Multi or Single Record using SQL

Hello All Master,

I have a question how to display multi or single record note using SQL effectively?

CREATE TABLE ZZZZ

  (DOCNBR VARCHAR(255), ZN VARCHAR(255)

  );

 

INSERT INTO ZZZZ VALUES ('138200806', 'A');

INSERT INTO ZZZZ VALUES ('138200807', 'C');

INSERT INTO ZZZZ VALUES ('138200807', 'C');

INSERT INTO ZZZZ VALUES ('138200808', 'B');

INSERT INTO ZZZZ VALUES ('138200808', 'B');

INSERT INTO ZZZZ VALUES ('138200808', 'D');

INSERT INTO ZZZZ VALUES ('138200808', 'C');

if DOCNBR having multiple ZN then return SINGLE else MULTI group by DOCNBR.

Like 138200807, even it has more than 1 record, query should return single because it only have 1 ZN