It depends on the kind of job you're looking for. For an early stage startup the "width" is relatively more important (but obviously the deeper the better too). Meanwhile for a bigger company with a lot more fellow coders the "depth" is relatively more important (but the more width the better too obviously).

best,

Lee

I think you should go deep, but not with a certain technology. Think about it a bit differently and go deep in an area.

For example, as you put it, instead of being "the best SQL guy" work your way on being "The best Database guy". Instead of being "The best Javascript guy", go towards being "The best Frontend" guy.

Remember, technologies change fast, there will always be new platforms, new DB types (just think about Big Data and their adoption rate in the latest years). You will not be safe if you master a technology, but if you master a domain you will have deep knowledge on the subject and you will be quick(er) to adapt to any new technology coming your way.

Best of luck!

As Lee said, it will really depend on what you are planning on doing with your career. If you really want to be a successful developer, you need to be familiar with more than just one language. That will not only help you get better jobs, but many of these programming languages need to interface with one another. For example, if you want to work for an e-commerce development company, chances are you will need PHP, MySQL, JavaScript, Jquery and HTML/CSS at a minimum. Learning just one of those will not really give you much traction. You'll probably find that you enjoy working with one of those more than the others, but you really need at least a basic understanding of all of them.