Foreign Key Constraints are used to model a parent/child relation in an entity relationship (ER) model:
SQL> CREATE TABLE team (
id TINYINT UNSIGNED NOT NULL AUTO_INCREMENT
, name VARCHAR(48) NOT NULL
, PRIMARY KEY (id)
);
SQL> CREATE TABLE employee (
id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT
, first_name VARCHAR(32) NOT NULL
, last_name VARCHAR(64) NOT NULL
, team_id BIGINT UNSIGNED NOT NULL
, PRIMARY KEY (id)
, CONSTRAINT `fk_employee_team` FOREIGN KEY (team_id)
REFERENCES team (id) ON …
