MySQL does not start when upgrading OSX to Yosemite or El Capitan

Open a terminal:

1. Check MySQL system pref panel, if it says something along the line "Warning, /usr/local/mysql/data is not owned by 'mysql' or '_mysql'

2. If yes, go to the mysql folder cd /usr/local/mysql

3. do a sudo chown -R _mysql data/

4. This will change ownership of the /usr/local/mysql/data and all of its content to own by user '_mysql'

5. Check MySQL system pref panel, it should be saying it's running now, auto-magically. If not start again.

6. Another way to confirm is to do a

   netstat -na | grep 3306

It should say:

tcp46      0      0  *.3306                 *.*                    LISTEN

To see the process owner and process id of the mysqld:

ps aux | grep mysql

Long story short you need to create a launch file. So, from Terminal:

sudo vi /Library/LaunchDaemons/com.mysql.mysql.plist

(If you are not familiar with vi, then press i to start inserting text)

This should be the content of your file:

<?xml version="1.0" encoding="UTF-8"?> 
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
  <dict>
    <key>KeepAlive</key>
    <true />
    <key>Label</key>
    <string>com.mysql.mysqld</string>
    <key>ProgramArguments</key>
    <array>
      <string>/usr/local/mysql/bin/mysqld_safe</string>
      <string>--user=mysql</string>
    </array>
  </dict>
</plist>

press esc then : wq!enter

Then you need to give the file the right permissions and set it to load on startup.

sudo chown root:wheel /Library/LaunchDaemons/com.mysql.mysql.plist 
sudo chmod 644 /Library/LaunchDaemons/com.mysql.mysql.plist 
sudo launchctl load -w /Library/LaunchDaemons/com.mysql.mysql.plist

And that is it.

In my case I fixed it doing a little permission change:

sudo chown -R _mysql:_mysql /usr/local/var/mysql
sudo mysql.server start

I hope it helps somebody else...

Note: As per Mert Mertin comment:

For el capitan, it is sudo chown -R _mysql:_mysql /usr/local/var/mysql

Solved by installing the latest mySQL release, following the instructions here http://coolestguidesontheplanet.com/get-apache-mysql-php-phpmyadmin-working-osx-10-10-yosemite/

EDIT
As Yosemite gets more popular, more people are stumbling on this question. The answer above has to do with upgrading MySQL, so that it runs. The answer linked by @doc in the comments has to do with getting MySQL to start automatically. These are 2 separate issues.