  Formal query building is an important part of complex question answering over
knowledge bases. It aims to build correct executable queries for questions.
Recent methods try to rank candidate queries generated by a state-transition
strategy. However, this candidate generation strategy ignores the structure of
queries, resulting in a considerable number of noisy queries. In this paper, we
propose a new formal query building approach that consists of two stages. In
the first stage, we predict the query structure of the question and leverage
the structure to constrain the generation of the candidate queries. We propose
a novel graph generation framework to handle the structure prediction task and
design an encoder-decoder model to predict the argument of the predetermined
operation in each generative step. In the second stage, we follow the previous
methods to rank the candidate queries. The experimental results show that our
formal query building approach outperforms existing methods on complex
questions while staying competitive on simple questions.
