<span>A search engine displays a list of webpage names that contain the search text. The term for that list is hit.
Basically, what this term refers to is the number of visits, or a number of downloads that happened on that particular webpage. It is a good tool to measure web traffic, or how much that page is visited.
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For the view of Exercise 4.18, explain why the database system would not allow a tuple to be inserted into the database through this view.
For reference
For the database of Figure 4.12, write a query to find the ID of each employee with no manager. Note that an employee may simply have no manager listed or may have a null manager. Write your query using an outer join and then write it again using no outer join at all.
OUTER JOIN
select e.ID from employee e left outer join manages m on e.ID = m.ID
where m.manager_id is null;
NO OUTER
select e.ID from employee e where e.ID not in (select m.ID from manages m) or e.ID in (select m.ID from manages m where m.manager_id is null);
Answer:
1 true 2 true 3 false 4 true
Explanation:
