a relationship where the record is active when 'start_date <= date-being-considered < end_date' is true. Attributes like weight, eye color, etc.