NVL

☆ NVL : null값 반환. 은근히 쓰임 정확하게 알아두기
-- 고객 별 총 구매액 
-- count 가능하지만 깨짐. null이 아니라 1로 출력됨. null때문에 sum도 불가능
-- NVL(SUM(saleprice),0) '안쪽에 null값이 있으면 0으로 변환하여 출력해라'

SELECT c.NAME, NVL(SUM(saleprice),0) FROM CUSTOMER c 
LEFT OUTER JOIN ORDERS o ON c.CUSTID = o.CUSTID 
GROUP BY c.NAME;

-- NULL : 문자가 아닌 키워드, 값
-- '='이 안됨 is로 표현

SELECT * FROM CUSTOMER WHERE PHONE = null; -- 출력안됨
SELECT * FROM CUSTOMER WHERE PHONE IS NULL; -- NULL값 출력
SELECT * FROM CUSTOMER WHERE PHONE IS NOT NULL; -- NULL 아닌 값 출력