Oracle 18c

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CREATE TABLE t(Warranty_Period  INTERVAL YEAR(2) TO MONTH)
BEGIN
  INSERT INTO t VALUES(TO_YMINTERVAL('00-00'));
  INSERT INTO t VALUES(TO_YMINTERVAL('00-03'));
  INSERT INTO t VALUES(TO_YMINTERVAL('01-00'));
  INSERT INTO t VALUES(TO_YMINTERVAL('01-01'));  
  INSERT INTO t VALUES(TO_YMINTERVAL('01-06'));  
END;  
/
1 rows affected

WITH t2 AS
(
 SELECT Warranty_Period,
        TO_NUMBER(REGEXP_SUBSTR( Warranty_Period, '[^-]+' )) AS Warranty_y,
        TO_NUMBER(REGEXP_SUBSTR( Warranty_Period, '[^-]+$')) AS Warranty_m 
   FROM t       
)
SELECT CASE Warranty_Period
         WHEN INTERVAL '0-0' YEAR TO MONTH THEN
          'No warranty'
         ELSE
          DECODE(SIGN(Warranty_y),1,NVL2(Warranty_y,Warranty_y||' year ',''))||  
          CASE WHEN Warranty_y*Warranty_m > 0 THEN 'and ' END||        
          DECODE(SIGN(Warranty_m),1,Warranty_m||' month')||
          CASE WHEN Warranty_m > 1 THEN 's' END
       END AS Warranty
  FROM t2


WARRANTY


No warranty

3 months

1 year

1 year and 1 month

1 year and 6 months