title: Older SQL servers and referential integrity thru foreign keys
body: <p>I am trying to gain an understanding of referential integrity they the use of foreign keys between an entity and relation table. </p>
  
  <p>The relational schema consist of 3 tables:</p>
  
  <ul>
  <li>Suppliers (S), </li>
  <li>Parts (P), and </li>
  <li>the quantity each has purchased (SP). </li>
  </ul>
  
  <p>The primary keys in each would be SNum in S, PNum in P and a composite primary and foreign keys of SNum and PNum in SP, referencing their respective tables.</p>
  
  <p>If my understanding of referential integrity is correct, if I were to input an illegal entry into the SP table( one that doesn’t exist in the primary key in S or P) such as “S7”, shouldn’t their be an error occurring because there is no “S7” in the S table? Is it because the server type may be outdated and not picking up the integrity or am I not understanding correctly? </p>