A primary key is a column, or set of columns, that uniquely identify a row, meaning that each value in a primary key column is unique. As a constraint, the primary key prevents duplicate (non-unique) and null values from being entered into the column.
A primary key could be something like an employee id column in an employee table or it could be a combination of the customer's id and the customer's phone number.
Notes:
•Only one Primary Key can be defined per table.
•Defining a Primary Key automatically enforces "not NULL" and "unique" constraints on the column(s).
•A Primary Key cannot be defined if any one of the columns included in the desired key already have a NULL or duplicate values.
•R:BASE automatically builds an index on the specified column(s) when a Primary Key is defined.
•Every table should have a column or set of columns that identify a row, and (in a well-designed database) should have a Primary Key.
•A Primary Key definition is used instead of a Rule to prevent duplicates, and has the advantage of faster performance. It is recommended to delete the rules and indexes that are no longer needed before defining a Primary Key constraint.
