Identification of primary keys is an important part of entity type identification
Ideally, primary keys should be 1) stable and 2) single purpose. Stable
means that a primary key should never change after it has been assigned to an entity.
means that a primary key attribute should have no purpose other than entity identification.
Typically, good choices for primary keys are integer values automatically generated by a DBMS. Access has the AutoNumber
data type for primary keys and
has the Sequence object for primary keys.
As noted in the preceding lesson, tables consist of
key columns and
Descriptor columns contain information about the person, place, or thing described by a particular row in the table.
Key columns contain information that distinguishes that person, place, or thing from every other item represented in the table.
A key column (or set of key columns) that distinguishes a record from every other record in a table is referred to as a primary key
A primary key should never be a value that is also used outside the database, such as a
- Social Security number (Social Insurance Number in Canada) or
- license-plate number.
Social Security numbers are especially bad choices
because they can be recycled, incorrectly assigned, or changed.
More important, the U.S. government strictly limits when Social Security numbers can be collected and used.
A better practice is to create
a unique identifier to represent each row in a table and not try to store meaningful data in the primary-key column.
Most database designers mark primary-key fields in an ER diagram with an asterisk (*).
Some Relational database management systems underline the key fields or write the field's name in bold-face type.
A column or set of columns that can used as a primary key
for a table.
A car's license-plate number is one example of a candidate key.
- Within a given state, only one car can have a particular license number, so a table describing
CAR might contain a column labeled PlateNo.
- In a nationwide database of license-plate numbers, where the same license plate number could be assigned to cars in
different states, the state and PlateNo key columns would form a candidate key.
The problem with using a license-plate number to identify a car is that the car's owner can request new plates with a
chosen message or the state may change their numbering system to accommodate more cars.
In either case, the license-plate number associated with the car would change.
Changing key values in a table can cause a great deal of confusion, especially if an update is done incorrectly and
the wrong value is assigned to an item.
When you design a table, look for candidate keys with the following two properties:
The value in the key column can not change.
The key column cannot contain a null value.
If the candidate key meets these two additional requirements, it can be considered a primary key
The next lesson defines concatenated primary keys
concatenated primary key:
A concatenated primary key is a database table key made up of more than one field.