title: SQL Foreign key to two different tables
body: <p>I have a problem like<br>
  Table A:<br>
  -- TableBCId<br>
  Table B:<br>
  -- Id<br>
  Table C:<br>
  -- Id</p>
  
  <p>I am looking for a way to create a foreign key table A where an entry can be either in table B or table C  </p>
  
  <p>Example entries:<br>
  Table A:<br>
   --  TableBCId: 1<br>
   --  TableBCId: 2<br>
  Table B:<br>
  -- Id: 1<br>
  Table C:<br>
  -- Id: 2  </p>
  
  <p>I want to avoid if possible:<br>
  - Two columns in table A<br>
  - Default values<br>
  - Additional tables<br>
  - Creation of an base entity is not possible  </p>
  
  <p>Every idea welcome</p>