Prior studies in the Computer Science education literature have illustrated that novices make many mistakes in composing SQL queries. Query formulation proves to be difficult for students. Only recently, some headway was made towards understanding why SQL leads to so many mistakes, by uncovering student misconceptions. In this article, we shed new light on SQL misconceptions by analyzing the hypotheses of SQL experts on the causes of student errors. By examining the experts' perceptions, we draw on their understanding of students' misconceptions and on their experiences with studying and teaching SQL. For our analysis, we chose the Policy Delphi, a questionnaire instrument specifically designed for gathering opinions and evidence. Through a two-round process, our nineteen participants proposed and voted on underlying causes for SQL errors which resulted in a set of hypotheses per error. Our main contribution to this article is this new set of possible misconceptions. With them, we can design more complete educational approaches to address misconceptions underlying SQL errors made by students, leading to more effective SQL education.