What does “binary string” means in MySQL

I'm reading the doc of MySQL. Where I found the UNHEX function is described as:

UNHEX

For a string argument str, UNHEX(str) interprets each pair of characters in the argument as a hexadecimal number and converts it to the byte represented by the number. The return value is a binary string.

mysql> SELECT UNHEX('4D7953514C');
-> 'MySQL'
mysql> SELECT X'4D7953514C';
-> 'MySQL'
mysql> SELECT UNHEX(HEX('string'));
-> 'string'
mysql> SELECT HEX(UNHEX('1267'));
-> '1267'


What is a "binary string"? Is it a general term or a term specifically used in MySQL?

I thought that if it is a "binary" data, then it shouldn't be called as a "string"(array of chars). So I'm wondering what is "binary string"?? does it means "array of bytes"?

I assume that eventually every datatype will be converted into binary data. And so does the "String Types" in MySQL. What makes the difference from a "string" and a "binary string" is that the "character string" has some information about what encoding schema and collation to be used when MySQL covert it from binary to a character string.

So what I don't quite understand is that why UNHEX('4D7953514C') returns a "character string" 'MySQL' but not a "binary string" represented by a sequence of 0s and 1s(0100 1101 0111 1001 0101 0011 0101 0001 0100 1100).

'MySQL'

2 Answers
2

The term "binary string" in general means a sequence of 0s and 1s. In the case of the MySQL function unhex, it takes a string of hexadecimal values and converts it into a binary number, which will be represented as a string. Nothing fancy here. The result will be a string, which is the result of converting from hexadecimal number to binary number. A hexadecimal number is of four digits when converting into binary. The resulting sequence of 0s and 1s will be the binary representation of the resulting string.

As mysql manual on binary and varbinary data type says, binary strings

contain byte strings rather than character strings. This means they have the binary character set and collation, and comparison and sorting are based on the numeric values of the bytes in the values.

Essentialy, they are arrays of bytes.

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