Relational DataBase Management Systems (RDBMS) perform more reliably than other systems because they function with an association established between common column values of two or more tables. There are three kinds of relationships:

> One-to-One
> One-to-Many
> Many-to-Many

In a one-to-one relationship, each row in Table A can have no more than one matching entry in Table B. If you are going to have a one-to-one relationship, then you probably need to include the columns from both tables in just one table. A possible exception to combining multiple one-to-one tables into one table would be if the second table’s data required higher security that would be applied only to the second table.