Safety of Expressions (Cont.) Consider again that query to find all students who have taken all courses offered in the Biology department 。 {t3rE student (t [ID]=r[ID]) (tu∈course(u[dept_name]=“Biology'→ 3s e takes (t [ID]=s [ID s [course_id]=u [course_id]))} Without the existential quantification on student,the above query would be unsafe if the Biology department has not offered any courses. Database System Concepts-7th Edition 27.11 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 27.11 ©Silberschatz, Korth and Sudarshan th Edition Safety of Expressions (Cont.) ▪ Consider again that query to find all students who have taken all courses offered in the Biology department • {t |  r  student (t [ID] = r [ID])  ( u  course (u [dept_name]=“Biology”   s  takes (t [ID] = s [ID ]  s [course_id] = u [course_id]))} ▪ Without the existential quantification on student, the above query would be unsafe if the Biology department has not offered any courses