The difference between MySQL primary key, foreign key, and index

• Defined
  Primary Key-uniquely identifies a record, cannot be duplicated, is not allowed to be empty
  Foreign key--the foreign key of the table is the primary key of another table, the foreign key can have duplicate, can be a null value
  Index--The field does not have duplicate values, but can have a null value
• Role
  Primary key--to ensure data integrity
  Foreign key--used to establish contact with other tables.
  Index--is to improve the speed of query sorting
• Number
  Primary key--the primary key can only have one
  Foreign key--a table can have multiple foreign keys
  Index--A table can have multiple unique indexes

The difference between MySQL primary key, foreign key, and index