Fields in red color are primary key fields.
Every table has at least one primary key field.
Primary key fields have the following meanings, to list a few:
When entering a new record, primary key fields must be entered with values. They can not be left blank.
By searching primary key fields values, we can find at most one matched record in a table. For example, we can locate 0 or one matched record in customer table by searching field customer#.
To search a record in a table, matching values with primary key fields always takes the least amount of time to get the result.
Social security number is another example. The computer system responds most quickly by looking for a specific social security number in its database than looking other fields like name. Besides, there are chances that multiple records associated with the provided name are returned by the search.
Refer to screen shot Figure 5-1.