Andy Niu �����ĵ�

1、double(5,2) 表示总的位数是5，小数点后面是2位。
2、插入数据，整数部分超出，报错：
    1264 - Out of range value for column 'v3' at row 1
    小数部分超出，不会报错，四舍五入只取两位

1、这里的int(11) 与int的大小和存储字节，没有一毛钱关系，int的存储字节是4个字节，最大值为 65536*65536 = 40多亿，
    对于有符号的int，是20多亿。
2、那么这里的(11) 表示什么意思？
    考虑下面的需求，ID字段显示宽度为2，宽度不够的补充0。
3、测试如下：
mysql> desc student;
    +-------+-------------+------+-----+---------+-------+
    | Field | Type        | Null | Key | Default | Extra |
    +-------+-------------+------+-----+---------+-------+
    | id    | int(11)     | NO   | PRI | 0       |       |
    | NAME  | varchar(16) | YES  | MUL | NULL    |       |
    | AGE   | int(11)     | YES  |     | NULL    |       |
    +-------+-------------+------+-----+---------+-------+
    3 rows in set (0.01 sec)
    
    mysql> select * from student;
    +-----+---------+------+
    | id  | NAME    | AGE  |
    +-----+---------+------+
    | 1   | Andy1   | NULL |
    | 11  | Andy11  | NULL |
    | 101 | Andy101 | NULL |
    +-----+---------+------+
    
    mysql> alter table student modify id int(2);
    Query OK, 0 rows affected (0.03 sec)
    Records: 0 Duplicates: 0 Warnings: 0
    
    mysql> desc student;
    +-------+-------------+------+-----+---------+-------+
    | Field | Type        | Null | Key | Default | Extra |
    +-------+-------------+------+-----+---------+-------+
    | id    | int(2)      | NO   | PRI | 0       |       |
    | NAME  | varchar(16) | YES  | MUL | NULL    |       |
    | AGE   | int(11)     | YES  |     | NULL    |       |
    +-------+-------------+------+-----+---------+-------+
    3 rows in set (0.01 sec)
    
    mysql> select * from student;
    +-----+---------+------+
    | id  | NAME    | AGE |
    +-----+---------+------+
    | 1   | Andy1   | NULL |
    | 11  | Andy11  | NULL |
    | 101 | Andy101 | NULL |
    +-----+---------+------+
    3 rows in set (0.00 sec)
    
    mysql> alter table student modify id int(2) zerofill;
    Query OK, 3 rows affected (0.04 sec)
    Records: 3 Duplicates: 0 Warnings: 0
    
    mysql> desc student;
    +-------+--------------------------+------+-----+---------+-------+
    | Field | Type                     | Null | Key | Default | Extra |
    +-------+--------------------------+------+-----+---------+-------+
    | id    | int(2) unsigned zerofill | NO   | PRI | 00      |       |
    | NAME  | varchar(16)              | YES  | MUL | NULL    |       |
    | AGE   | int(11)                  | YES  |     | NULL    |       |
    +-------+--------------------------+------+-----+---------+-------+
    3 rows in set (0.01 sec)
    
    mysql> select * from student;
    +-----+---------+------+
    | id  | NAME    | AGE  |
    +-----+---------+------+
    | 01  | Andy1   | NULL |
    | 11  | Andy11  | NULL |
    | 101 | Andy101 | NULL |
    +-----+---------+------+
    3 rows in set (0.00 sec)
4、得出结果：
    a、int(N) 必须与zerofill 结合，才能有效果。
    b、只有宽度不够的，才补充0，宽度超过的，显示实际宽度。
    c、使用mysql客户端有效果，使用navicat等第三方软件可能没有效果。