Answer to Question #238088 in Databases | SQL | Oracle | MS Access for Owen

Question

2.3.2 Why do we care that the expressions are equivalent?

Answer 1

Given the relations R (A, B) and S (B, C) where values are all integers. Examine the undermentioned three relational algebra expressions

a. A, C(R⋈B=1S)

b. A(B = 1R)XC(B = 1S)

c. A, C (ARXB = 1S)

Two of the three expressions are equivalent (i.e., they produce the same answer on all databases), while one of them can produce a different answer

1. Which query can produce a contrasting answer? Give the simplest database instance you can think of where a different answer is produced
2. Why do we care that the expressions are equivalent?
3. How can we determine that the expressions are equivalent?