In your example, filename is set to the empty string (mind the spaces after the = sign!). You need

filename=$(inotifywait --format "%f" -e create /var/www/media2net/torrent)

Similarly,

date_field=$(date +"%F:%T")

and be careful, you have a typo in your mysql command (date_field and note date_feild):

mysql --host=localhost --user=root --password=admin Media2net <<EOF
insert into video (title, description, url_video, upload_date)
values('testing','default_description','$filename', '$date_field');
EOF

Now I hope that you're controlling the filenames. Imagine a filename that contains a single quote e.g., hello'howdy. You'll have a problem in your query. Worse, an evil user who puts a file named whatever','something'); evil_mysql_command; whatever, you'll have the evil command performed! One possibility is to sanitize the filename using printf thus:

printf -v filename '%q' "$(inotifywait --format "%f" -e create /var/www/media2net/torrent)"

This will at least escape the single quotes that could appear in a filename. See Gordon Davisson's comment: the printf trick will not prevent from all the possible attacks, so I really hope you absolutely control the name of the files!

All these suggestions yield the following script:

#!/bin/bash

while true; do
    printf -v filename '%q' "$(inotifywait --format "%f" -e create /var/www/media2net/torrent)"
    date_field=$(date +"%F:%T")
    mysql --host=localhost --user=root --password=admin Media2net <<EOF
    insert into video (title, description, url_video, upload_date) 
    values('testing','default_description','$filename', '$date_field');
    EOF
    echo "$filename"
done

Edit.

To answer your question in the comment:

why did the script properly echo $filename to my terminal but not send it properly to MySQL, does that have to do with string starting with a space? or something else completely?

That's because when you do something like:

whatever= command

then the variable whatever is set to the empty string, and the command command is executed (with the whatever variable set as environment variable). E.g.,

$ IFS='c' read a b c <<< "AcBcC"
$ echo "$a $b $c"
A B C
$ echo $IFS

$

In your script, the variable filename was in fact never globally set. You can check it by doing this:

$ filename= "false"
$ echo "$filename"

$

What happens is that the environment variable filename is set to empty string then the command false (which happens to exist) is launched using that environment variable and we're done.

When you do this:

filename= inotifywait --format "%f" -e create /var/www/media2net/torrent

the variable filename is set to the empty string, and then the command inotifywait ... is executed with filename as an environment variable (but inotifywait doesn't really care about it). And that's what you saw on your terminal! it was the output of this command. Then you probably saw an empty line, that was the output of

echo $filename

which was equivalent to

echo

since the variable filename expanded to an empty string.

Hope this helps.