Subquery: Any SQL query can be written in the sub query approach. The SQL subquery returns one or more column as the result. Most SQL systems return a pair wise joining of one and only one column in an inner query with one column of an outer query. The data displayed form the table referenced in outer query only. Correlated subquery: It is a subquery that depends on the outer query. Here, the “WHERE” clause of the correlated subquery uses the information of the outer query. Processing order of correlated sub queries: The inner query is computed for each outer row in the processing order of correlated sub query. First, the outer sub query will get executed and it returns the result. Based on the result of the outer query, the inner subquery will be evaluated (i.e. the subquery will use the value of the outer query).

Question

If a UDP datagram is sent from host A, port P to host B, port Q, but at host B there is no process listening to port Q, then B is to send back an ICMP Port Unreachable message to A. Like all ICMP messages, this is addressed to A as a whole, not to port P on A. (a) Give an example of when an application might want to receive such ICMP messages. (b) Find out what an application has to do, on the operating system of your choice, to receive such messages. (c) Why might it not be a good idea to send such messages directly back to the originating port P on A?

Answer 1

Question

Chapter 7, Problem 15RQ

Program Plan Intro

Subquery:

Any SQL query can be written in the sub query approach.

The SQL subquery returns one or more column as the result.

Most SQL systems return a pair wise joining of one and only one column in an inner query with one column of an outer query.

The data displayed form the table referenced in outer query only.

Correlated subquery: It is a subquery that depends on the outer query. Here, the “WHERE” clause of the correlated subquery uses the information of the outer query.

Processing order of correlated sub queries:

The inner query is computed for each outer row in the processing order of correlated sub query.

First, the outer sub query will get executed and it returns the result.

Based on the result of the outer query, the inner subquery will be evaluated (i.e. the subquery will use the value of the outer query).

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