摘要
SQL语句在表达具有全部值语义时需要使用谓词NOT EXISTS构建复杂的多层嵌套结构,学习者学习起来难度大,易出错。元组关系演算可以充分表达全部值语义,通过应用实例探讨了元组关系演算表达式转换为SQL语句的转换规则,对学习SQL语句者非常有帮助。
SQL phrases require the use of predicate NOT EXISTS to construct complicated nested query in order to express the full value meaning.Practitioners find it very difficult to write and easy to make errors.Tuple relational calculus can fully express such meaning.This paper uses examples to discuss and demonstrate transformation rules when transforming a tuple relational calculus expression to a SQL query,which is very helpful to SQL query learners.
出处
《计算机系统应用》
2010年第1期169-170,50,共3页
Computer Systems & Applications
关键词
SQL
元组关系演算
全称量词
存在量词
转换规则
SQL
tuple relational calculus
universal quantifier
existential quantifier
transformation rules
