RDBMS Concepts: Freshers Resource Center
RDBMS Concepts: Freshers Resource Center
RDBMS Concepts: Freshers Resource Center
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RDBMS Concepts
1. What is database?
representing some aspect of real world and which is designed, built and populated
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.
4. Advantages of DBMS?
Redundancy is controlled.
Unauthorised access is restricted.
Providing multiple user interfaces.
Enforcing integrity constraints.
Providing backup and recovery.
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
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.
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)) ]
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.
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
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.
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.
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.
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
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.
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.
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.
(a) i & iii because theta joins are joins made on keys that are not primary
keys.
94. Select 'NORTH', CUSTOMER From CUST_DTLS Where REGION = 'N' Order
By
CUSTOMER Union Select 'EAST', CUSTOMER From CUST_DTLS Where
REGION = 'E' Order By CUSTOMER
The above is
a) Not an error
b) Error - the string in single quotes 'NORTH' and 'SOUTH'
c) Error - the string should be in double quotes
d) Error - ORDER BY clause
(d) Error - the ORDER BY clause. Since ORDER BY clause cannot be used
in UNIONS
that he/she wants to back up the tablespace files, he/she can use the operating system
to copy those files to the desired backup destination.
The database must be running in ARCHIVELOG mode for the hot
backup option.
If a data loss failure does occur, the lost database files can be
restored using the hot backup and the online and offline redo logs created since the
backup was done. The database is restored to the most consistent state without any
loss of committed transactions.
103.What are Armstrong rules? How do we say that they are complete and/or sound
The well-known inference rules for FDs
Reflexive rule :
If Y is subset or equal to X then X Y.
Augmentation rule:
If X Y then XZ YZ.
Transitive rule:
If {X Y, Y Z} then X Z.
Decomposition rule :
If X YZ then X Y.
Union or Additive rule:
If {X Y, X Z} then X YZ.
Pseudo Transitive rule :
If {X Y, WY Z} then WX Z.
Of these the first three are known as Amstrong Rules. They are sound because
it is enough if a set of FDs satisfy these three. They are called complete because using
these three rules we can generate the rest all inference rules.