2. 创建一张表（name非空，age可空）
    3. [root@localhost][test]: create table user (name varchar(20) not null ,age int);
    4. Query OK, 0 rows affected (0.69 sec)
    6. 可以插入空字符串
    7. [root@localhost][test]: insert into user values('',19);
    8. Query OK, 1 row affected (0.05 sec)
    10. 插入空字符串，null值
    11. [root@localhost][test]: insert into user values('',null);
    12. Query OK, 1 row affected (0.08 sec)
    14. 不能插入null值
    15. [root@localhost][test]: insert into user values(null,null);
    16. ERROR 1048 (23000): Column 'name' cannot be null
    18. 查看字符串占用空间
    19. [root@localhost][test]: select length(name),length(age) from user where age is null;
    20. +--------------+-------------+
    21. | length(name) | length(age) |
    22. +--------------+-------------+
    23. |            0 |        NULL |
    24. +--------------+-------------+
    25. 1 row in set (0.02 sec)
    27. 查看字符串长度
    28. [root@localhost][test]: select char_length(name),char_length(age) from user where age is null;
    29. +-------------------+------------------+
    30. | char_length(name) | char_length(age) |
    31. +-------------------+------------------+
    32. |                 0 |             NULL |
    33. +-------------------+------------------+
    34. 1 row in set (0.05 sec)
    36. 判断字符串是否为空，如果为空，则isnull(字段)=1
    37. [root@localhost][test]: select isnull(name),isnull(age) from user where age is null;
    38. +--------------+-------------+
    39. | isnull(name) | isnull(age) |
    40. +--------------+-------------+
    41. |            0 |           1 |
    42. +--------------+-------------+
    43. 1 row in set (0.00 sec)