What is a primary Key?
What a primary key is depends, somewhat, on the database. However, in its simplest form a primary key:
- Is a field (Column) or combination of Fields (columns) which uniquely identifies every row.
- Is an index in database systems which use indexes for optimization
- Is a type of table constraint
- Is applied with a data definition language (DDL) alter command
- And, depending on the data model can, define parent-Child relationship between tables