如何在 MySQL 中将字符串修剪为特定字节长度？

Question

我有一个包含 253 个字符的字符串，但由于某些字符长于一个字节（在这种情况下，它包含几个零宽度空格），它占用了 261 个字节。

我想将此字符串插入到utf8mb4 表中的VARCHAR(255) 列中，但它会引发此错误：

INSERT INTO mytable(mycolumn) VALUES("This string contains 253 ​characters which should ​fit inside of a varchar(255) column, but because ​of the zero width spaces, it takes up more than 255 bytes and therefore throws an error when you try to insert it into the table and that makes me ​sad.")

/*Error 1406: Data too long for column*/

这是两个长度函数输出的示例：

/* Returns 253 */
SELECT CHAR_LENGTH("This string contains 253 ​characters which should ​fit inside of a varchar(255) column, but because ​of the zero width spaces, it takes up more than 255 bytes and therefore throws an error when you try to insert it into the table and that makes me ​sad.");

/* Returns 261 */
SELECT LENGTH("This string contains 253 ​characters which should ​fit inside of a varchar(255) column, but because ​of the zero width spaces, it takes up more than 255 bytes and therefore throws an error when you try to insert it into the table and that makes me ​sad.");

要解决这个问题，我需要剪掉足够的字符以使字符串在字节限制内。

不幸的是，LEFT() 函数似乎不适合这个：

/* Still returns 261 because it counts characters, not bytes */
SELECT LENGTH(LEFT("This string contains 253 ​characters which should ​fit inside of a varchar(255) column, but because ​of the zero width spaces, it takes up more than 255 bytes and therefore throws an error when you try to insert it into the table and that makes me ​sad.", 255));

那么，我该如何修剪字符串呢？

Answer 1

一种解决方法是使用另一种语言提前处理字符串。

例如，这是一个使用 Python 的可行解决方案：

def trim_string_to_x_bytes(string, max_bytes, character_encoding):
    string = string[:max_bytes]
    
    while (len(string.encode(character_encoding)) > max_bytes):
        string = string[:-1]
    
    return string

long_string = "This string contains 253 ​characters which should ​fit inside of a varchar(255) column, but because ​of the zero width spaces, it takes up more than 255 bytes and therefore throws an error when you try to insert it into the table and that makes me ​sad."

long_string = trim_string_to_x_bytes(long_string, 255, 'utf8')

cursor.execute("INSERT INTO mytable(mycolumn) VALUES(%s)", (long_string,))

也就是说，如果存在一个纯 MySQL 解决方案会更好。