Left join returns all the observations in the left data set regardless of their key values but only observations with matching key values from the right data set. Considering the same example as above,
PROC SQL;
CREATE TABLE C AS
SELECT A.*, B.CC_NUMBER, B.START_DATE
FROM CUSTOMER A LEFT JOIN CC_DETAILS B
ON A.CUSTOMERID=B.CUSTOMERID
QUIT;
Dataset C contains all the values from the left table, plus matched values from the right table or missing values in the case of no match.