Quality is delighting customers
It differs from Organization to organization , at times you may not need to have even basics and sometimes you must have sound knowledge of SQL Queries . even I have attended an Interview where they started asking questions from PL-SQL too. So it depends on several factors like what kind of Project you will be assigned to work on and what kind of working environment is there in the organization but in general a tester must have basic knowledge of SQL.
I used to ask few SQL Queries like following before I hire any QA Engineer:
Write Code to display second maximum?
Difference between Group By and Order By?