SQL Antipatterns, Volume 1

Using Inline References Syntax

Standard SQL and most implementations support syntax to define a foreign key for a single column on the same line as the column. However, MySQL doesn’t support inline foreign key syntax. If you try to define a foreign key this way, you get no error, but the foreign key is not added to the table.^[36]

Foreign-Key-Checklist/inline-ignored.sql

CREATE TABLE Parent (
  parent_id VARCHAR(10) PRIMARY KEY
);

CREATE TABLE Child (
  child_id INT PRIMARY KEY,
  parent_id VARCHAR(10) NOT NULL REFERENCES Parent(parent_id)
);

If you subsequently view the table definition by running SHOW CREATE TABLE Child, you see the foreign key constraint is missing, as if you had not defined it at all.