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A primary key is a unique identifier for a database record. When a table is created, one of the fields is typically assigned as the primary key. While the primary key is often a number, it may also be a text field or other data type. For example, if a database contains definitions of computer terms, it would make sense that each term is only listed once in the database. By defining the Term field as the primary key, it would ensure that no term is listed more than once in the database.
While a tables primary key is usually assigned to a specific field, it can also be comprised of multiple values. For example, a database of news articles might use both the title and date fields to uniquely identify each entry. By combining the Title and Date fields as the primary key, it would ensure no entries have the same title on the same day.
If multiple fields in a table cannot have duplicate values, most database management system (DBMS) also allow fields to be defined as Unique. This is similar to setting a field as the primary key, but is not exactly the same thing. The primary key also acts as a pointer to the record and provides a simple way for the DBMS to search, sort, and access the records. It also helps relate records across multiple tables. Therefore, while multiple fields within a table can be defined as Unique, each table can only have one primary key. The primary key field must also be defined for each record and not set to null. After all, a undefined identifier is not very useful.