title: mysql error operand should have 1 columns stored procedure
body: <p>i try to create stored procedure but when i try my code it isnt work 
  my troubel is on @tgl when i execute the SP why it error operand should have 1 column</p>
  
  <pre><code>BEGIN
  
  SET @book = booking_id;
  
  
  
  set @tgl = concat(("d"),('SELECT RIGHT(booking_checkin ,2) as date FROM pt_bookings WHERE booking_id = ',@book));
  
  set @month =concat('SELECT mid(booking_checkin ,6,2) as month FROM pt_bookings WHERE booking_id = ',@book);
  
  set @year = concat('SELECT IF(left(booking_checkin ,4)  = "2019", "0", "1") as year FROM pt_bookings WHERE booking_id = ',@book);
  
  
  PREPARE stmt FROM @year;
  EXECUTE stmt;
  DEALLOCATE PREPARE stmt;
  
  PREPARE stmt FROM @month;
  EXECUTE stmt;
  DEALLOCATE PREPARE stmt;
  
  PREPARE stmt FROM @tgl;
  EXECUTE stmt;
  DEALLOCATE PREPARE stmt;
  
  
  END
  </code></pre>
  
  <p>my expetation for @tgl is number + d (22d)</p>