Database-Frequently Asked Questions!
1. 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.
2. 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.
3. What is a Database system?
The database and DBMS software together is called as Database
system.
4. Advantages of DBMS?
� Redundancy is controlled.
� Unauthorised access is restricted.
� Providing multiple user interfaces.
� Enforcing integrity constraints.
� Providing backup and recovery.
5. Disadvantage in File Processing System?
� Data redundancy & inconsistency.
� Difficult in accessing data.
� Data isolation.
� Data integrity.
� Concurrent access is not possible.
� Security Problems.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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
11. 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
12. 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.
13. What is Data Model?
A collection of conceptual tools for describing data, data
relationships data semantics and constraints.
14. 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.
15. 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.
16. What is an Entity?
It is a ‘thing’ in the real world with an independent existence.
17. What is an Entity type?
It is a collection (set) of entities that have same attributes.
18. What is an Entity set?
It is a collection of all entities of particular entity type in
the database.
19. What is an Extension of entity type?
The collections of entities of a particular entity type are grouped
together into an entity set.
20. 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.
21. What is an attribute?
It is a particular property, which describes the entity.
22. 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).
23. What is degree of a Relation?
It is the number of attribute of its relation schema.
24. What is Relationship?
It is an association among two or more entities.
25. What is Relationship set?
The collection (or set) of similar relationships.
26. What is Relationship type?
Relationship type defines a set of associations or a relationship
set among a given set of entity types.
27. What is degree of Relationship type?
It is the number of entity type participating.
25. What is DDL (Data Definition Language)?
A data base schema is specifies by a set of definitions expressed
by a special language called DDL.
26. What is VDL (View Definition Language)?
It specifies user views and their mappings to the conceptual schema.
27. What is SDL (Storage Definition Language)?
This language is to specify the internal schema. This language may
specify the mapping between two schemas.
28. What is Data Storage - Definition Language?
The storage structures and access methods used by database system
are specified by a set of definition in a special type of DDL called
data storage-definition language.
29. What is DML (Data Manipulation Language)?
This language that enable user to access or manipulate data as
organised by appropriate data model.
� Procedural DML or Low level: DML requires a user to specify what
data are needed and how to get those data.
� Non-Procedural DML or High level: DML requires a user to specify
what data are needed without specifying how to get those data.
31. What is DML Compiler?
It translates DML statements in a query language into low-level
instruction that the query evaluation engine can understand.
32. What is Query evaluation engine?
It executes low-level instruction generated by compiler.
33. What is DDL Interpreter?
It interprets DDL statements and record them in tables containing
metadata.
34. What is Record-at-a-time?
The Low level or Procedural DML can specify and retrieve each record
from a set of records.This retrieve of a record is said to be Record-
at-a-time.
35. What is Set-at-a-time or Set-oriented?
The High level or Non-procedural DML can specify and retrieve many
records in a single DML statement.This retrieve of a record is said
to be Set-at-a-time or Set-oriented.
36. What is Relational Algebra?
It is procedural query language.It consists of a set of operations
that take one or two relations as input and produce a new relation.
37. What is Relational Calculus?
It is an applied predicate calculus specifically tailored for
relational databases proposed by E.F. Codd. E.g. of languages based
on it are DSL ALPHA, QUEL.
38. How does Tuple-oriented relational calculus differ from domain-
oriented relational calculus
The tuple-oriented calculus uses a tuple variables i.e., variable
whose only permitted values are tuples of that relation. E.g. QUEL
The domain-oriented calculus has domain variables i.e., variables
that range over the underlying domains instead of over relation. E.g.
ILL, DEDUCE.
39. What is normalization?
It is a process of analysing the given relation schemas based on
their Functional Dependencies (FDs) and primary key to achieve the
properties
� Minimizing redundancy
� Minimizing insertion, deletion and update anomalies.
40. What is Functional Dependency?
A Functional dependency is denoted by X Y between two sets of
ttributes X and Y that are subsets of R specifies a constraint on
the possible tuple that can form a relation state r of R. The
constraint is for any two tuples t1 and t2 in r if t1[X] = t2[X]
then they have t1[Y] = t2[Y]. This means the value of X component
of a tuple uniquely determines the value of component Y.
41. When is a functional dependency F said to be minimal?
� Every dependency in F has a single attribute for its right hand
side.
� We cannot replace any dependency X A in F with a dependency Y A
where Y is a proper subset of X and still have a set of dependency
that is equivalent to F.
� We cannot remove any dependency from F and still have set of
dependency that is equivalent to F.
42. What is Multivalued dependency?
Multivalued dependency denoted by X Y specified on relation schema R,
where X and Y are both subsets of R, specifies the following
constraint on any relation r of R: if two tuples t1 and t2 exist in
r such that t1[X] = t2[X] then t3 and t4 should also exist in r with
the following properties
� t3[x] = t4[X] = t1[X] = t2[X]
� t3[Y] = t1[Y] and t4[Y] = t2[Y]
� t3[Z] = t2[Z] and t4[Z] = t1[Z]
where [Z = (R-(X U Y)) ]
43. What is Lossless join property?
It guarantees that the spurious tuple generation does not occur with
respect to relation schemas after decomposition.
44. What is 1 NF (Normal Form)?
The domain of attribute must include only atomic (simple, indivisible)
values.
45. What is Fully Functional dependency?
It is based on concept of full functional dependency. A functional
dependency X Y is full functional dependency if removal of any
attribute A from X means that the dependency does not hold any more.
46. What is 2NF?
A relation schema R is in 2NF if it is in 1NF and every non-prime
attribute A in R is fully functionally dependent on primary key.
47. What is 3NF?
A relation schema R is in 3NF if it is in 2NF and for every FD X A
either of the following is true
� X is a Super-key of R.
� A is a prime attribute of R.
In other words, if every non prime attribute is non-transitively
dependent on primary key.
48. What is BCNF (Boyce-Codd Normal Form)?
A relation schema R is in BCNF if it is in 3NF and satisfies an
additional constraint that for every FD X A, X must be a candidate
key.
49. What is 4NF?
A relation schema R is said to be in 4NF if for every Multivalued
dependency X Y that holds over R, one of following is true
� X is subset or equal to (or) XY = R.
� X is a super key.
50. What is 5NF?
A Relation schema R is said to be 5NF if for every join dependency
{R1, R2, …, Rn} that holds R, one the following is true
� Ri = R for some i.
� The join dependency is implied by the set of FD, over R in which
the left side is key of R.
51. What is Domain-Key Normal Form?
A relation is said to be in DKNF if all constraints and dependencies
that should hold on the the constraint can be enforced by simply
enforcing the domain constraint and key constraint on the relation.
52. What are partial, alternate,, artificial, compound and natural
key?
Partial Key:
It is a set of attributes that can uniquely identify weak entities
and that are related to same owner entity.It is sometime called as
Discriminator.
Alternate Key:
All Candidate Keys excluding the Primary Key are known as Alternate
Keys.
Artificial Key:
If no obvious key, either stand alone or compound is available,then
the last resort is to simply create a key, by assigning a unique
number to each record or occurrence.Then this is known as developing
an artificial key.
Compound Key:
If no single data element uniquely identifies occurrences within a
construct, then combining multiple elements to create a unique
identifier for the construct is known as creating a compound key.
Natural Key:
When one of the data elements stored within a construct is utilized
as the primary key, then it is called the natural key.
53. What is indexing and what are the different kinds of indexing?
Indexing is a technique for determining how quickly specific data
can be found.
Types:
� Binary search style indexing
� B-Tree indexing
� Inverted list indexing
� Memory resident table
� Table indexing
54. What is system catalog or catalog relation? How is better
known as?
A RDBMS maintains a description of all the data that it contains,
information about every relation and index that it contains. This
information is stored in a collection of relations maintained by
the system called metadata. It is also called data dictionary.
55. What is meant by query optimization?
The phase that identifies an efficient execution plan for evaluating
a query that has the least estimated cost is referred to as query
optimization.
56. What is join dependency and inclusion dependency?
Join Dependency:
A Join dependency is generalization of Multivalued dependency.A JD
{R1, R2, …, Rn} is said to hold over a relation R if
R1,R2,R3,…, Rn is a lossless-join decomposition of R . There is
no set of sound and complete inference rules for JD.
Inclusion Dependency:
An Inclusion Dependency is a statement of the form that some columns
of a relation are contained in other columns.A foreign key constraint
is an example of inclusion dependency.
57. What is durability in DBMS?
Once the DBMS informs the user that a transaction has successfully
completed, its effects should persist even if the system crashes
before all its changes are reflected on disk. This property is
called durability.
58. What do you mean by atomicity and aggregation?
Atomicity:
Either all actions are carried out or none are. Users should not
have to worry about the effect of incomplete transactions. DBMS
ensures this by undoing the actions of incomplete transactions.
Aggregation:
A concept which is used to model a relationship between a collection
of entities and relationships. It is used when we need to express a
relationship among relationships.
59. What is a Phantom Deadlock?
In distributed deadlock detection, the delay in propagating local
information might cause the deadlock detection algorithms to identify
deadlocks that do not really exist. Such situations are called
phantom deadlocks and they lead to unnecessary aborts.
60. What is a checkpoint and When does it occur?
A Checkpoint is like a snapshot of the DBMS state. By taking
checkpoints, the DBMS can reduce the amount of work to be done
during restart in the event of subsequent crashes.
61. What are the different phases of transaction?
Different phases are
� Analysis phase
� Redo Phase
� Undo phase
62. What do you mean by flat file database?
It is a database in which there are no programs or user access
languages. It has no cross-file capabilities but is user-friendly
and provides user-interface management.
63. What is “transparent DBMS”?
It is one, which keeps its Physical Structure hidden from user.
64. Brief theory of Network, Hierarchical schemas and their
properties?
Network schema uses a graph data structure to organize records
example for such a database management system is CTCG while a
hierarchical schema uses a tree data structure example for such
a system is IMS.
65. What is a query?
A query with respect to DBMS relates to user commands that are
used to interact with a data base. The query language can be
classified into data definition language and data manipulation
language.
66. What do you mean by Correlated subquery?
Subqueries, or nested queries, are used to bring back a set of rows
to be used by the parent query. Depending on how the subquery is
written, it can be executed once for the parent query or it can be
executed once for each row returned by the parent query. If the
subquery is executed for each row of the parent, this is called a
correlated subquery.
A correlated subquery can be easily identified if it contains any
references to the parent subquery columns in its WHERE clause.
Columns from the subquery cannot be referenced anywhere else in the
parent query. The following example demonstrates a non-correlated
subquery.
E.g. Select * From CUST Where ‘10/03/1990′ IN (Select ODATE From
ORDER Where CUST.CNUM = ORDER.CNUM)
67.What are the primitive operations common to all record management
systems?
Addition, deletion and modification.
68. Name the buffer in which all the commands that are typed in are
stored
�Edit� Buffer
69. What are the unary operations in Relational Algebra?
PROJECTION and SELECTION.
70. Are the resulting relations of PRODUCT and JOIN operation the
same?
No.
PRODUCT: Concatenation of every row in one relation with every row
in another.
JOIN: Concatenation of rows from one relation and related rows from
another.
71. What is RDBMS KERNEL?
Two important pieces of RDBMS architecture are the kernel,which is
the software, and the data dictionary,which consists of the system-l
evel data structures used by the kernel to manage the database
You might think of an RDBMS as an operating system (or set of
subsystems), designed specifically for controlling data access; its
primary functions are storing, retrieving, and securing data. An
RDBMS maintains its own list of authorized users and their associated
privileges; manages memory caches and paging; controls locking for
concurrent resource usage; dispatches and schedules user requests;
and manages space usage within its table-space structures.
72. Name the sub-systems of a RDBMS?
I/O, Security, Language Processing, Process Control, Storage
Management,Logging and Recovery,Distribution Control,Transaction
Control, Memory Management, Lock Management
73. Which part of the RDBMS takes care of the data dictionary? How?
Data dictionary is a set of tables and database objects that is
stored in a special area of the database and maintained exclusively
by the kernel.
74. What is the job of the information stored in data-dictionary?
The information in the data dictionary validates the existence of
the objects, provides access to them, and maps the actual physical
storage location.
75. Not only RDBMS takes care of locating data it also
determines an optimal access path to store or retrieve the data.
76. How do you communicate with an RDBMS?
You communicate with an RDBMS using Structured Query Language (SQL).
77. Define SQL and state the differences between SQL and other
conventional programming Languages?
SQL is a nonprocedural language that is designed specifically for
data access operations on normalized relational database structures.
The primary difference between SQL and other conventional programming
languages is that SQL statements specify what data operations should
be performed rather than how to perform them.
78. Name the three major set of files on disk that compose a database
in Oracle?
There are three major sets of files on disk that compose a database.
All the files are binary. These are:
� Database files
� Control files
� Redo logs
The most important of these are the database files where the actual
data resides. The control files and the redo logs support the
functioning of the architecture itself.
All three sets of files must be present, open, and available to
Oracle for any data on the database to be useable. Without these
files, you cannot access the database,and the database administrator
might have to recover some or all of the database using a backup,
if there is one.
79. What is an Oracle Instance?
The Oracle system processes, also known as Oracle background
processes, provide functions for the user processes�functions that
would otherwise be done by the user processes themselves
Oracle database-wide system memory is known as the SGA, the system
global area or shared global area. The data and control structures
in the SGA are shareable, and all the Oracle background processes
and user processes can use them.
The combination of the SGA and the Oracle background processes is
known as an Oracle instance
80. What are the four Oracle system processes that must always be up
and running for the database to be useable
The four Oracle system processes that must always be up and running
for the database to be useable include DBWR (Database Writer), LGWR
(Log Writer), SMON (System Monitor), and PMON (Process Monitor).
81. What are database files, control files and log files. How many
of these files should a database have at least? Why?
Database Files:
The database files hold the actual data and are typically the largest
in size. Depending on their sizes, the tables (and other objects) for
all the user accounts can go in one database file�but that’s not
an ideal situation because it does not make the database structure
very flexible for controlling access to storage for different users,
putting the database on different disk drives, or backing up and
restoring just part of the database.
You must have at least one database file but usually, more than one
files are used. In terms of accessing and using the data in the
tables and other objects, the number (or location) of the files is
immaterial.
The database files are fixed in size and never grow bigger than the
size at which they were created
Control Files
The control files and redo logs support the rest of the architecture.
Any database must have at least one control file, although you
typically have more than one to guard against loss. The control file
records the name of the database, the date and time it was created,
the location of the database and redo logs, and the synchronization
information to ensure that all three sets of files are always in step.
Every time you add a new database or redo log file to the database,
the information is recorded in the control files.
Redo Logs
Any database must have at least two redo logs.These are the journals
for the database; the redo logs record all changes to the user
objects or system objects.If any type of failure occurs, the changes
recorded in the redo logs can be used to bring the database to a
consistent state without losing any committed transactions. In the
case of non-data loss failure, Oracle can apply the information in
the redo logs automatically without intervention from the DBA.
The redo log files are fixed in size and never grow dynamically from
the size at which they were created.
82. What is ROWID?
The ROWID is a unique database-wide physical address for every row
on every table. Once assigned (when the row is first inserted into
the database),it never changes until the row is deleted or the table
is dropped.
The ROWID consists of the following three components,the combination
of which uniquely identifies the physical storage location of the row.
� Oracle database file number, which contains the block with the
rows
� Oracle block address, which contains the row
� The row within the block (because each block can hold many rows)
The ROWID is used internally in indexes as a quick means of
retrieving rows with a particular key value. Application developers
also use it in SQL statements as a quick way to access a row once
they know the ROWID.
83.What is Oracle Block?Can two Oracle Blocks have the same address?
Oracle “formats” the database files into a number of Oracle blocks
when they are first created�making it easier for the RDBMS software
to manage the files and easier to read data into the memory areas.
The block size should be a multiple of the operating system block
size.Regardless of the block size,the entire block is not available
for holding data;Oracle takes up some space to manage the contents
of the block.This block header has a minimum size, but it can grow.
These Oracle blocks are the smallest unit of storage.Increasing the
Oracle block size can improve performance,but it should be done only
when the database is first created.
Each Oracle block is numbered sequentially for each database file
starting at 1. Two blocks can have the same block address if they
are in different database files.
84. What is database Trigger?
A database trigger is a PL/SQL block that can defined to automatically
execute for insert, update, and delete statements against a table.
The trigger can e defined to execute once for the entire statement or
once for every row that is inserted, updated, or deleted.For any one
table, there are twelve events for which you can define database
triggers. A database trigger can call database procedures that are
also written in PL/SQL.
85. Name two utilities that Oracle provides,which are use for backup
and recovery.
Along with the RDBMS software, Oracle provides two utilities that you
can use to back up and restore the database. These utilities are
Export and Import.
The Export utility dumps the definitions and data for the specified
part of the database to an operating system binary file. The Import
utility reads the file produced by an export,recreates the definitions
of objects, and inserts the data.
If Export and Import are used as a means of backing up and recovering
the database,all the changes made to the database cannot be recovered
since the export was performed. The best you can do is recover the
database to the time when the export was last performed.
86. What are stored-procedures? And what are the advantages of using
them?
Stored procedures are database objects that perform a user defined
operation.A stored procedure can have a set of compound SQL statements.
A stored procedure executes the SQL commands and returns the result
to the client. Stored procedures are used to reduce network traffic.
87. How are exceptions handled in PL/SQL? Give some of the internal
exceptions’ name?
PL/SQL exception handling is a mechanism for dealing with run-time
errors encountered during procedure execution. Use of this mechanism
enables execution to continue if the error is not severe enough to
cause procedure termination.
The exception handler must be defined within a subprogram
specification. Errors cause the program to raise an exception with a
transfer of control to the exception-handler block. After the
exception handler executes, control returns to the block in which
the handler was defined. If there are no more executable statements
in the block, control returns to the caller.
User-Defined Exceptions:
PL/SQL enables the user to define exception handlers in the declarations area of subprogram specifications. User accomplishes this by naming an exception as in the following example:
ot_failure EXCEPTION;
In this case, the exception name is ot_failure. Code associated with
this handler is written in the EXCEPTION specification area as follows:
EXCEPTION
when OT_FAILURE then
out_status_code := g_out_status_code;
out_msg := g_out_msg;
The following is an example of a subprogram exception:
EXCEPTION
when NO_DATA_FOUND then
g_out_status_code := ‘FAIL’;
RAISE ot_failure;
Within this exception is the RAISE statement that transfers control
back to the ot_failure exception handler.This technique of raising
the exception is used to invoke all user-defined exceptions.
System-Defined Exceptions
Exceptions internal to PL/SQL are raised automatically upon error.
NO_DATA_FOUND is a system-defined exception. Table below gives a
complete list of internal exceptions.
PL/SQL internal exceptions.
Exception Name Oracle Error
CURSOR_ALREADY_OPEN ORA-06511
DUP_VAL_ON_INDEX ORA-00001
INVALID_CURSOR ORA-01001
INVALID_NUMBER ORA-01722
LOGIN_DENIED ORA-01017
NO_DATA_FOUND ORA-01403
NOT_LOGGED_ON ORA-01012
PROGRAM_ERROR ORA-06501
STORAGE_ERROR ORA-06500
TIMEOUT_ON_RESOURCE ORA-00051
TOO_MANY_ROWS ORA-01422
TRANSACTION_BACKED_OUT ORA-00061
VALUE_ERROR ORA-06502
ZERO_DIVIDE ORA-01476
In addition to this list of exceptions, there is a catch-all
exception named OTHERS that traps all errors for which specific
error handling has not been established.
88. Does PL/SQL support “overloading”? Explain
The concept of overloading in PL/SQL relates to the idea that you
can define procedures and functions with the same name. PL/SQL does
not look only at the referenced name, however, to resolve a procedure
or function call. The count and data types of formal parameters are
also considered.
PL/SQL also attempts to resolve any procedure or function calls in
locally defined packages before looking at globally defined packages
or internal functions.To further ensure calling the proper procedure,
you can use the dot notation.Prefacing a procedure or function name
with the package name fully qualifies any procedure or function
reference.
89. Tables derived from the ERD
a) Are totally unnormalised
b) Are always in 1NF
c) Can be further denormalised
d) May have multi-valued attributes
(b) Are always in 1NF
90. Spurious tuples may occur due to
i. Bad normalization
ii. Theta joins
iii. Updating tables from join
a) i & ii b) ii & iii
c) i & iii d)
