Advanced Database Management System - Tutorials and Notes: Which is the equivalent SQL query for the given relational algebra expression

Monday, 12 September 2016

Which is the equivalent SQL query for the given relational algebra expression

Which is the equivalent SQL query for the given relational algebra expression?


Question:


7. Consider relations(tables) R(A, B, C) and S(B, D). Which of the given SQL queries are equivalent to the Relational Algebra expression "ΠA, R.B, C, S.B, D(R X S)"?

(a) SELECT * FROM R, S;
(b) SELECT * FROM R, S WHERE R.B = S.B;
(c) SELECT A, R.B, C, S.B, D FROM R, S;
(d) SELECT A, R.B, C, D FROM R, S WHERE R.B = S.B;



Answer:

Both (a) and (C)

SELECT with * or SELECT with all attributes are equal to the Projection operation in relational algebra. Hence, for the given relational algebra projection on R X S, the equivalent SQL queries are both (a) and (c)

The queries in options (b) and (d) are operations involving a join condition. These two queries are equivalent to a SELECTION operation in relational algebra with a JOIN condition or PROJECTION operation with a JOIN condition.
The query "SELECT * FROM R, S WHERE R.B = S.B;" is equivalent to "σR.B = S.B(R X S)".
The query "SELECT A, R.B, C, D FROM R, S WHERE R.B = S.B;" is equivalent to "σR.B = S.B(R X S)".







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