오라클공부 181. NULL의 이해

NULL의 이해

 

SELECT MGR FROM EMP WHERE ENAME='SCOTT';                       7566

 

SELECT MGR FROM EMP WHERE ENAME='KING';                           NULL

 

SELECT NVL(MGR,0) FROM EMP WHERE ENAME='KING';                 0

 

SELECT MGR FROM EMP WHERE ENAME='KANG';                          공집합

 

SELECT NVL(MGR,'X') FROM EMP WHERE ENAME='KANG';             공집합

 

SELECT MAX(MGR) FROM EMP WHERE ENAME='KANG';                  NULL

 

SELECT NVL(MAX(MGR),99) FROM EMP WHERE ENAME='KANG;'      99