length() 결과로 0 과 null 의 차이점이 무엇인가여?

mysql> select length(CD1) from test_a where a_no = '2100000090' ;
+------------------------+
| length(CD)             |
+------------------------+
|                   NULL    |
+------------------------+
 

mysql> select length(CD2) from test_a where a_no = '2100000090' ;
+------------------------+
| length(CD)             |
+------------------------+
|                   0             |
+------------------------+