for what purpose # (hash) sign is used in Joomla?

Question

In joomla MVC i came around this SQL query. I was unable to understand the purpose of #sign ??

INSERT INTO `#__helloworld` (`greeting`) VALUES
    ('Hello World!'),
    ('Good bye World!');

Answer 1

It is replaced by the prefix for that installation when it is run. This way you can have multiple Joomla installations running in the same database, as long as they all have different prefixes. You can find the prefix for your installation in the Global Settings as well as in the configuration.php file.

The entire #__ part is necessary for the replace to work properly. So just a # will not work as intended.

*Note that this is a Joomla specific construct, so don't expect this to work with just PHP/MySQL. It will work for all Joomla versions and is highly recommended.