Database Management Systems (DBMSs) are fundamental to numerous enterprise applications. Due to the significance of DBMSs, various testing techniques have been proposed to detect DBMS bugs. However, to trigger deep bugs, most of the existing techniques focus on generating lengthy and complex queries which burdens developers with the difficult of debugging. Therefore, SQL query simplification, which aims to reduce lengthy SQL queries without compromising their ability to detect bugs, is highly demanded. To bridge this gap, we introduce SQLess, an innovative approach that employs a dialect-agnostic method for efficient and semantically correct SQL query simplification tailored for various DBMSs. Unlike previous works that have to depend on DBMS-specific grammar, SQLess utilizes an adaptive parser, which leverages error recovery and grammar extension to support DBMS dialects. Moreover, SQLess performs a semantics-sensitive SQL query trimming, which leverages alias and dependency analysis to simplify SQL queries with preserving bug-triggering capability. We evaluate SQLess using two datasets from the state-of-the-art database bug detection studies, encompassing six widely-used DBMSs and over 32,000 complex SQL queries. The results demonstrate SQLess superior performance it achieves an average simplification rate of 72.45% in the PINOLO Dataset, which significantly outperforms the state-of-the-art approaches by 84.91%.