Can I use SQL to develop an SMS sending site?
Yes with combination of another programming language like PHP. No just with SQL. SQL is a query language to interact with supported DBM's and has nothing to do with send an sms, you can use it to store/retrieve messages, number, time etc...
Hello, Yes, you can use it as a query language, if you have a high quality gateway. I used Ozeki NG SMS Gateway when I developed a bulk SMS site in SQL, and it still works great.