Difference between primary key, foreign key and index

definition:

Primary key – uniquely identifies a record, cannot have duplicates, and is not allowed to be empty

Foreign key – the foreign key of a table is the primary key of another table, the foreign key can have duplicates and can be null

index – the field has no duplicate values, but can have a null value

effect:

Primary key – used to ensure data integrity

Foreign keys – used to connect with other tables

Indexes – are to increase the speed of query sorting

Number:

Primary Key – There can only be one primary key

Foreign keys – a table can have multiple foreign keys

Indexes – a table can have multiple unique indexes