MySQL supports foreign keys, which permit cross-referencing related
data across tables, and foreign key constraints, which help keep the
related data consistent.

A foreign key relationship involves a parent table that holds the
initial column values, and a child table with column values that
reference the parent column values. A foreign key constraint is defined
on the child table.

The essential syntax for a defining a foreign key constraint in a
CREATE TABLE or ALTER TABLE statement includes the following:

[CONSTRAINT [symbol]] FOREIGN KEY
    [index_name] (col_name, ...)
    REFERENCES tbl_name (col_name,...)
    [ON DELETE reference_option]
    [ON UPDATE reference_option]

reference_option:
    RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT

URL: https://dev.mysql.com/doc/refman/5.7/en/create-table-foreign-keys.html

