tE Unit 2 The Relational Model A Relational model H Relational algebra(关系代数) A Relational calculus(关系演算) Haichang Gao, Software School, Xidian University 3DataBase System Haichang Gao , Software School , Xidian University 3 Unit 2 The Relational Model Relational Model Relational Algebra(关系代数) Relational Calculus(关系演算)