MySQL 中的 CAST 函数详解及常见用法

CAST(expression AS type)

CONVERT(expression, type)

SELECT CAST('123' AS SIGNED);      -- 转换为有符号整数，结果为 123
SELECT CONVERT('45.67', DECIMAL(5,2)); -- 转换为带2位小数的十进制数，结果为 45.67

SELECT CAST(123 AS CHAR);          -- 转换为字符串，结果为 '123'
SELECT CAST(123.45 AS CHAR(10));   -- 转换为长度为10的字符串，结果为 '123.45'

SELECT CAST('2023-08-20' AS DATE); -- 转换为日期类型
SELECT CAST(NOW() AS CHAR);        -- 将当前日期时间转为字符串

SELECT CAST(1 AS UNSIGNED);        -- 结果为 1
SELECT CAST(0 AS SIGNED);          -- 结果为 0

SELECT CAST('MySQL' AS BINARY);    -- 转换为二进制字符串
SELECT CAST(123 AS BINARY);        -- 将数字转为二进制

SELECT CAST(NULL AS SIGNED);       -- 结果为 NULL

SELECT CAST('123.456' AS DECIMAL(5,2)); -- 结果为 123.46 (四舍五入)
SELECT CAST('123.456' AS DECIMAL(5,1)); -- 结果为 123.5 (四舍五入)

SELECT CAST('abc' AS SIGNED);      -- 结果为 0 (无法转换时返回0)
SELECT CAST('2023-02-30' AS DATE); -- 结果为 NULL (无效日期)

-- 避免字符串和数字的隐式转换
SELECT * FROM products WHERE CAST(price AS CHAR) LIKE '12%';

SELECT 
    product_name,
    CONCAT('$', CAST(price AS DECIMAL(10,2))) AS formatted_price
FROM products;

-- 将字符串列转为数字进行计算
UPDATE orders 
SET total = CAST(subtotal AS DECIMAL(10,2)) + CAST(tax AS DECIMAL(10,2));

SET @str_value = '123';
SELECT * FROM table WHERE id = CAST(@str_value AS SIGNED);