This is not the answer you are looking for, but... read it and think about it, at least if security is of interest.
You should never access an SQL Server from the internet due to the security risks, and I do not even recommend a solution where you place a webserver in DMZ and use an SQL server inside the firewall.
I would recommend you to place the webserver in one DMZ, the SQL server in a second DMZ and make sure the traffic to SQL server is only allowed from the first DMZ or from inside the firewall, never from the Internet.
If you really want to ignore recommendations, make sure you use a VPN tunnel from the Webserver and the SQL server, but remember it will not save you from SQL injections and you have to prevent it by yourself.
I have seen some examples, where it where possible to gain control of a webserver and use it to attack the SQL server with xp_cmdshell activated inside the firewall. Basicly it where possible to backup the complete database, send it somewhere and drop the database, and even use DEL . in the filesystem (with xp_cmdshell)
You can do a lot of nasty things with an SQL Server if you gain control over it.