title: Failed to add the foreign key constraint. Missing index for constraint Error Code: 1822
body: <p>So I'm trying to create a table 'Prerequisite_to', which is basically a relation that shows what classes are considered as a prerequisite for a specific class. Here are my SQL tables defections:</p>
  
  <pre><code>CREATE TABLE Class(
  infs CHAR(4) NOT NULL,
  course_number CHAR(3) NOT NULL,
  PRIMARY KEY (infs,course_number));
  
  CREATE TABLE Prerequisite_to(
      infs CHAR(4),
      course_number CHAR(3),
      PRIMARY KEY (infs,course_number),
      FOREIGN KEY (infs) REFERENCES Class(infs),
      FOREIGN KEY (course_number) REFERENCES Class(course_number)
  )
  </code></pre>
  
  <p>However, when I execute the script I get this error:</p>
  
  <blockquote>
    <p>17:29:43  CREATE TABLE Prerequisite_to(  infs CHAR(4),  course_number CHAR(3),  PRIMARY KEY (infs,course_number),  FOREIGN KEY (infs) REFERENCES Class(infs),  FOREIGN KEY (course_number) REFERENCES Class(course_number) )  Error Code: 1822. Failed to add the foreign key constraint. Missing index for constraint 'prerequisite_to_ibfk_2' in the referenced table 'Class'   0.00038 sec</p>
  </blockquote>