Full join selects all the observations from both data sets but there are missing values where the key value in each observation is found in one table only.
PROC SQL;
CREATE TABLE C AS
SELECT A.*, B.CC_NUMBER, B.START_DATE
FROM CUSTOMER A FULL JOIN CC_DETAILS B
ON A.CUSTOMERID=B.CUSTOMERID
QUIT;
Dataset C will contain all records from both the tables and fill in .
for missing matches on either side.