11.What is "index covering" of a query?

　　Answer:

　　A nonclustered index that includes (or covers) all columns used in a query is called a covering index. When SQL server can use a nonclustered index to resolve the query, it will prefer to scan the index rather than the table, which typically takes fewer data pages. If your query uses only columns included in the index, then SQL server may scan this index to produce the desired output.

　　12.What is a SQL view?

　　Answer:

　　View is a precomplied SQL query which is used to select data from one or more tables. A view is like a table but it doesn't physically take any space. View is a good way to present data in a particular format if you use that query quite often.

　　View can also be used to restrict users from accessing the tables directly.

　　A view otherwise known as a virtual table is a mere window over the base tables in the database. This helps us gain a couple of advantages:

　　1) Inherent security exposing only the data that is needed to be shown to the end user

　　2) Views are updateable based on certain conditions. For example, updates can only be directed to one underlying table of the view. After modification if the rows or columns don't comply with the conditions that the view was created with, those rows disappear from the view. You could use the CHECK OPTION with the view definition, to make sure that any updates to make the rows invalid will not be permitted to run.

　　3) Views are not materialized (given a physical structure) in a database. Each time a view is queried the definition stored in the database is run against the base tables to retrieve the data. One exception to this is to create a clustered index on the view to make it persistent in the database. Once you create a clustered index on the view, you can create any number of non-clustered indexes on the view.