UniRel: Relation-Centric Knowledge Graph Question Answering with RL-Tuned LLM Reasoning

Knowledge Graph Question Answering (KGQA) has largely focused on entity-centric queries that return a single answer entity. However, many real-world questions are inherently relational, aiming to understand how entities are associated rather than which entity satisfies a query. In this work, we introduce relation-centric KGQA, a complementary setting in which the answer is a subgraph that represents the semantic relations among entities. The main challenge lies in the abundance of candidate subgraphs, where trivial or overly common connections often obscure the identification of unique and informative answers. To tackle this, we propose UniRel, a unified modular framework that combines a subgraph retriever with an LLM fine-tuned using reinforcement learning. The framework uses a reward function to prefer compact and specific subgraphs with informative relations and low-degree intermediate entities. Experiments show that UniRel not only produces more coherent and informative relational answers than existing baselines, but also generalizes effectively to unseen entities and relations. Additionally, UniRel can be applied to conventional entity-centric KGQA, achieving competitive or improved performance in several settings. A human evaluation further shows that participants prefer UniRel's answers over the baselines.