28. List the possible operations is Relational Algebra.

• Select operation

• Project operation

• Union operation

• Set Difference operation

• Cartesian Product operation

• Rename operation

• Set-Intersection operation

• Natural-join operation

• Division

• Assignment operation

29. Define Aggregate Functions. 

Aggregate functions are functions that take a collection of values as input and return a single value. SQL offers five built-in aggregate functions: 

• Average: avg

• Minimum: min

• Maximum: max

• Total: sum

• Count: count

30. Define Null Values.

SQL allows the use of null values to indicate absence of information about the value of an attribute. 

31. Define Nested Sub queries. 

SQL provides a mechanism for the nesting of sub queries. A sub query is a select-from-where expression that is nested within another query. A common use of sub queries is to perform tests for set membership, set comparisons, and set cardinality. 

32. Define Embedded SQL. 

The SQL standard defines embeddings of SQL in a variety of programming languages, such as Pascal, PL/I, Fortran, C, and COBOL. A language in which SQL queries are embedded is referred to as a host language, and the SQL structures permitted in the host language constitute embedded SQL. 

33. Define Integrity Constraints. 

Integrity constraints provide a means of ensuring that changes made to the database by authorized users do not result in a loss of data consistency. Thus Integrity Constraints guard against accidental damage to the database. The constraints were in the following forms: Key declarations, and Form of a relationship. 

34. Define Referential Integrity. 

Often, we wish to ensure that a value that appears in one relation for a given set of attributes also appears for a certain set of attributes in another relation. This condition is called referential integrity. 

35. Define Assertions. 

An assertion is a predicate expressing a condition that we wish the database always satisfied. E.g.) create assertion check 

36. Define Triggers. A trigger is a statement that is executed automatically by the system as a side effect of a modification to the database. To design a trigger mechanism, we must meet two requirements: 

1. Specify the conditions under which the trigger is to be executed.

2. Specify the actions to be taken when the trigger executes.

37. Define Functional Dependency. 

Functional dependencies are constraints on the set of legal relations. They allow us to express facts about the enterprise that we are modeling with our database. Syntax: A -> B e.g.) account no -> balance for account table. 

38. List the pitfalls in Relational Database Design.

1. Repetition of information

2. Inability to represent certain information

39. Define normalization.

By decomposition technique we can avoid the Pitfalls in Relational Database Design. This process is termed as normalization. 

40. List the properties of decomposition. 

1. Lossless join

2. Dependency Preservation

3. No repetition of information

41. Define First Normal Form.

If the Relation R contains only the atomic fields then that Relation R is in first normal form. 

E.g.) R = (account no, balance) – first normal form. 

42. Define Second Normal Form. 

A relation schema R is in 2 NF with respect to a set F of FD’s if for all FD’s of the form A -> B, where A is contained in R and B is contained in R, and A is a superkey for schema R. 

43. Define BCNF. 

A relation schema R is in BCNF with respect to a set F of FD’s if for all FD’s of the form A -> B, where A is contained in R and B is contained in R, at least one of the following holds: 

1. A -> B is a trivial FD

2. A is a superkey for schema R.

44. Define 3 Normal Form.

A relation schema R is in 3 NF with respect to a set F of FD’s if for all FD’s of the form A -> B, where A is contained in R and B is contained in R, at least one of the following holds: 

1. A -> B is a trivial FD

2. A is a superkey for schema R.

3. Each attribute in B – A is contained in a candidate key for R. 

45. Define Fourth Normal Form. 

A relation schema R is in 4NF with respect to a set F of FD’s if for all FD’s of the form A ->> B (Multi valued Dependency), where A is contained in R and B is contained in R, at least one of the following holds: 

1. A ->> B is a trivial MD

2. A is a superkey for schema R.

3. Define 5NF or Join Dependencies.

Let R be a relation schema and R1, R2, …, Rn be a decomposition of R. The join dependency *(R1, R2, …Rn) is used to restrict the set of legal relations to those for which R1,R2,…Rn is a lossless-join decomposition of R. Formally, if R= R1 U R2U …U Rn, we say that a relation r® satisfies the join dependency *(R1, R2, …Rn) if 

R = A join dependency is trivial if one of the Ri is R itself. 

47. Define Cache? 

The cache is the fastest and most costly form of storage. Cache memory is small; its use is managed by the operating system. 

48. Explain Optical Storage Device? 

The most popular form of optical storage is the compact disk read-only memory, can be read by a laser. Optical storage is the write-once, read-many disk, which allows data to be written once, but does not allow them to be erased and rewritten. 

49. Define disk controller? 

It is an interface between the computer system and the actual hardware of the disk drive. Accept high-level command to read or write a sector. It attaches checksums to each sector that is written. It also performs remapping of bad sectors. 

50. Define RAID. 

It is collectively called redundant arrays of inexpensive disk, have been proposed to address the performance and reliability issues. Raids are used for their higher reliability and higher data transfer rate. RAID stands for independent, instead of inexpensive. 

51. Define file organization 

A file is organized logically as a sequence of records. These records are mapped onto disk blocks. Files are provided as a basic construct in operating system.

52. Define Hash indices? 

Indices are based on the values being distributed uniformly across a range of buckets. The bucket to which a value is assigned is determined by a function, called a hash function. 

53. Define dense index? 

An index record appears for every search-key value in the file. The index record contains the search-key value and pointer to the first data record with that search-key value. 

54. Define sparse index?

An index record is created for only some of the values. Each index record contains a search-key value and a pointer to the first data record with that search-key value. To locate a record we find the index entry with the largest search-key value that is less than or equal to the search-key value. 

55. Explain B+ -tree index structure?

The B+ -tree index structure is the most widely used of several index structures that maintain their efficiency despite insertion and deletion of data. A B+ -tree index takes the form of a balanced tree in which every path from the root of the tree to a leaf of 

The tree is the same length. 

56. Define Static Hashing? 

File organization based on the technique of hashing allow us to avoid accessing an index structure. Hashing also provides a way of constructing indices.

57. Define Query processing? 

Query processing refers to the range of activities involved in extracting data form a database. These activities include translation of queries expressed in high-level database language into expression that can be implemented at the physical level of the file system. 

58. Define Merge-join? 

The merge-join algorithm can be used to compute natural joins and equi-joins. 

59. Explain Hybrid Hash-join? 

The hybrid hash-join algorithm performs another optimization; it is useful when memory size is relatively large, but not all the build relation fits in memory. The partitioning phase of the hash-join algorithm needs one block of memory as a buffer for each partition that is created, and one block of memory as an input buffer. 

60. Define hash-table overflow? 

Hash-table overflow occurs in partition i of the build relation s if the hash index on H is larger than main memory. Hash-table overflow can occur if there are many tuples in the build relation with the same values for the join attributes. 

61. What is transaction? 

A transaction is a unit of program execution that accesses and possibly updates various data items. A transaction usually results from the execution of a user program written in a high-level data-manipulation language or programming language, and is delimited by statements of the form begin transaction and end transaction. The transaction consists of all operations executed between the begin and end of the transaction. 

62. List the properties of transaction.

1. Atomicity

2. Consistency

3. Isolation

4. Durability

63. List the possible transaction states.

1. Active

2. Partially committed

3. Aborted

4. Committed

13. What is database?

A database is a logically coherent collection of data with some inherent meaning, representing some aspect of real world and which is designed, built and populated with data for a specific purpose. 

14. What is DBMS?

It is a collection of programs that enables user to create and maintain a database. In other words it is general-purpose software that provides the users with the processes of defining, constructing and manipulating the database for various applications. 

15. What is a Database system?

The database and DBMS software together is called as Database system.

16. Advantages of DBMS?

Redundancy is controlled.

Unauthorised access is restricted.

Providing multiple user interfaces.

Enforcing integrity constraints.

Providing backup and recovery.

17. Disadvantage in File Processing System?

Data redundancy & inconsistency.

Difficult in accessing data.

Data isolation.

Data integrity.

Concurrent access is not possible. 

Security Problems.

18. Describe the three levels of data abstraction?

The are three levels of abstraction:

Physical level: The lowest level of abstraction describes how data are stored.

Logical level: The next higher level of abstraction, describes what data are stored in database and what relationship among those data. 

View level: The highest level of abstraction describes only part of entire database.

19. Define the "integrity rules"

There are two Integrity rules.

Entity Integrity: States that “Primary key cannot have NULL value”

Referential Integrity: States that “Foreign Key can be either a NULL value or should be Primary Key value of other relation. 

20. What is extension and intension?

Extension - 

It is the number of tuples present in a table at any instance. This is time dependent.

Intension - 

It is a constant value that gives the name, structure of table and the constraints laid on it.

21. What is System R? What are its two major subsystems?

System R was designed and developed over a period of 1974-79 at IBM San Jose Research Center. It is a prototype and its purpose was to demonstrate that it is possible to build a Relational System that can be used in a real life environment to solve real life problems, with performance at least comparable to that of existing system. 

Its two subsystems are 

Research Storage 

System Relational Data System.

22. How is the data structure of System R different from the relational structure? 

Unlike Relational systems in System R 

Domains are not supported

Enforcement of candidate key uniqueness is optional

Enforcement of entity integrity is optional

Referential integrity is not enforced

23. What is Data Independence?

Data independence means that “the application is independent of the storage structure and access strategy of data”. In other words, The ability to modify the schema definition in one level should not affect the schema definition in the next higher level. 

Two types of Data Independence:

Physical Data Independence: Modification in physical level should not affect the logical level.

Logical Data Independence: Modification in logical level should affect the view level. 

NOTE: Logical Data Independence is more difficult to achieve

24. What is a view? How it is related to data independence?

A view may be thought of as a virtual table, that is, a table that does not really exist in its own right but is instead derived from one or more underlying base table. In other words, there is no stored file that direct represents the view instead a definition of view is stored in data dictionary. Growth and restructuring of base tables is not reflected in views. Thus the view can insulate users from the effects of restructuring and growth in the database. Hence accounts for logical data independence. 

14. What is Data Model?

A collection of conceptual tools for describing data, data relationships data semantics and constraints. 

16. What is E-R model?

This data model is based on real world that consists of basic objects called entities and of relationship among these objects. Entities are described in a database by a set of attributes. 

17. What is Object Oriented model?

This model is based on collection of objects. An object contains values stored in instance variables with in the object. An object also contains bodies of code that operate on the object. These bodies of code are called methods. Objects that contain same types of values and the same methods are grouped together into classes. 

28. What is an Entity?

It is a 'thing' in the real world with an independent existence. 

29. What is an Entity type?

It is a collection (set) of entities that have same attributes. 

30. What is an Entity set?

It is a collection of all entities of particular entity type in the database. 

31. What is an Extension of entity type?

The collections of entities of a particular entity type are grouped together into an entity set. 

32. What is Weak Entity set?

An entity set may not have sufficient attributes to form a primary key, and its primary key compromises of its partial key and primary key of its parent entity, then it is said to be Weak Entity set. 

33. What is an attribute?

It is a particular property, which describes the entity. 

34. What is a Relation Schema and a Relation?

A relation Schema denoted by R(A1, A2, …, An) is made up of the relation name R and the list of attributes Ai that it contains. A relation is defined as a set of tuples. Let r be the relation which contains set tuples (t1, t2, t3, ..., tn). Each tuple is an ordered list of n-values t=(v1,v2, ..., vn). 

35. What is degree of a Relation?

It is the number of attribute of its relation schema. 

36. What is Relationship?

It is an association among two or more entities. 

37. What is Relationship set? 

The collection (or set) of similar relationships

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