Validating LLM-Generated SQL Queries through Metamorphic Prompting

Large Language Models (LLMs) can translate natural language (NL) into SQL, enabling non-experts to query databases via conversational interfaces. However, the generated SQL often contains intent-violating hallucinations —queries that are syntactically valid and executable, yet semantically misaligned with the user’s question. These failures are especially risky in real-world settings where users cannot verify the correctness. In this paper, we propose MRSQLGen, a framework for detecting intent-violating hallucinations, built on the metamorphic prompting paradigm. MRSQLGen rewrites the input prompt using task-specific transformation rules derived from a hallucination taxonomy, and validates the generated SQL by checking behavioral consistency across multiple executions. Each transformation is associated with a metamorphic relationship (MR) that defines the expected relation between results; discrepancies are aggregated through a majority-vote strategy to robustly flag hallucinations without ground-truth SQL. We evaluate MRSQLGen on two benchmarks (Spider and Bird) using five representative LLMs, including GPT-4o. Experimental results demonstrate that MRSQLGen consistently outperforms state-of-the-art hallucination detection techniques, achieving higher precision and recall in detecting hallucinated SQL queries.