[Mysql] CAST函数

CAST函数用于将值从一种数据类型转换为表达式中指定的另一种数据类型

语法

CAST(value AS datatype)

AS关键字用于分隔两个参数，在AS之前的是要处理的数据，在AS之后的是要转换的数据类型 

参数说明

value: 要转换的值

datatype: 要转换成的数据类型

示例

1.将值转换为DATE数据类型

-- 2017-08-29SELECT CAST('2017-08-29' AS DATE); -- 2022-04-27 15:42:19SELECT NOW();-- 2022-04-27SELECT CAST(NOW() AS DATE);

将值转换为DATETIME数据类型

-- 2022-04-27 00:00:00SELECT CAST('2022-04-27' AS DATETIME); 

将值转换为TIME数据类型

-- 14:06:10SELECT CAST('14:06:10' AS TIME); -- 14:06:10SELECT CAST('2022-04-27 14:06:10' AS TIME); 

将值转换为CHAR数据类型

-- '150'SELECT CAST(150 AS CHAR); -- 'Hello World437'SELECT CONCAT('Hello World',CAST(437 AS CHAR));

将值转换为SIGNED数据类型

-- 5SELECT CAST('5.0' AS SIGNED);-- 2SELECT (1 + CAST('3' AS SIGNED))/2;-- -5SELECT CAST(5-10 AS SIGNED); -- 6SELECT CAST(6.4 AS SIGNED);-- -6SELECT CAST(-6.4 AS SIGNED);-- 7SELECT CAST(6.5 AS SIGNED); -- -7SELECT CAST(-6.5 AS SIGNED);

将值转换为UNSIGNED数据类型

-- 5SELECT CAST('5.0' AS UNSIGNED);-- 6SELECT CAST(6.4 AS UNSIGNED);-- 0SELECT CAST(-6.4 AS UNSIGNED);-- 7SELECT CAST(6.5 AS UNSIGNED); -- 0SELECT CAST(-6.5 AS UNSIGNED);

 7.将值转换为DECIMAL数据类型

-- 9SELECT CAST('9.0' AS DECIMAL);-- DECIMAL(数值精度，小数点保留长度)-- DECIMAL(10,2)可以存储最多具有8位整数和2位小数的数字-- 精度与小数位数分别为10与2-- 精度是总的数字位数，包括小数点左边和右边位数的总和-- 小数位数是小数点右边的位数-- 9.50SELECT CAST('9.5' AS DECIMAL(10,2));-- 99999999.99SELECT CAST('1234567890.123' AS DECIMAL(10,2));-- 220.232SELECT CAST('220.23211231' AS DECIMAL(10, 3));-- 220.232SELECT CAST(220.23211231 AS DECIMAL(10, 3));

练习案例

导入数据 

DROP TABLE IF EXISTS `Material`;CREATE TABLE `Material` (  `ID` varchar(255) DEFAULT NULL,  `Material` varchar(255) DEFAULT NULL,  `FileName` varchar(255) DEFAULT NULL) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;INSERT INTO `Material` VALUES ('1', '07-100215-01', 'FD03-FCZ-220430012624541.txt');INSERT INTO `Material` VALUES ('2', '07-100390-01', 'FD03-FCZ-210318012624541.txt');INSERT INTO `Material` VALUES ('3', '07-1043-01', 'FD03-FCZ-170430012624541.txt');INSERT INTO `Material` VALUES ('4', '07-1044-01', 'FD03-FCZ-191225012624541.txt');INSERT INTO `Material` VALUES ('5', '07-1045-01', 'FD03-FCZ-200601012624541.txt');INSERT INTO `Material` VALUES ('6', '07-1046-01', 'FD03-FCZ-180705012624541.txt');

Material表 

Material:料号  FileName:文件名称(存储相应料号位置)

注明: ID，Material，FileName都是VARCHAR类型 

FileName取值举例说明:'FD03-FCZ-220430012624541.txt'，红色标注数字代表年月日，其余都取值相同(不用考虑具体含义)

问题:查询小于220401(整数类型年月日数值)的Material

SELECT MaterialFROM (SELECT Material,(CAST(SUBSTRING(FileName,10,6)AS SIGNED))AS 'time'      FROM Material      WHERE 'time' < 220401)AS filter_material;

解题思路
利用SUBSTRING函数截取FileName值中代表年月日的数字
由于FileName中的值为varchar类型，220401为整数类型，需要使用CAST函数进行类型转换 

结果展示:

来源地址：https://blog.csdn.net/Hudas/article/details/124399908