完全相同的 MySQL 表结构/数据，相同查询的不同结果

Question

Here's my situation.

I'm migrating from one server to another. As part of this, I'm moving across database data.

The migration method involved running the same CREATE TABLE query on the new server, then using a series of INSERT commands to insert the data row by row. It's possible this resulted in different data, however, the CHECKSUM command was used to validate the contents. CHECKSUM was done on the whole table after the transfer, on a new table with that row isolated, and after truncation of the string by applying the LEFT operator. Every time, the result was identical between the old and new server, indicating the raw data should be exactly identical at the byte level.

CHECKSUM TABLE `test`

I've checked the structure and it's exactly the same as well.

SHOW CREATE TABLE `test`

Here is the structure:

CREATE TABLE test ( item varchar(32) COLLATE utf8_unicode_ci NOT NULL, amount mediumint(5) NOT NULL ) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

The field is of type:

`item` varchar(32) COLLATE utf8_unicode_ci NOT NULL

Here is my connection code in PHP:

$sql = new mysqli($db_host, $db_user, $db_pass, $db_name);
if ($sql->connect_error) {
  die('Connect Error ('.$sql->connect_errno.') '.$sql->connect_error);
}

When I go to retrieve the data in PHP with a simple query:

SELECT * FROM `test`

The data displays like this:

§lO

On the old server/host, I get this sequence of raw bytes:

Decimal: -194-167-108-79-
HEX: -C2-A7-6C-4F-

And on the new server, I get a couple of extra bytes at the beginning:

Decimal: -195-130-194-167-108-79-
HEX: -C3-82-C2-A7-6C-4F-

Why might the exact same raw data, table structure, and query, return a different result between the two servers? What should I do to ensure that results are as consistent as possible in the future?

Answer 1

§lO is "Mojibake" for §lO . I presume the latter (3-character) is "correct"?

The raw data looks like this (in both cases when I display it)

is bogus because the technique used for displaying it probably messed with the encoding.

Since the 3 characters became 4 and then became 6, you probably have "double-encoding".

This discusses how "double encoding" can occur: Trouble with UTF-8 characters; what I see is not what I stored

If you provide some more info ( CREATE TABLE , hex, method of migrating the data, etc), we may be able to further unravel the mess you have.

More

When using mysqli, do $sql->set_charset('utf8');

(The HEX confirms my analysis.)

The migration method involved running the same CREATE TABLE query on the new server

Was it preceded by some character set settings, as in mysqldump ?

then using a series of INSERT commands to insert the data row by row.

Can you get the HEX of some accented character in the file?

... CHECKSUM...

OK, being the same rules out one thing.

CHECKSUM was done on... a new table with that row isolated

How did you do that? SELECTing the row could have modified the text, thereby invalidating the test.

indicating the raw data should be exactly identical at the byte level.

For checking the data in the table, SELECT HEX(col)... is the only way to bypass all possible character set conversions that could happen. Please provide the HEX for some column with a non-ascii character (such as the example given). And do the CHECKSUM against the HEX output.

And provide SHOW VARIABLES LIKE 'char%';