title: Warning: Trigger created with compilation errors. (ORACLE
body: <p>I'm try to create a trigger who check if a teacher already teach this class in this session: </p>
  
  <pre><code>CREATE OR REPLACE TRIGGER C3
  BEFORE INSERT ON classGroup
  FOR EACH ROW
  DECLARE teach NUMBER(10);
  BEGIN
      SELECT count(*) INTO teach;
      from classGroup
      WHERE classId = :new.classId
      AND teacherId = :new.teacherId,
      AND sessionId = :new.sessionId;
          RAISE_APPLICATION_ERROR(-20002,'you cant teach 2 time the same class');
  
  exception
      when no_data_found then
          -- do insert
  END;
  /
  </code></pre>
  
  <p>but i'm always getting this message : </p>
  
  <pre><code>Warning: Trigger created with compilation errors.
  </code></pre>
  
  <p>the table Classgroup contain: classId, groupId, sessionId, teacherId, desc</p>