# 创建具有DECIMAL字段的表 验证decimal默认是decimal(10,0)
mysql> create table decimal_tb (col1 decimal,col2 decimal(5,2));
Query OK, 0 rows affected (0.04 sec)
mysql> show create table decimal_tb\G
*************************** 1. row ***************************
    Table: decimal_tb
Create Table: CREATE TABLE `decimal_tb` (
 `col1` decimal(10,0) DEFAULT NULL,
 `col2` decimal(5,2) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

# 插入数据测试
# 结论：超出存储范围会报错，小数位不足会自动补0，首位数字为0自动忽略，小数位超出会截断 并按四舍五入处理。
mysql> insert into decimal_tb (col1,col2) values (100,100);
Query OK, 1 row affected (0.05 sec)
mysql> insert into decimal_tb (col2) values (1.23);
Query OK, 1 row affected (0.01 sec)
mysql> insert into decimal_tb (col2) values (10.2);
Query OK, 1 row affected (0.01 sec)
mysql> insert into decimal_tb (col2) values (09.9);
Query OK, 1 row affected (0.01 sec)
mysql> select * from decimal_tb;
+------+--------+
| col1 | col2  |
+------+--------+
| 100 | 100.00 |
| NULL |  1.23 |
| NULL | 10.20 |
| NULL |  9.90 |
+------+--------+
4 rows in set (0.00 sec)
mysql> insert into decimal_tb (col2) values (9999);
ERROR 1264 (22003): Out of range value for column 'col2' at row 1
mysql> insert into decimal_tb (col2) values (12.233); 
Query OK, 1 row affected, 1 warning (0.01 sec)
mysql> show warnings;
+-------+------+-------------------------------------------+
| Level | Code | Message                  |
+-------+------+-------------------------------------------+
| Note | 1265 | Data truncated for column 'col2' at row 1 |
+-------+------+-------------------------------------------+
1 row in set (0.00 sec)
mysql> insert into decimal_tb (col2) values (12.2300);
Query OK, 1 row affected (0.01 sec)

# 变量范围测试
# 结论：M范围是1到65，D范围是0到30，且D不大于M
mysql> alter table decimal_tb add column col3 decimal(6,6);
Query OK, 0 rows affected (0.12 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table decimal_tb add column col4 decimal(6,7); 
ERROR 1427 (42000): For float(M,D), double(M,D) or decimal(M,D), M must be >= D (column 'col4').
mysql> alter table decimal_tb add column col4 decimal(65,2);
Query OK, 0 rows affected (0.11 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table decimal_tb add column col4 decimal(66,2);
ERROR 1426 (42000): Too-big precision 66 specified for 'col4'. Maximum is 65.
mysql> alter table decimal_tb add column col5 decimal(60,30); 
Query OK, 0 rows affected (0.13 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table decimal_tb add column col6 decimal(60,31);
ERROR 1425 (42000): Too big scale 31 specified for column 'col6'. Maximum is 30.