CREATE TEMPORARY TABLE TMP_TEST1 (CUSTOMER_NAME varchar(255));

INSERT INTO TMP_TEST1 (CUSTOMER_NAME) 

VALUES ('JOHN'),('MIKE'),('SMITH');

DELIMITER //

BEGIN NOT ATOMIC

  DECLARE done INT DEFAULT FALSE;

  -- DECLARE CUSTOMER_NAME varchar(255); -- not works

  DECLARE V_CUSTOMER_NAME varchar(255); -- works

  DECLARE CUSTOMER_NAME_LIST text DEFAULT '';

  DECLARE cur1 CURSOR FOR SELECT CUSTOMER_NAME FROM TMP_TEST1;

  DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

  OPEN cur1;

  read_loop: LOOP

    -- FETCH cur1 INTO CUSTOMER_NAME; -- not works

    FETCH cur1 INTO V_CUSTOMER_NAME; -- works

    IF done THEN

      LEAVE read_loop;

    END IF;

    -- set CUSTOMER_NAME_LIST = CONCAT_WS('|',CUSTOMER_NAME_LIST,CUSTOMER_NAME); -- not works

    set CUSTOMER_NAME_LIST = CONCAT_WS('|',CUSTOMER_NAME_LIST,V_CUSTOMER_NAME); -- works

  END LOOP read_loop;

  CLOSE cur1;

  SELECT CUSTOMER_NAME_LIST;

END; //

DELIMITER ;

DROP TEMPORARY TABLE TMP_TEST1;