Oracle Exists

 FOR R IN (SELECT T.USER_NAME
              FROM T_USER T
             WHERE EXISTS (SELECT 1
                      FROM T_USER_ROLE TR
                     WHERE T.USER_ID = TR.USER_ID)) LOOP
    DBMS_OUTPUT.PUT_LINE(R.USER_NAME);
  
  END LOOP;
  
  --用 WHERE T.USER_ID = TR.USER_ID 来判断 T_USER 表中的USER_ID是否存在于T_USER_ROLE表中