Depends what you mean by developer, but there a heaps of ways to accidentally write code that doesn't use an index, and then you need to understand db internals and how indexing works, and so you are on your way to being a SQL dev. A non-developer I wouldn't expect to understand db internals. You can get quite far though not understanding db internals as a non-dev for simple querying until you write some complex mission-critical query that must continue running, but needs maintenance/starts over-burdening the db/etc.