Mysql select from mediumint(8) to unique Binary(2)

Question

I am trying to create a query to populate unique data from one table into part of another table. The two rows from the source table I'm selecting are of types mediumint(8) and varchar(20). In the destination table I have a Binary(2) and a varchar(20) that I'm trying to populate from the source table. I'm also only selecting the top 100 unique values.

The problem I have is with the mediumint(8) to Binary(2) conversion. I can see all my mediumint(8) values are unique, and they are all less than 200 numerically, but for some reason I can't get them to fit in a 2 byte unique binary column. My query keeps rejecting the insert because it says the values are larger than 2 bytes.

sample query:

select distinct id, part from partsTable
group by id
limit 100;

This comes back with a table that looks kind of like this

 id| part
_________
1  |  A
2  |  B
3  |  C
21 |  AB
22 |  AC
...

I have tried doing things like ( id & 0xFFFF) to force my id column as 2 bytes, and CAST(id as UNSIGNED) or CAST(id as BINARY(2)), but these always truncate the data so that i get duplicate values. Not really sure what I'm missing or how my unique id values that I can see are all unique get translated to non unique values when I try to cast them into binary.

What's even more confusing is as a test I just inserted two records into my destination table:

INSERT into table1 (id, part) VALUES (0xFFFF, 'A');
INSERT into table1 (id, part) VALUES (0xFFFE, 'B');

And then I just wrote a query to see what that data looked like:

SELECT bin(id) as id, hex(id) as hex, part FROM table1;

Which returns a result like:

id  | hex   | part
__________________
0   | 0xFFFF| A
0   | 0xFFFE| B

So I have no idea what's going on

Answer 1

2 bytes would be a numeric range of 65535. The problem here is the binary data type in MySQL still contains a string, not a number, but with collation of binary instead of something like utf8 as the char data type would. The difference is that the binary data type restricts the value based on the number of bytes rather than the number of characters in the string that it is storing. In other words, a 3 digit value such as 150 as a string would be 3 characters, totaling 3 bytes. So if this value was converted to BINARY(2), it would lose anything past its 2nd byte, making it 15.

Because it stores a string, the binary data type is not restricted to numeric characters (0,1) as you would expect either. So you could make the values fit in a BINARY(2) by using HEX() . HEX(150) would be '96', and would therefore fit in a BINARY(2) . You would then have to use UNHEX() to retreive any of these values. If this table already has other values stored in it I doubt this would work for you though.