Differences between primary key and Index

Differences between primary key and Index

1.
The primary key must be a unique index, and the unique index is not necessarily a primary key.
A primary key uniquely identifies a row in a table. A table can have only one primary key, but multiple candidate indexes. Because the primary key can uniquely identify a row of records, it can ensure that no error is reported when data is updated or deleted. In addition to the above functions, the primary key often forms a reference Integrity Constraint with the foreign key to prevent data inconsistency. During database design, primary keys play an important role.
The primary key can ensure that the record is unique and the primary key field is not empty. The database management system automatically generates a unique index for the primary key, so the primary key is also a special index.
2.
A table can have multiple unique indexes, but only one primary key.
3.
The primary key column does not allow null values, while the unique index column does.