Mysql CAST函数的具体使用

CAST(value AS datatype)

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

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

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

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

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

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

-- 9
SELECT 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
-- 9.50
SELECT CAST('9.5' AS DECIMAL(10,2));
-- 99999999.99
SELECT CAST('1234567890.123' AS DECIMAL(10,2));
 
-- 220.232
SELECT CAST('220.23211231' AS DECIMAL(10, 3));
-- 220.232
SELECT CAST(220.23211231 AS DECIMAL(10, 3));

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');

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