ACE-DBMS

Question

Answer 1

It is an example of nested query with correlation .Following are the 3 salient features of nested query with correlation :

a) Outer query uses the result of inner query

b) Inner query uses the attributes mentioned in outer query

c) So for each  instance of outer query is used and compared with every tuple resulting in the subquery..So the execution sequence is in zigzag manner.

Such nested queries are used generally with EXISTS , NOT EXISTS clauses..

So in the given query  we see for a given value of A attribute of R which is in outer query , it is compared with every attribute value D of relation S in the inner query(or subquery) ..

So for 1 attribute instance of A , inner query will execute once and then we will have those tuples where A = D is satisfied

So total number of times inner query is executed            = No of values of A(or tuples of R)  

                                                                                  =  10  

                                          

Hence B) should be the correct option. 

Comments

Ok sir so the answer should be 10 right??

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yes..

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@Arjun sir your are correct .As question asked for no of times the query executed but not evaluated and it is evaluated 200 times but executed 10 times only.