I have a Golang program that may connect to databases with different character sets or collation.
For example the default at the time of writing of the Golang MYSQL driver is utf8mb4_general_ci https://github.com/go-sql-driver/mysql#collation
However if I connect to a database configured like so:
CREATE DATABASE example character set utf8mb4 collate utf8mb4_unicode_ci;
Can I expect "bad things to happen"? Indexes not to work?
In most case, there are no problem. For example, column collation is used regardless connection collation when WHERE column=? is used.
See also: https://dev.mysql.com/doc/refman/5.6/en/charset-collation-coercibility.html
But I can't speak it's 100% safe. It's safe to use one collation all place.
Related
I started working on a wordpress on my dev machine. mysql version is 5.6, and worpdress is 4.7 so its already using the utf8mb4_unicode_520_ci encoding if it detects its possible.
My problem is that on my hosting (mysql 5.5) utf8mb4_unicode_520_ci is not recognized as a valid encoding. So I'm trying to target utf8mb4_unicode_ci encoding as my hosting knows about this one, and if I understand correctly, this would - in opposition to going to utf8 - allow me to keep the 4 bytes.
I tried several different combinaison of encoding and collation set up for the db, but nothing successful (from here How to convert an entire MySQL database characterset and collation to UTF-8?).
I tried several combination of encoding and collation in the wp-config, but nothing.
Everything that is coming from the database (like post titles and post contents displays badly encoded char for all diatrics, anything else is displayed appropriately )
menu label from the database display incorrectly, where the hardcoded/translated label display correctly
I think I need to convert the actual content of the database, changing charset and collation does not seems to be enough.
I found this but it does not address my problem directly, or if it does I missed it.
Any help would be appreciated
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UPDATE :
here is the precise procedure I went through:
Initial situation:
I installed a wordpress (4.6.1) locally (on my dev machine, mysql 5.6.28).
I worked on the theme and plugin locally
(at this moment I have, locally, a database that is utf8_general_ci and tables that are utf8mb4_unicode_520_ci
Problem:
I want to deploy my wordpress on my hosting (mysql: 5.5 - db collation seems to be utf8mb4_unicode_ci).
I mysqldump the db locally, then try to import it on my hostings' phpmyadmin.
This gives error :
Unknown collation: 'utf8mb4_unicode_520_ci'
solution 1 change the tables charset to utf8mb4_unicode_ci:
On my hosting sql server, utf8mb4_unicode_520_ci is not available and I can't get a more recent version of mysql.
utf8mb4_unicode_ci seems like the closest and is available on my hosting sql server.
from various so question, I adapt a bash script to change charset and collation of my tables
for tbl in wp_sij2017_commentmeta wp_sij2017_comments wp_sij2017_cwa wp_sij2017_links wp_sij2017_options wp_sij2017_postmeta wp_sij2017_posts wp_sij2017_term_relationships wp_sij2017_term_taxonomy wp_sij2017_termmeta wp_sij2017_terms wp_sij2017_usermeta wp_sij2017_users wp_sij2017_woocommerce_api_keys wp_sij2017_woocommerce_attribute_taxonomies wp_sij2017_woocommerce_downloadable_product_permissions wp_sij2017_woocommerce_order_itemmeta wp_sij2017_woocommerce_order_items wp_sij2017_woocommerce_payment_tokenmeta wp_sij2017_woocommerce_payment_tokens wp_sij2017_woocommerce_sessions wp_sij2017_woocommerce_shipping_zone_locations wp_sij2017_woocommerce_shipping_zone_methods wp_sij2017_woocommerce_shipping_zones wp_sij2017_woocommerce_tax_rate_locations wp_sij2017_woocommerce_tax_rates; do
mysql --execute="ALTER TABLE wp_sij_2017_original_copy.${tbl} CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;"
done
I run this script on the local db
I now have all my tables set to collation utf8mb4_unicode_ci
My db collation is still utf8
I mysqldump the db, then import it to my hosting and...
Import is successful.
I search and replace siteurl in the db.
I then visit the online website, I got SOME diatrics that renders a "question mark char"
Any text coming from the db has decoding issue AT SOME POINT
The source/html markup also has those "question mark char"
I have no idea where to look or what to do next
Clarification: CHARACTER SETs utf8 and utf8mb4 specify how characters are encoded into bytes. COLLATIONs *_unicode_*, etc, specify how those character compare.
The encoding for utf8mb4_unicode_ci and utf8mb4_unicode_520_ci are the same because they are encoded in the character set utf8mb4.
"database that is utf8_general_ci and tables that are utf8mb4_unicode_520_ci" -- that probably means that new tables in that database, unless specifically stated, will be CHARACTER SET utf8 COLLATION utf8_general_ci. That is the database setting is just a default for CREATE TABLE. Since your tables are already CHARACTER SET utf8mb4 COLLATION utf8mb4_unicode_520_ci, the database default is not relevant to them.
As long as the CHARACTER SET stays utf8mb4, no Emoji, Chinese, etc will be lost or otherwise mangled.
Do not use mysql40; it did not know about any CHARACTER SETs. Do not use CONVERT or CAST. Etc.
I assume the 520 is coming from the output of mysqldump? Do you have an editor that can handle a file that big? If so, simply edit it to change utf8mb4_unicode_520_ci to utf8mb4_unicode_ci throughout. Then load the dump. Problem solved?
Your fix
You did ALTER ... CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci on your local machine. That is probably an even better way -- since it will put your dev and prod machine in line with each other. That should have worked. Don't worry about what the "database" claims.
I'm find 'utf8mb4_unicode_520_ci' and replace with 'utf8mb4_unicode_ci' in .sql file.
Its simplest why to solve this.
I understand that collations are a set of rules for making comparisons over a character set. MySQL / MariaDB has table and database collations in addition to column collation. I was wondering what was the difference between a collation on these three (database, table and column).
Thanks.
MySQL's character sets and collations can be interpreted as a top-down list of prioritized items. The topmost is least priority and the bottommost is most priority.
Order of precedence with topmost being least precedence:
Server collation
Connection-specific collation
Database collation
Table collation
Column collation
Query collation (using CAST or CONVERT)
The server collation is set by the server, which is set either inside of my.cnf or when the server was built from source code. By default, this will usually be latin1 or utf8, depending on your platform.
The connection-specific collation is set by the client using a query like SET NAMES 'utf8' COLLATE 'utf8_unicode_ci';. Most clients don't set a connection-specific collation, so the server will use its own default as explained above.
The database collation is set during database creation, or manually by updating it later. If you don't specify one, it will use the next higher-level collation, which would either be the connection-specific or the server collation.
The table collation is the same as the database collation, except if left blank, it will use the database as its default, then connection-specific, and then finally the server's collation.
The column collation uses the table's collation as its default, and if there is no collation set, it will then follow up the chain to find a collation to use, stopping at server if all of the others weren't set.
The query collation is specified in the query by using CAST or CONVERT, but otherwise will use the next available collation in the chain. There's no way to set this unless you use a function.
Please also refer to the manual page Character Set Support.
In short. When you set Server collation. to UTF-8. All Databases created without defining collation will inherit it from Server.
column iherits from table
table inherits from database
database inherits from server
However, you can overwrite default server at one of those points. Then everything will inherit from it.
This seems like I'm just missing something trivial, but I'm not able to change the collation of a table from Schema Default. Selecting anything from the dropdown just reverts to Schema Default.
I have utf8 - utf8_general_ci set as the schema collation, which I can change without issue.
However, even though the schema default is set to utf8_general_ci and the tables supposedly use the schema default, when I export the SQL CREATE script and import it in phpMyAdmin, the collation is set to latin1_swedish_ci.
The script itself contains a correct CREATE SCHEMA statement:
CREATE SCHEMA IF NOT EXISTS 'my_table' DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci ;
But the CREATE TABLE statements only include the engine assignment.
I'm using MySQL Workbench 6.0, and the server I'm using is running MySQL 5.5.34 and phpyMyAdmin 3.4.11.1. The server default collation is also utf8_general_ci.
EDIT: As I suspected, something stupid. I created the database via the cPanel beforehand rather than through the SQL script, and the default collation was set to latin1_swedish_ci.
However, that doesn't explain why I couldn't set specific collations on the tables in Workbench.
Collations when not given the way they're supposed to be given will certainly revert to Schema Default.
Where did you set the collation in MySQL Workbench actually? There are 2 places, one in modeling and one for live objects (existing db objects in a server).
If the latter, did you apply your changes?
In my database, the collation was originally utf8_general_ci. However, I noticed that utf8_unicode_ci is necessary because of better sorting accuracy.
So I exported all database using phpmyadmin and checked that the word "COLLATION" does not appear in the exported sql file (except for only 2 times in one table where it is set to binary) so generally this script is collation agnostic and should not imply any specific collation when importing but use database default.
After dropping all tables, the database collation was changed to utf8_unicode_ci and then the import script was run from phpmyadmin. But as a result, all tables and all columns are shown again with utf8_general_ci collation (and sorting is incorrect). Why?? And what to do to change it?
P.S. The export/import script contains commented line at the beginning:
/*!40101 SET #OLD_COLLATION_CONNECTION=##COLLATION_CONNECTION */;
I don't know if it has any impact while importing, but after opening mysql console, the command show variables like 'collation_connection'shows COLLATION_CONNECTION as cp852_general_ci.
However, in phpmyadmin->variables the variable 'collation_connection' is set to utf8_general_ci. But there is no way to change it.
That happens because the database export is setting the character set on every table, and such a clause comes with a default collation that depends on the character set, not on the collation of your connection. utf8_general_ci is the default collation for utf8.
You'll have to convert your tables with something like ALTER TABLE tablename CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci; or edit your database export if this is affordable.
As for the MySQL console: the command-line client is pretty much broken on Windows. It'll never support, display or read Unicode, and you're getting a per-connection collation for that client that matches your Windows so-called OEM character set for your locale. This is a Windows misfeature that's difficult to workaround in portable software. PHPMyAdmin uses a web server and doesn't suffer from this problem. I advise you to use a UNIX-like operating system like GNU/Linux for any serious work in any case, not just for this reason. As an added benefit, MySQL, Apache and your whole application stack perform better on Linux.
The MySQL database used by my Rails application currently has the default collation of latin1_swedish_ci. Since the default charset of Rails applications (including mine) is UTF-8, it seems sensible to me to use the utf8_general_ci collation in the database.
Is my thinking correct?
Assuming it is, what would be the best approach to migrate the collation and all the data in the database to the new encoding?
UTF-8, as well as any other Unicode encoding scheme, can store characters in any language, so it is an excellent choice of codepage for your database.
The collation setting, on the other hand, is a completely separate issue from the encoding scheme. It involves sort orders, upper/lowercase conversions, string equality comparisons, and things like that which are language-specific. The collation setting should match the language that is used in the database.
The UTF-8 general collation is (I am assuming here—I'm not familiar with MySQL in particular) used for situations where the language is unknown and some simple default ordering is needed. It probably corresponds to the Unicode code point ordering, which is almost certainly not what you want if you're storing Swedish.
Convert to UTF-8 as the charset.
Collation settings are only used for sorting and stuff like that. Choose the collation that most of your users would expect.
Providing your existing data in the database is CORRECTLY encoded in latin1, converting the tables to utf8 (using ALTER TABLE, as described in the docs) should just work.
Then all your application needs to do is continue doing whatever it did before. If your application wants to use unicode characters, it should set its connection encoding to utf8 and use utf8, but that's its own problem.
The problem is that a large number of crap web apps have historically sent utf8 data to mysql and told it to treat it as latin1. MySQL will honour this perfectly and save junk into the tables, as instructed.
Converting the tables from latin1 to utf8 will NOT repair this mistake, as you genuinely do have total rubbish in there. Repairing them is nontrivial, particularly if during the lifetime of the app it's been talking different types of rubbish to the database.
Use below mysql query to convert your column :
ALTER TABLE users MODIFY description VARCHAR(255) CHARACTER SET utf8 COLLATE utf8_unicode_ci;
To see full details about your table :
SHOW FULL COLUMNS FROM users;