How do I create a foreign key in MySQL?

Following are the syntax of the ALTER TABLE statement to add a foreign key in the existing table:
  1. ALTER TABLE table_name.
  2. ADD [CONSTRAINT [symbol]] FOREIGN KEY.
  3. [index_name] (column_name, )
  4. REFERENCES table_name (column_name,)
  5. ON DELETE referenceOption.
  6. ON UPDATE referenceOption.

How do you insert a foreign key?

If you are inserting data into a dependent table with foreign keys:
  1. Each non-null value you insert into a foreign key column must be equal to some value in the corresponding parent key of the parent table.
  2. If any column in the foreign key is null, the entire foreign key is considered null.

Can foreign key be null?

Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). Null by definition is not a value.

Can a primary key be a foreign key?

Foreign keys are almost always “Allow Duplicates,” which would make them unsuitable as Primary Keys. It is perfectly fine to use a foreign key as the primary key if the table is connected by a one-to-one relationship, not a one-to-many relationship.