A primary keyA field attribute that uniquely identifies a record (eg, social security number). The field(s) with the primary key attribute must have a unique value for every record in a table and cannot be blank. is a field attribute that uniquely identifies a record (eg, social security number, invoice number, customer ID, employee ID). The field or fields with the primary key attribute must have a unique value for every record in a table and cannot be blank.

A secondary keyA field(s) in a table that was not selected as the primary key but provides a way to uniquely identify a record. It may be used as an alternative means of searching for and accessing data in addition to the primary key. (also known as the alternative key) is a field or combination of fields in a table that was not selected as the primary key but provides a way to identify a record or set of records and does not need to identify a unique record (Choice C). It may be used as an alternative means of searching for and accessing data in addition to the primary key (eg, the Employee_ID columns in the above image).

(Choice A) A foreign keyA field or set of fields in a table that are used as primary keys within other tables in a database. Foreign keys help ensure that data integrity is maintained among all tables that contain the associated foreign key. refers to a field or set of fields in a table that are used as primary keys within other tables in a database. When a record is changed or removed, foreign keys help ensure that data integrity is maintained among all tables that contain the associated foreign key.

(Choice D) A schema is a model describing the fields and structure (ie, logical configuration) of data in relational tables.

Things to remember:
A primary key is a field attribute that uniquely identifies a record. It can be a single field, such as an ID number, or several fields that, when combined, produce a unique row identifier.