在mysql中，null与''是不相同的，''会插入一个空字符串
mysql> select version();
+------------+
| version()  |
+------------+
| 5.5.18-log |
+------------+
1 row in set (0.00 sec)

mysql> create table goolen (name varchar(10) primary key); 
Query OK, 0 rows affected (0.03 sec)

mysql> insert into goolen values('goolen');               
Query OK, 1 row affected (0.00 sec)

mysql> insert into goolen values('');      
Query OK, 1 row affected (0.00 sec)

mysql> insert into goolen values(null);
ERROR 1048 (23000): Column 'name' cannot be null
mysql> insert into goolen values('scott');
Query OK, 1 row affected (0.00 sec)

mysql> select * from goolen;
+--------+
| name   |
+--------+
|        |
| goolen |
| scott  |
+--------+
3 rows in set (0.00 sec)

在oracle中，null跟''是一样的，oracle都会理解为是null值；

SQL> select * from v$version where rownum=1;

BANNER
--------------------------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi

SQL> create table goolen(name varchar2(10) primary key);

Table created.

SQL> insert into goolen values('goolen');  

1 row created.

SQL> insert into goolen values(''); 
insert into goolen values('')
                          *
ERROR at line 1:
ORA-01400: cannot insert NULL into ("SYS"."GOOLEN"."NAME")

SQL> insert into goolen values(null);
insert into goolen values(null)
                          *
ERROR at line 1:
ORA-01400: cannot insert NULL into ("SYS"."GOOLEN"."NAME")

SQL> insert into goolen values('scott');

1 row created.

SQL> select * from goolen;

NAME
------------------------------
goolen
scott