55. What is sub-query?

Sol.Sub-query is a query within a Query.

56. Explain the use of group by clause.

"Group By" is used to derive aggegate values by grouping similar data.

57. Difference between clustered and non-clustered index.

Both stored as B-tree structure. The leaf level of a clustered index is the actual data where as leaf level of a non-clustered index is pointer to data. We can have only one clustered index in a table but we can have many non-clustered index in a table. Physical data in the table is sorted in the order of clustered index while not with the case of non-clustered data.  

58. Define aggregate and scalar functions.

Aggregate Functions return a single value by operating against a group of values. Scalar functions operate against a single value. 

59. What are the restrictions applicable while creating views?

Views can be created referencing tables and views only in the current database.
A view name must not be the same as any table owned by that user.
You can build views on other views and on procedures that reference views.
Rules or DEFAULT definitions can't be associated with views.
Only INSTEAD OF triggers can be associated with views.
The query that defines the view cannot include the ORDER BY, COMPUTE, or COMPUTE BY clauses or the INTO keyword.
You cannot define full-text index definitions for views.
You cannot create temporary views
You cannot create views on temporary tables.

60.Explain the difference between an explicit and an implicit lock..

Explicit Lock :- Lock is explicitly requested for a record or table.
Implicit Lock :- Lock is implied but is not acquired.