MySQL is running out of memory in DigitalOcean

Hey, after update from MySQL 5.5 to 5.6, my life is getting worse. There are many 'Database connection error' when I try to open my website. And I even can not access to phpMyAdmin. What's happening.
OK, I turn on the SSH, connect to my server and BOOM, mysql is having 'Out of Memory' error, and everything crash.
That time, I need to restart mySQL:
$> sudo service mysql start

Next, I need to find out how much memory is being used, because Digital Ocean does not support this feature.
$> sudo apt-get install htop

$> htop

Now there is my memory usage:

Wow, mysqld service is consuming 48.9% memory, what!!!

There are 2 things I can do: Upgrade RAM (means upgrade Dropplet) or continue to check for another solution, my VPS is 1GB RAM.

Next, I check the log of mysql at /var/log/mysql: 

There are alot of errors:

There are some problems that we can find out:

+ InnoDB can not allocate memory 

+ InnoDB memory heap is disabled

Let's check the swap by this command:

$> free -m 

Swap = 0 means there is no swap setup. 

---> I need to setup the swap File by following steps:

https://www.digitalocean.com/community/tutorials/how-to-add-swap-on-ubuntu-14-04

There might be some errors when you try command in above pages, like:

sysctl: cannot stat /proc/sys/50: No such file or directory

What you need to do is to:

+  move to the /proc/sys/vm folder: $> cd /proc/sys/vm

Then type again

Now my system start using swap file and no out of memory error. However I always recommend you to upgrade RAM if possible because using swap file will degrade your performance.