Since SQL is a declarative language, you have to use it very differently than procedural or functional languages. I've met many people who don't understand the difference and just write procedural code in their SQL queries (cursors, etc). The output may be correct, but performance on anything bigger than a toy dataset is terrible.
If your team builds massively parallel systems in Erlang, you need to make sure a candidate understands at least the basics of its process model and message passing. If your team build high-performance web apps, you need to make sure a candidate understands at least the basics of HTTP and the difference between client and server. For SQL, the same is true: they need to understand at least the basics of the relational model and declarative programming.
If your team builds massively parallel systems in Erlang, you need to make sure a candidate understands at least the basics of its process model and message passing. If your team build high-performance web apps, you need to make sure a candidate understands at least the basics of HTTP and the difference between client and server. For SQL, the same is true: they need to understand at least the basics of the relational model and declarative programming.