  Knowledge base question answering (KBQA) aims to answer a question over a
knowledge base (KB). Recently, a large number of studies focus on semantically
or syntactically complicated questions. In this paper, we elaborately summarize
the typical challenges and solutions for complex KBQA. We begin with
introducing the background about the KBQA task. Next, we present the two
mainstream categories of methods for complex KBQA, namely semantic
parsing-based (SP-based) methods and information retrieval-based (IR-based)
methods. We then review the advanced methods comprehensively from the
perspective of the two categories. Specifically, we explicate their solutions
to the typical challenges. Finally, we conclude and discuss some promising
directions for future research.
