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How to use Nested Subqueries in SQL Programming

A subquery can contain one or more subqueries. Subqueries are used when the condition of a query is dependent on the result of another query, which in turn is dependent on the result of another subquery.

Consider an example. You need to view the DepartmentID of an employee whose e-mail address is Sales Agent. To perform this task, you can use the following query:

SELECT DepartmentID FROM HumanResources.EmployeeDepartmentHistory
WHERE BusinessEntityID = /* Level 1 inner query */
(SELECT BusinessEntityID FROM Person.BusinessEntityContact
WHERE ContactTypeID = /* Level 2 inner query */
(SELECT ContactTypeID FROM Person.ContactType WHERE Name= 'Sales Agent')

In the preceding example, two queries are nested within another query. The level 2 inner query returns the Contact ID of an employee based on the e-mail address of the employee from the Person table.

The level 1 inner query uses this ContactID to search for the BusinessEntityID of the employee with the given e-mail address. The main query uses the BusinessEntityID returned by level 1 inner query to search for the DepartmentID from the EmployeeDepartmentHistory table.

You can implement subqueries upto 32 levels. However, the number of levels that can be used depends on the memory available on the database server.


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             In memory the linked list is stored in scattered cells (locations).The memory for each node is allocated dynamically means as and when required. So the Linked List can increase as per the user wish and the size is not fixed, it can vary.

               Suppose first node of linked list is allocated with an address 1008. Its graphical representation looks like the figure shown below:

      Suppose next node is allocated at an address 506, so the list becomes,

  Suppose next node is allocated with an address with an address 10,s the list become,

The other way to represent the linked list is as shown below:

 In the above representation the data stored in the linked list is “INDIA”, the information part of each node contains one character. The external pointer root points to first node’s address 1005. The link part of the node containing information I contains 1007, the address of next node. The last node …