Both in my current job and in my previous job, there are a number of areas of the application to test that require mining data from a relational database, specifically Microsoft SQL Server of various versions.
For the most part, I've gotten by with running simple SELECT queries with various WHERE filters. With my current position, though, I'm finding that adding more complex JOINs are necessary to really get what I need out of the database. Additionally, the database I'm currently working with has a lot more in the way of foreign key constraints, views, and other such things built into it making the relationships between the various normalized tables really slick (but at the same time, a bit of a pain) to navigate.
When looking for a software tester for your database application, what level of proficiency are you looking for?