MySQL数据库——数据库的数据类型（一）

四、数据类型

1.数据类型分类

|-----------|--------------------------------|----------------------------------------------------------------------------------------|
| 分类 | 数据类型 | 说明 |
| 数值类型 | BIT（M） | 位类型。指定位数，默认值1，范围1-64 |
| 数值类型 | TINYINT [UNSIGNED] | 带符号的范围-128127，无符号范围0255.默认有符号 |
| 数值类型 | BOOL | 使用0和1表示真和假 |
| 数值类型 | SMALLINT [UNSIGNED] | 带符号是-2^15次方到2^15-1，无符号是2^16-1 |
| 数值类型 | INT [UNSINGED] | 带符号是-2^31次方到2^31-1，无符号是2^32-1 |
| 数值类型 | BIGINT [UNSIGNED] | 带符号是-2^63次方到2^63-1，无符号是2^64-1 |
| 数值类型 | FLOAT [(M,D)] [UNSIGNED] | M指定显示长度，d指定小数位数，占用4字节 |
| 数值类型 | DOUBLE [(M,D)] [UNSIGNED] | 表示比f1oat精度更大的小数，占用空间8字节 |
| 数值类型 | DECIMAL [(M,D)] [UNSIGNED] | 定点数指定长度，D表示小数点的位数 |
| 文本、二进制类型 | CHAR(size) | 固定长度字符串，最大255 |
| 文本、二进制类型 | VARCHAR(SIZE) | 可变长度字符串，最大长度65535 |
| 文本、二进制类型 | BLOB | 二进制数据 |
| 文本、二进制类型 | TEXT | 大文本，不支持全文索引，不支持默以值 |
| 时间日期 | DATE/DATETIME/TIMESTAMP | 日期类型(yyyy-mm-dd) (yyyy-mm-dd hh:mm:ss) timestamp时间戳 |
| String类型 | ENUM类型 | EUM是一个字符串对象，其值来自表创建时在列规定中显示枚举的一列值 |
| String类型 | SET类型 | SET是一个字符串对象，可以有零或多个值，其值来自表创建时规定的允许的一列值。指定包括多个set成员的set列值时各成员之间用逗号间隔开。这样set成员值本身不能包含逗号。 |

2.数值类型

|-----------|----|----------------------|---------------------|-----|----------------------|
| 类型 | 字节 | 带符号的 || 无符号的 ||
| | | 最大值 | 最小值 | 最大值 | 最小值 |
| TINYINT | 1 | -128 | 127 | 0 | 255 |
| SMALLINT | 2 | -32768 | 32767 | 0 | 65535 |
| MEDIUMINT | 3 | -8388608 | 8388607 | 0 | 16777215 |
| INT | 4 | -2147483648 | 2147483647 | 0 | 4294967295 |
| BIGINT | 8 | -9223372036854775808 | 9223372036854775807 | 0 | 18446744073709551615 |

①tinyint类型

sql 复制代码

mysql> create table t1(
    -> num tinyint
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> desc t1;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| num   | tinyint(4) | YES  |     | NULL    |       |
+-------+------------+------+-----+---------+-------+
1 row in set (0.00 sec)

mysql> show tables;
+-------------------+
| Tables_in_test_db |
+-------------------+
| t1                |
+-------------------+
1 row in set (0.00 sec)

mysql> show create table t1;
+-------+------------------------------------------------------------------------------------------+
| Table | Create Table                                                                             |
+-------+------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `num` tinyint(4) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show create table t1\G
*************************** 1. row ***************************
       Table: t1
Create Table: CREATE TABLE `t1` (
  `num` tinyint(4) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)


mysql> insert into t1 values (-128);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t1 values (127);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t1 values (0);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t1 values (-1);
Query OK, 1 row affected (0.01 sec)

mysql> insert into t1 values (1);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t1;
+------+
| num  |
+------+
| -128 |
|  127 |
|    0 |
|   -1 |
|    1 |
+------+
5 rows in set (0.00 sec)

mysql> insert into t1 values (-129);
ERROR 1264 (22003): Out of range value for column 'num' at row 1
mysql> insert into t1 values (128);
ERROR 1264 (22003): Out of range value for column 'num' at row 1

在MySQL中，整型可以指定是有符号的和无符号的，默认是有符号的。

可以通过UNSIGNED来说明某个字段是无符号的

注意：尽量不使用unsigned,对于int类型可能存放不下的数据，int unsigned同样可能存放不下，与其如此，还不如设计时，将int类型提升为bigint类型。

sql 复制代码

mysql> create table t2(
    -> num tinyint unsigned
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> show tables;
+-------------------+
| Tables_in_test_db |
+-------------------+
| t1                |
| t2                |
+-------------------+
2 rows in set (0.00 sec)

mysql> desc t2;
+-------+---------------------+------+-----+---------+-------+
| Field | Type                | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| num   | tinyint(3) unsigned | YES  |     | NULL    |       |
+-------+---------------------+------+-----+---------+-------+
1 row in set (0.00 sec)

mysql> insert into t2 values (255);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t2 values (0);
Query OK, 1 row affected (0.01 sec)

mysql> insert into t2 values (100);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t2 values (-1);
ERROR 1264 (22003): Out of range value for column 'num' at row 1
mysql> insert into t2 values (256);
ERROR 1264 (22003): Out of range value for column 'num' at row 1
mysql> select * from t2;
+------+
| num  |
+------+
|  255 |
|    0 |
|  100 |
+------+
3 rows in set (0.00 sec)

如果我们向mysql特定的类型中插入不合法的数据，MySQL一般都是直接拦截我们的操作，如果我们的数据成功插入到mysql中了，一定插入的时候是合法的。所以在mysql中，一般而言，数据类型本身也就是一种约束（约束使用者，保证数据插入的合法性），这样就能保证数据库中的数据是可预期的、完整的

但是在C/C++中，如果较大的数存入范围较小的数据类型中（如char中存入1234567），这种情况下一般不会报错，顶多会警告，这时候存入了数据，但是数据会被截断或发生了隐式类型转换

②bit类型

基本语法：bit[(M)]:位字段类型，M表示每个值的位数，范围从1到64，如果M被忽略，默认为1

"位数 " 指的是字段可以存储的位的数量。在这种情况下，BIT(1) 意味着字段只能存储一个位。如果需要存储更多的位，比如 8 位，您可以使用 BIT(8)，这将允许存储 8 位的二进制数，范围从 00000000 到 11111111。
bit字段在显示时，是按照ASC码对应的值显示。

sql 复制代码

mysql> show tables;
+-------------------+
| Tables_in_test_db |
+-------------------+
| t1                |
| t2                |
+-------------------+
2 rows in set (0.00 sec)

-- 创建表t3，一行存储id，一行存储在线状态online
mysql> create table t3(
    -> id int,
    -> online bit(1)
    -> );
Query OK, 0 rows affected (0.02 sec)

-- BIT(1) 表示这个字段可以存储一个位（bit），即 0 或 1

mysql> desc t3;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| id     | int(11) | YES  |     | NULL    |       |
| online | bit(1)  | YES  |     | NULL    |       |
+--------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> insert into t3 (id,online) values (123,0);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t3 (id,online) values (124,1);
Query OK, 1 row affected (0.00 sec)

-- 超出了 BIT(1) 字段所能存储的范围

mysql> insert into t3 (id,online) values (123,3);
ERROR 1406 (22001): Data too long for column 'online' at row 1
mysql> insert into t3 (id,online) values (123,5);
ERROR 1406 (22001): Data too long for column 'online' at row 1
mysql> insert into t3 (id,online) values (123,2);
ERROR 1406 (22001): Data too long for column 'online' at row 1

-- 这里发现online的数据没有出现，因为bit字段显示时，是按照ASCII码的值显示
mysql> select * from t3;
+------+--------+
| id   | online |
+------+--------+
|  123 |        |
|  124 |       |
+------+--------+
2 rows in set (0.00 sec)
-- 以十进制显示
mysql> select id,hex(online) from t3;
+------+-------------+
| id   | hex(online) |
+------+-------------+
|  123 | 0           |
|  124 | 1           |
+------+-------------+
2 rows in set (0.00 sec)

-- 更改bit的位数
mysql> alter table t3 modify online bit(10);
Query OK, 2 rows affected (0.04 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> desc t3;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| id     | int(11) | YES  |     | NULL    |       |
| online | bit(10) | YES  |     | NULL    |       |
+--------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> select * from t3;
+------+--------+
| id   | online |
+------+--------+
|  123 |        |
|  124 |       |
+------+--------+
2 rows in set (0.00 sec)
这次插入之前不能插入的数据可以插入了
mysql> insert into t3 (id,online) values (123,3);
Query OK, 1 row affected (0.01 sec)

mysql> insert into t3 (id,online) values (123,5);
Query OK, 1 row affected (0.00 sec)

mysql> select id,hex(online) from t3;
+------+-------------+
| id   | hex(online) |
+------+-------------+
|  123 | 0           |
|  124 | 1           |
|  123 | 3           |
|  123 | 5           |
+------+-------------+
4 rows in set (0.00 sec)

-- 且可以插入字符，但其实这里的字符是以97的形式存储
mysql> insert into t3 (id,online) values (123,'a');
Query OK, 1 row affected (0.00 sec)

mysql> select * from t3;
+------+--------+
| id   | online |
+------+--------+
|  123 |        |
|  124 |       |
|  123 |       |
|  123 |       |
|  123 |  a     |
+------+--------+
5 rows in set (0.00 sec)
-- 如果插入97，显示的时候也是字符a
mysql> insert into t3 (id,online) values (123,97);
Query OK, 1 row affected (0.01 sec)

mysql> select * from t3;
+------+--------+
| id   | online |
+------+--------+
|  123 |        |
|  124 |       |
|  123 |       |
|  123 |       |
|  123 |  a     |
|  123 |  a     |
+------+--------+
6 rows in set (0.00 sec)

-- 创建表t4，给他指定位数为65，发现创建失败，因为bit的位数范围为1~64
mysql> create table t4(
    -> id int,
    -> online bit(65)
    -> );
ERROR 1439 (42000): Display width out of range for column 'online' (max = 64)
-- 表示online的最大范围为64
mysql> create table t4( id int, online bit(64) );
Query OK, 0 rows affected (0.01 sec)

mysql> desc t4;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| id     | int(11) | YES  |     | NULL    |       |
| online | bit(64) | YES  |     | NULL    |       |
+--------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

-- 删除掉表t4重新创建，这次不指定位数
mysql> drop table t4;
Query OK, 0 rows affected (0.01 sec)

mysql> create table t4( id int, online bit ); // 如果M被忽略，默认为1
Query OK, 0 rows affected (0.01 sec)

mysql> desc t4;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| id     | int(11) | YES  |     | NULL    |       |
| online | bit(1)  | YES  |     | NULL    |       |
+--------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

③小数类型

1>float

基本语法：float[(m, d)] [unsigned] : M指定显示长度（整数位+小数位），d指定小数位数，占用空间4个字节

小数：float(4,2)表示的范围是-99.99 ~ 99.99，MySQL在保存值时会进行四舍五入。

sql 复制代码

mysql> show tables;
+-------------------+
| Tables_in_test_db |
+-------------------+
| t1                |
| t2                |
| t3                |
| t4                |
+-------------------+
4 rows in set (0.01 sec)
-- 创建表t5，一行存储int，一行存储浮点数，总长度为4，小数位位数为2
mysql> create table t5(
    -> id int,
    -> salary float(4,2)
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> desc t5;
+--------+------------+------+-----+---------+-------+
| Field  | Type       | Null | Key | Default | Extra |
+--------+------------+------+-----+---------+-------+
| id     | int(11)    | YES  |     | NULL    |       |
| salary | float(4,2) | YES  |     | NULL    |       |
+--------+------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> show create table t5\G
*************************** 1. row ***************************
       Table: t5
Create Table: CREATE TABLE `t5` (
  `id` int(11) DEFAULT NULL,
  `salary` float(4,2) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

-- 向表中插入数据
mysql> insert into t5 (id,salary) values (1,99.99);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t5 (id,salary) values (1,-99.99);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t5 (id,salary) values (1,12.34);
Query OK, 1 row affected (0.00 sec)
-- 都OK
mysql> select * from t5;
+------+--------+
| id   | salary |
+------+--------+
|    1 |  99.99 |
|    1 | -99.99 |
|    1 |  12.34 |
+------+--------+
3 rows in set (0.00 sec)

-- 插入的数据要符合定义的属性
-- 99.999四舍五入后是100.000，不符合
mysql> insert into t5 (id,salary) values (1,99.999);
ERROR 1264 (22003): Out of range value for column 'salary' at row 1
-- 100.00不是四位，不符合
mysql> insert into t5 (id,salary) values (1,100.00);
ERROR 1264 (22003): Out of range value for column 'salary' at row 1
-- 当插入 100.0 时，MySQL 会自动将其视为 100.00
-- 因为 MySQL 总是会确保小数部分符合 D 的要求
mysql> insert into t5 (id,salary) values (1,100.0);
ERROR 1264 (22003): Out of range value for column 'salary' at row 1
-- 符合要求，这里存储10.0实际上存储的是10.00
mysql> insert into t5 (id,salary) values (1,10.0);
Query OK, 1 row affected (0.01 sec)

mysql> select * from t5;
+------+--------+
| id   | salary |
+------+--------+
|    1 |  99.99 |
|    1 | -99.99 |
|    1 |  12.34 |
|    1 |  10.00 |
+------+--------+
4 rows in set (0.00 sec)

mysql> insert into t5 (id,salary) values (1,23.45);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t5;
+------+--------+
| id   | salary |
+------+--------+
|    1 |  99.99 |
|    1 | -99.99 |
|    1 |  12.34 |
|    1 |  10.00 |
|    1 |  23.45 |
+------+--------+
5 rows in set (0.00 sec)

-- 当插入小数位数超过定义的小数位数（例如 FLOAT(4,2)）的数据时
-- MySQL 会对数据进行四舍五入处理
mysql> insert into t5 (id,salary) values (1,23.456); -- 保存为了23.46
Query OK, 1 row affected (0.00 sec)

mysql> insert into t5 (id,salary) values (1,23.454); -- 保存为了23.45
Query OK, 1 row affected (0.00 sec)

mysql> select * from t5;
+------+--------+
| id   | salary |
+------+--------+
|    1 |  99.99 |
|    1 | -99.99 |
|    1 |  12.34 |
|    1 |  10.00 |
|    1 |  23.45 |
|    1 |  23.46 |
|    1 |  23.45 |
+------+--------+
7 rows in set (0.00 sec)

-- 都保存为了99.99
mysql> insert into t5 (id,salary) values (1,99.994);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t5 (id,salary) values (1,99.993);
Query OK, 1 row affected (0.01 sec)

mysql> insert into t5 (id,salary) values (1,99.991);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t5;
+------+--------+
| id   | salary |
+------+--------+
|    1 |  99.99 |
|    1 | -99.99 |
|    1 |  12.34 |
|    1 |  10.00 |
|    1 |  23.45 |
|    1 |  23.46 |
|    1 |  23.45 |
|    1 |  99.99 |
|    1 |  99.99 |
|    1 |  99.99 |
+------+--------+
10 rows in set (0.00 sec)

-- 这几个在四舍五入的时候超过了规定的范围
mysql> insert into t5 (id,salary) values (1,99.995);
ERROR 1264 (22003): Out of range value for column 'salary' at row 1
mysql> insert into t5 (id,salary) values (1,99.996);
ERROR 1264 (22003): Out of range value for column 'salary' at row 1
mysql> insert into t5 (id,salary) values (1,99.997);
ERROR 1264 (22003): Out of range value for column 'salary' at row 1
mysql> desc t5;
+--------+------------+------+-----+---------+-------+
| Field  | Type       | Null | Key | Default | Extra |
+--------+------------+------+-----+---------+-------+
| id     | int(11)    | YES  |     | NULL    |       |
| salary | float(4,2) | YES  |     | NULL    |       |
+--------+------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

sql 复制代码

mysql> create table t6(
    -> id bigint,
    -> salary float(4,2) unsigned
    -> );
Query OK, 0 rows affected (0.02 sec)

mysql> desc t6;
+--------+---------------------+------+-----+---------+-------+
| Field  | Type                | Null | Key | Default | Extra |
+--------+---------------------+------+-----+---------+-------+
| id     | bigint(20)          | YES  |     | NULL    |       |
| salary | float(4,2) unsigned | YES  |     | NULL    |       |
+--------+---------------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

mysql> insert into t6(id,salary) values (1,0);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t6;
+------+--------+
| id   | salary |
+------+--------+
|    1 |   0.00 |
+------+--------+
1 row in set (0.00 sec)

mysql> insert into t6(id,salary) values (1,99.99);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t6(id,salary) values (1,99.996);
ERROR 1264 (22003): Out of range value for column 'salary' at row 1
mysql> insert into t6(id,salary) values (1,99.999);
ERROR 1264 (22003): Out of range value for column 'salary' at row 1
mysql> insert into t6(id,salary) values (1,99.99);
Query OK, 1 row affected (0.00 sec)

mysql> insert into t6(id,salary) values (1,-99.99);
ERROR 1264 (22003): Out of range value for column 'salary' at row 1
mysql> insert into t6(id,salary) values (1,-0.99);
ERROR 1264 (22003): Out of range value for column 'salary' at row 1
mysql> insert into t6(id,salary) values (1,-0.01);
ERROR 1264 (22003): Out of range value for column 'salary' at row 1
mysql> insert into t6(id,salary) values (1,34.567);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t6;
+------+--------+
| id   | salary |
+------+--------+
|    1 |   0.00 |
|    1 |  99.99 |
|    1 |  99.99 |
|    1 |  34.57 |
+------+--------+
4 rows in set (0.00 sec)

float可能会出现丢失精度的情况，例如：

sql 复制代码

mysql> desc t5;
+--------+---------+------+-----+---------+-------+
| Field  | Type    | Null | Key | Default | Extra |
+--------+---------+------+-----+---------+-------+
| id     | int(11) | YES  |     | NULL    |       |
| salary | float   | YES  |     | NULL    |       |
+--------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec)

-- 这里插入了35434663.23但是保存的是35434700
mysql> insert into t5(id,salary) values (1,35434663.23);
Query OK, 1 row affected (0.00 sec)

mysql> select * from t5;
+------+----------+
| id   | salary   |
+------+----------+
|    1 |    99.99 |
|    1 |   -99.99 |
|    1 |    12.34 |
|    1 |       10 |
|    1 |    23.45 |
|    1 |    23.46 |
|    1 |    23.45 |
|    1 |    99.99 |
|    1 |    99.99 |
|    1 |    99.99 |
|    1 |  3456540 |
|    1 |  3456540 |
|    1 |  543.244 |
|    1 |  3543.24 |
|    1 |  3543460 |
|    1 | 35434700 |
+------+----------+
16 rows in set (0.00 sec)

salary 列在实际的表结构中是一个普通的 FLOAT****类型，允许插入大数值 ，但由于浮点数的精度限制，部分大数值在存储和检索时可能会出现精度丢失 ，显示为不同的数值。要避免这种精度问题，应该使用 DECIMAL 类型来代替 FLOAT，尤其是当你需要精确存储货币或其他精度敏感的数据时。

2>decimal

基本语法：decimal(m, d) [unsigned] : 定点数m指定长度，d表示小数点的位数

sql 复制代码

mysql> create table t7(
    -> f1 float(10,8),
    -> f2 decimal(4,2)
    -> );
Query OK, 0 rows affected (0.01 sec)

mysql> desc t7;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| f1    | float(10,8)  | YES  |     | NULL    |       |
| f2    | decimal(4,2) | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
-- 插入数据
mysql> insert into t7(f1,f2) values (10.0,99.99);
Query OK, 1 row affected (0.01 sec)

mysql> insert into t7(f1,f2) values (10.0,-99.99);
Query OK, 1 row affected (0.00 sec)

-- 数据不符合小数位2位的要求
mysql> insert into t7(f1,f2) values (10.0,-99.999);
ERROR 1264 (22003): Out of range value for column 'f2' at row 1
mysql> insert into t7(f1,f2) values (10.0,99.999);
ERROR 1264 (22003): Out of range value for column 'f2' at row 1

mysql> insert into t7(f1,f2) values (10.0,0.1);
Query OK, 1 row affected (0.00 sec)

-- 同样会四舍五入
mysql> insert into t7(f1,f2) values (10.0,99.994);
Query OK, 1 row affected, 1 warning (0.01 sec)

mysql> insert into t7(f1,f2) values (10.0,99.935);
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> select * from t7;
+-------------+--------+
| f1          | f2     |
+-------------+--------+
| 10.00000000 |  99.99 |
| 10.00000000 | -99.99 |
| 10.00000000 |   0.10 |
| 10.00000000 |  99.99 |
| 10.00000000 |  99.94 |
+-------------+--------+
5 rows in set (0.00 sec)

-- 改变decimal的属性，变得和float一样
mysql> alter table t7 modify f2 decimal(10,8);
Query OK, 5 rows affected (0.04 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> desc t7;
+-------+---------------+------+-----+---------+-------+
| Field | Type          | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| f1    | float(10,8)   | YES  |     | NULL    |       |
| f2    | decimal(10,8) | YES  |     | NULL    |       |
+-------+---------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

-- 再次插入数据，且float和decimal数据一致
mysql> insert into t7 (f1,f2) values (23.12345678,23.12345678);
Query OK, 1 row affected (0.01 sec)

-- 一个是23.12345695，另一个是23.12345678，精度差距很大
mysql> select * from t7;
+-------------+--------------+
| f1          | f2           |
+-------------+--------------+
| 10.00000000 |  99.99000000 |
| 10.00000000 | -99.99000000 |
| 10.00000000 |   0.10000000 |
| 10.00000000 |  99.99000000 |
| 10.00000000 |  99.94000000 |
| 23.12345695 |  23.12345678 |
+-------------+--------------+
6 rows in set (0.00 sec)

3>float和decimal比较

1. 精度 vs. 性能

DECIMAL（定点数）：

①高精度：DECIMAL 类型在内部以字符串或整数的形式存储数值，能够精确地表示小数点后的位数，因此特别适合用于对精度要求高的场景，例如金融、货币、科学计算等。

②性能较低：由于 DECIMAL 以定点形式存储 ，需要进行较多的计算处理，尤其是在涉及大量计算的情况下，性能可能低于 FLOAT。

FLOAT 和 DOUBLE（浮点数）：

①高性能：浮点数使用科学计数法存储 ，在硬件层面支持的情况下，计算速度非常快，适合于需要处理大量浮点数的场景，如图形计算、物理模拟、工程计算等。

②精度较低：由于浮点数的存储方式，它们只能近似表示某些数值，可能在非常大的或非常小的数值时产生精度丢失。这在要求精度的场景下可能导致问题。

2.存储空间

DECIMAL：

由于其精度，它通常占用更多的存储空间，尤其是在处理大范围的数值时。

FLOAT 和 DOUBLE：

这些类型相对于 DECIMAL 占用的存储空间较小，特别是在存储范围大的数值时。FLOAT 通常使用 4 字节，而 DOUBLE 使用 8 字节。