datatype參數(shù)取值
值	描述
DATE	將value轉(zhuǎn)換成'YYYY-MM-DD'格式
DATETIME	將value轉(zhuǎn)換成'YYYY-MM-DD HH：MM：SS'格式
TIME	將value轉(zhuǎn)換成'HH：MM：SS'格式
CHAR	將value轉(zhuǎn)換成CHAR(固定長(zhǎng)度的字符串)格式
SIGNED	將value轉(zhuǎn)換成INT(有符號(hào)的整數(shù))格式
UNSIGNED	將value轉(zhuǎn)換成INT(無(wú)符號(hào)的整數(shù))格式
DECIMAL	將value轉(zhuǎn)換成FLOAT(浮點(diǎn)數(shù))格式
BINARY	將value轉(zhuǎn)換成二進(jìn)制格式

示例

1.將值轉(zhuǎn)換為DATE數(shù)據(jù)類(lèi)型

-- 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);

2.將值轉(zhuǎn)換為DATETIME數(shù)據(jù)類(lèi)型

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

3.將值轉(zhuǎn)換為T(mén)IME數(shù)據(jù)類(lèi)型

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

4.將值轉(zhuǎn)換為CHAR數(shù)據(jù)類(lèi)型

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

5.將值轉(zhuǎn)換為SIGNED數(shù)據(jù)類(lèi)型

-- 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);

6.將值轉(zhuǎn)換為UNSIGNED數(shù)據(jù)類(lèi)型

-- 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);

7.將值轉(zhuǎn)換為DECIMAL數(shù)據(jù)類(lèi)型

-- 9
SELECT CAST('9.0' AS DECIMAL);
 
-- DECIMAL(數(shù)值精度，小數(shù)點(diǎn)保留長(zhǎng)度)
-- DECIMAL(10,2)可以存儲(chǔ)最多具有8位整數(shù)和2位小數(shù)的數(shù)字
-- 精度與小數(shù)位數(shù)分別為10與2
-- 精度是總的數(shù)字位數(shù)，包括小數(shù)點(diǎn)左邊和右邊位數(shù)的總和
-- 小數(shù)位數(shù)是小數(shù)點(diǎn)右邊的位數(shù)
-- 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));

練習(xí)案例

導(dǎo)入數(shù)據(jù)

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:料號(hào) FileName:文件名稱(chēng)(存儲(chǔ)相應(yīng)料號(hào)位置)

注明: ID，Material，F(xiàn)ileName都是VARCHAR類(lèi)型

FileName取值舉例說(shuō)明:'FD03-FCZ-220430012624541.txt'，紅色標(biāo)注數(shù)字代表年月日，其余都取值相同(不用考慮具體含義)

問(wèn)題:查詢(xún)小于220401(整數(shù)類(lèi)型年月日數(shù)值)的Material

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

解題思路
利用SUBSTRING函數(shù)截取FileName值中代表年月日的數(shù)字
由于FileName中的值為varchar類(lèi)型，220401為整數(shù)類(lèi)型，需要使用CAST函數(shù)進(jìn)行類(lèi)型轉(zhuǎn)換

結(jié)果展示: