Category: "MySQL"

How to check what MySQL version I am using?

In order to determine which MySQL version you're running you can type the following command: mysql -V

Sample results:

# mysql -V
mysql Ver 14.7 Distrib 4.1.11, for pc-linux-gnu (i386)

# mysql -V
mysql Ver 14.12 Distrib 5.0.32, for pc-linux-gnu (i486) using readline 5.2

Now, if you want to query for the version number while beign logged in (PHP script for example), just go with: SELECT VERSION(); .

I believe this is only working since MySQL 4 though.

Charsets in MySQL 4.1

Once you start messing around with charsets in MySQL you eventually get to a situation where the default charset for your database is UTF-8 but you want to import old backups in their own charset, for instance ISO-LATIN-1.

But when you import it, MySQL thinks it's UTF-8 and all your specials chars get messed up.

Simple solution, add this at the end of your CREATE TABLE statements:

DEFAULT CHARACTER SET latin1 COLLATE latin1_general_ci

More in the MySQL Manual.

Note: this does not convert your data, this does tell MySQL that those particular tables are in another charset. I think, if you want to convert, you need to change the charset of your connection at the time you do the import (using the --default-character-set switch for example). Gotta re-check that though... but I think it can be overridden like this:

SET NAMES latin1

MySQL Data Integrity Enforcement Caveats

Okay, I desperately lack time to write full articles posts lately, so I'm going to make this quick! :P

Background: Suppose you have a table named Songs and a table named Genres. Genres only contains Genre IDs and Genre Names. Songs contains all sorts of data, but at some point there is one field (let's call it Song_Genre_ID) that points to the Genre ID. Get the picture? I'm sorry, I don't have the time for an actual picture of this. That "pointer" is called a Foreign Key. The Genre_ID, being the Primary key.

Data Integrity Enforcement means that MySQL is going to prevent you from putting any crappy value into the Foreign Key (FK) that would not exist in the refered Primary Key (PK). It will also prevent you from deleting a Genre if there are songs pointing to it. The bottom line is: if you're serious about your DB, you can't do without!

Now how would you enforce the integrity? Basically with something like this (but you may want to check the MySQL manual for details, right? :P):

REFERENCES Genres (Genre_ID)

Now, here's my point: from my experience I have found that a hell lot of things can go wrong when you try to add that constraint, and it can be a nightmare to find out what's going wrong! As a matter of fact, you'll find out MySQL's error messages aren't very helpful >:XX...

So here's a checklist to follow in order to find out what's been going wrong:

  • If the error message is even less helpful than you could possibly imagine, check your version of MySQL. Versions 4.1+ definitely have better messages than previous versions.
  • Check that both of your tables are of type InnoDB. Don't even try it otherwise.
  • Check that the PK you are refering to is actually defined as the Primary Key for its table.
  • On versions < 4.1, check that the the FK is being properly INDEXed before creating the foreign key.
  • Check the data types of the FK and the PK. They should be exactly the same. And I really mean *exactly*! Check the "UNSIGNED" attribute also. If it's signed on one side and unsigned on the other, it won't work! (I recommend using INT UNSIGNED for most of your FKs and PKs).
  • Check that any existing values in the PK column have matches in the FK column at the time you try to create the constraint.
  • Check that the DEFAULT value for your FK also has a match in the FK column at the time you try to create the constraint.

Okay I hope I didn't forget anything. Of course, if you find another reason for the constraint creation to fail, I'd love to hear about it! ;)

Good luck! :>>

MySQL features by version breakdown

I could not find anything like this on the net, so I thought I'd make my own chart and share it... ;)

MySQL version 3.23.58
4.0.22 4.1.7 5.0
Release date 11-sept-03 27-oct-04 23-oct-04 not stable yet
Row level locking yes yes yes yes
Transactions yes yes yes yes
Foreign keys yes yes yes yes
UNION no yes yes yes
Subqueries (derived tables) no no yes yes
Multiple table DELETEs no yes yes yes
Unicode UTF-8 support yes yes yes yes
Column level charset support no no yes yes
Timezone handling no no yes yes
Full text indexing (MyISAM only) no UTF-8 no UTF-8 yes yes
Stored procedures no no no yes
Rudimentary triggers no no no yes
Updatatable views no no no yes
Server side cursors no no no yes
Master/slave one way replication yes yes yes yes
Replication over SSL no no yes yes
Clustering (not available on Windows) no no yes yes

Note: I have not included MySQL 3.23 without InnoDB because that version can do vitually nothing interesting. It's a pure joke. Unfortunately, it is the kind of joke hosting providers like to try on you when they claim they offer you a "DBMS"... yeah right... call it an electronic rollodex, no more :>>

I'm a little bit too lazy for styling this right now, sorry.
If you notice an error, please be so nice and report it. Thanks ;)

Exporting MySQL databases

PhpMyAdmin does a pretty decent job at exporting MySql database with their structure and/or their data to a plain SQL file. One thing bugged me though: it encloses every table/column/whatever name in backquotes like in `addr_city` varchar(50). Who needs this? Are there really people who put spaces, commas, quotes and special chars in their database object names?? |-|

Today, I finally fixed this. You need to open and change this line:

$cfg['Export']['sql_backquotes'] = FALSE;

Life is good! :D