By the standard transaction model(composed of read and write operations),the scheduler can not exploit the semantics of application programs,thus not enhancing concurrency effectively.In this paper,a new transaction model based on SQL language are presented, which can exploit the semantics of high-level operations.Depending on the the transaction model and Two-Phase Locking method,the authors design a concurrency control mechanism-Condition-Locking, which can,(1)prevent phantoms from database,(2)exploit semantic information of application programs and the knowledge of the integrity constraints of the database system to enhance. cononrrency,(3)reduce the chances of producing deadlock.Hence,it is a practical concurrency control mechanism.