Primary key. Every record typically has a field, or group of fields
forming a composite key,
that uniquely identifies it. This is called the primary key.
Typically all tables have a primary key. The primary key is how that record
is identified by foreign keys in records in other tables. Primary keys must be unique and
non-null. To guarantee uniqueness, they are often arbitrary numbers.
Primary keys are usually indexed, which is simply a physical
database implementation to make it quick to find a record given a key.
Primary keys are sometimes marked with "PK" in relational schema diagrams.
A typical name for a key is the table name followed by "_id" if it is an arbitrary number or
"_code" if it is human readable. Other names are possible.