Final answer:
A primary key is a unique identifier for each record in a table. It ensures each record can be uniquely identified and accessed. Multiple fields are necessary for the primary key when a composite key is needed.
Step-by-step explanation:
In the context of databases, a primary key is a unique identifier for each record in a table. It is important because it ensures that each record in the table can be uniquely identified and accessed. A primary key is necessary to include multiple fields when there is a need for a composite key, where multiple fields together uniquely identify a record.
For example, in a student table, a primary key could be the student ID, as it uniquely identifies each student. However, if the table needs to handle students from multiple schools, combining the student ID and the school ID could form a composite primary key.