Short Answer
Yes.
Long Answer
Ruby is a scripting language that executes on the server-side. SQL is a language that allows Ruby to communicate with the database. In order to build working websites, however, you will also need to understand at least HTML (Hypertext Markup Language). HTML, unlike Ruby, is not a scripting language; it is a markup language. This means that it describes the data that should be displayed on the client-side.
In addition, Web Designers (though not always Web Developers) are expected to have background in CSS (Cascading Style Sheets), a stylesheet language which defines how the data in an HTML document should be shown. Furthemore, you will usually need at least minor understanding with JavaScript, a (primarily) client-side scripting language usually used to modify the DOM (Document Object Model) defined in an HTML page and the rules specified in a CSS stylesheet.
In total, that is 5 languages that you would have to be familiar with:
- Ruby
- SQL
- HTML
- CSS
- JavaScript
While you don't need to know all these languages to construct basic working websites, HTML, CSS, and JavaScript are the basis of most modern day sites on the client-side. The languages used on the server-side are much more diverse in choice. You do not have to chose Ruby and SQL, but they are certainly a possible pair to work with.