INTRODUCTION TO SQL
Pronounced as SEQUEL: Structured
English QUERY Language
·
Pure non-procedural query language
·
Designed and developed by IBM, Implemented by Oracle·
1978 System/R IBM- 1st Relational DBMS
·
1979 Oracle and Ingres
·
1982 SQL/DS and DB2 IBM
·
Accepted by both ANSI + ISO as Standard
Query Language for any RDBMS
·
SQL86 (SQL1) : first by ANSI and ratified by ISO
(SQL-87), minor revision on 89 (SQL-89)
·
SQL92 (SQL2) : major revision
·
SQL99 (SQL3) : add recursive query, trigger, some OO features, and
non-scholar type
·
SQL2003 : XML, Window functions, and sequences (Not free)
·
Supports all the three sublanguages of DBMS: DDL, DML, DCL
·
Supports Aggregate functions, String Manipulation
functions, Set theory operations, Date Manipulation functions, rich set of
operators ( IN, BETWEEN, LIKE, IS NULL,
EXISTS)
·
Supports REPORT writing features and Forms for designing GUI based applications
DATA
DEFINITION, CONSTRAINTS, AND SCHEMA CHANGES
Used to CREATE, ALTER, and DROP
the descriptions of the database tables (relations)
table…………………………………….……relation
row……………………………………..…….tuple column………………………………….……attribute
DATA
TYPES
·
Numeric: NUMBER, NUMBER(s,p), INTEGER, INT, FLOAT, DECIMAL
·
Character: CHAR(n), VARCHAR(n), VARCHAR2(n), CHAR VARYING(n)
·
Bit String: BLOB, CLOB
·
Boolean: true, false, and null
·
Date and Time: DATE (YYYY-MM-DD) TIME( HH:MM:SS)
·
Timestamp: DATE + TIME
·
USER Defined types
CREATE
SCHEMA
Specifies a new database schema
by giving it a name
Ex: CREATE SCHEMA COMPANY
AUTHORIZATION Jsmith;
CREATE
TABLE
·
Specifies a new base relation by giving it a name, and
specifying each of its attributes and their data types
Syntax of CREATE Command:
CREATE TABLE
<table name> ( <Attribute A1> <Data Type D1> [< Constarints>],
<Attribute A2> <Data Type D2> [< Constarints>],
…….
<Attribute An> <Data Type Dn> [< Constarints>],
[<integrity-constraint1>, <integrity-constraint k> ] );
- A constraint
NOT NULL may be specified on an attribute A constraint NOT NULL may be
specified on an attribute Ex: CREATE TABLE DEPARTMENT (
DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER NOT NULL,
MGRSSN CHAR(9), MGRSTARTDATE
CHAR(9) );
·
Specifying the unique, primary key attributes,
secondary keys, and referential integrity constraints (foreign keys).
Ex: CREATE TABLE
DEPT ( DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER NOT NULL, MGRSSN CHAR(9),
MGRSTARTDATE CHAR(9), PRIMARY
FOREIGN KEY (MGRSSN) REFERENCES
EMP(SSN));
·
We can specify RESTRICT, CASCADE, SET NULL or SET
DEFAULT on referential integrity constraints (foreign keys)
Ex: CREATE TABLE
DEPT ( DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER NOT NULL,
MGRSSN CHAR(9),
MGRSTARTDATE CHAR(9), PRIMARY KEY (DNUMBER),
UNIQUE (DNAME),
FOREIGN KEY (MGRSSN) REFERENCES
EMP
ON DELETE SET DEFAULT ON UPDATE
CASCADE);
DROP
TABLE
·
Used to remove a relation (base table) and its definition.
·
The relation can no longer be used in queries,
updates, or any other commands since its description no longer exists
Example: DROP TABLE DEPENDENT;
ALTER
TABLE:
·
Used to add an attribute to/from one of the base
relations drop constraint -- The new attribute will have NULLs in all the tuples
of the relation right after the command is executed; hence, the NOT NULL
constraint is not allowed for such an
attribute. Example: ALTER TABLE
EMPLOYEE ADD JOB VARCHAR2 (12);
·
The database users must still enter a value for the
new attribute JOB for each EMPLOYEE tuple. This can be done using the UPDATE command.
DROP
A COLUMN (AN ATTRIBUTE)
·
ALTER TABLE COMPANY.EMPLOYEE DROP ADDRESS CASCADE; All
constraints and views that reference the column are dropped automatically,
along with the column. ALTER TABLE COMPANY.EMPLOYEE DROP ADDRESS RESTRICT;
Successful if no views or constraints reference the column. ALTER TABLE
COMPANY.DEPARTMENT ALTER MGRSSN DROP DEFAULT;
KEY (DNUMBER), UNIQUE (DNAME),
FOREIGN KEY (MGRSSN) REFERENCES
EMP(SSN));
·
We can specify RESTRICT, CASCADE, SET NULL or SET
DEFAULT on referential integrity constraints (foreign keys)
Ex: CREATE TABLE
DEPT ( DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER NOT NULL,
MGRSSN CHAR(9),
MGRSTARTDATE CHAR(9), PRIMARY KEY (DNUMBER),
UNIQUE (DNAME),
FOREIGN KEY (MGRSSN) REFERENCES
EMP
ON DELETE SET DEFAULT ON UPDATE
CASCADE);
DROP
TABLE
·
Used to remove a relation (base table) and its definition.
·
The relation can no longer be used in queries,
updates, or any other commands since its description no longer exists
Example: DROP TABLE DEPENDENT;
ALTER
TABLE:
·
Used to add an attribute to/from one of the base
relations drop constraint -- The new attribute will have NULLs in all the tuples
of the relation right after the command is executed; hence, the NOT NULL
constraint is not allowed for such an
attribute. Example: ALTER TABLE
EMPLOYEE ADD JOB VARCHAR2 (12);
·
The database users must still enter a value for the
new attribute JOB for each EMPLOYEE tuple. This can be done using the UPDATE command.
DROP
A COLUMN (AN ATTRIBUTE)
·
ALTER TABLE COMPANY.EMPLOYEE DROP ADDRESS CASCADE; All
constraints and views that reference the column are dropped automatically,
along with the column. ALTER TABLE COMPANY.EMPLOYEE DROP ADDRESS RESTRICT;
Successful if no views or constraints reference the column. ALTER TABLE
COMPANY.DEPARTMENT ALTER MGRSSN DROP DEFAULT;
·
ALTER TABLE COMPANY.DEPARTMENT ALTER MGRSSN SET DEFAULT
―333445555‖;
BASIC
QUERIES IN SQL
·
SQL has one basic statement for retrieving information
from a database; the SLELECT statement
·
This is not the same as the
SELECT operation of the relational algebra
·
Important distinction between SQL and the formal relational model;
·
SQL allows a table (relation) to have two or more
tuples that are identical in all their attribute values
·
Hence, an SQL relation (table) is a multi-set (sometimes called a bag) of
tuples; it is not a set of tuples
·
SQL relations can be constrained to be sets by using
the CREATE UNIQUE INDEX command, or by using the DISTINCT option
·
Basic form of the SQL SELECT statement is called a mapping of a SELECT-FROM- WHERE block
SELECT <attribute list>
FROM <table list> WHERE <condition>
·
<attribute list> is a list of attribute names whose values are to
be retrieved by the query
·
<table list > is a list of the relation names required to process
the query
·
<condition> is a conditional (Boolean)
expression that identifies the tuples to be retrieved by the query
SIMPLE
SQL QUERIES
Basic SQL queries correspond to using the following operations of the
relational algebra: SELECT
PROJECT JOIN
All subsequent examples uses COMPANY database as shown below:
Example of a simple query on one
relation
Query 0: Retrieve the birth date and address of the
employee whose name is 'John B. Smith'.
Q0: SELECT BDATE, ADDRESS FROM EMPLOYEE
Viva Questions
1.
What is SQL?
Structured Query Language
2.
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.
3. 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.
What is a Database system?
The database and DBMS software
together is called as Database system.
5.
Advantages of DBMS?
Ø
Redundancy is controlled.
Ø
Unauthorized access is restricted.
Ø
Providing multiple user interfaces.
Ø
Enforcing integrity constraints.
Ø
Providing backup and recovery.
6.
Disadvantage in File Processing System?
Ø
Data redundancy & inconsistency.
Ø
Difficult in accessing data.
Ø
Data isolation.
Ø
Data integrity.
Ø
Concurrent access is not possible.
Ø
Security Problems.
7.
Describe the three levels of data abstraction?
There are three levels of
abstraction:
Ø
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
Ø
View level:The highest level of abstraction describes only part of
entire database.
2.
Define the "integrity rules"
There are two Integrity rules.
Ø
Entity Integrity:States that ―Primary key cannot have NULL value‖
Ø Referential
Integrity:States
that
―Foreign
Key
can
be
either
a
NULL
value
or
should be Primary Key value of other relation.
3.
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.
4. 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:
Ø Physical
Data Independence: Modification in physical level should not affect the logical level.
Ø Logical
Data Independence: Modification in logical level should affect the view level.
NOTE: Logical Data Independence
is more difficult to achieve
5. 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.
6. What
is Data Model?
A collection of conceptual tools for describing data, data relationships
data semantics and constraints.
1. 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.
2. What
is Object Oriented model?
This model is based on collection of objects. An
object contains values stored in instance variables within 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.
3. What
is an Entity?
It is an 'object' in the real
world with an independent existence.
4. What
is an Entity type?
It is a collection (set) of
entities that have same attributes.
5. What
is an Entity set?
It is a collection of all
entities of particular entity type in the database.
6. What
is an Extension of entity type?
The collections of entities of a
particular entity type are grouped together into an entity
set.
7. What
is an attribute?
It is a particular property,
which describes the entity.
8. 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).
9. What
is degree of a Relation?
It is the number of attribute of
its relation schema.
10. What
is Relationship?
It is an association among two or
more entities.
11. What
is Relationship set?
The collection (or set) of
similar relationships.
1.
What is Relationship type?
Relationship type defines a set of associations or a relationship set
among a given set of entity types.
2. What
is degree of Relationship type?
It is the number of entity type
participating.
3. What
is DDL (Data Definition Language)?
A data base schema is specified by a set of definitions expressed by a
special language called DDL.
4. What
is VDL (View Definition Language)?
It specifies user views and their
mappings to the conceptual schema.
5. What
is SDL (Storage Definition Language)?
This language is to specify the internal schema. This language may
specify the mapping between two schemas.
6. 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.
7. What
is DML (Data Manipulation Language)?
This language that enable user to access or manipulate
data as organized by
appropriate data model.
Ø Procedural
DML or Low level: DML requires a user to specify what data are needed and how
to get those data.
Ø Non-Procedural
DML or High level: DML requires a user to specify what data are needed without
specifying how to get those data.
8. What
is DML Compiler?
It translates DML statements in a query language into low-level instruction that the query evaluation engine
can understand.
9.
What
is Relational Algebra?
It is a procedural query language. It
consists of a set of operations that take one or two relations as input and produce a
new relation.
1. What
is normalization?
It is a process of analyzing the given relation
schemas based on their Functional Dependencies (FDs) and primary
key to achieve the properties
Ø
Minimizing redundancy
Ø
Minimizing insertion, deletion and update anomalies.
2. What
is Functional Dependency?
A Functional dependency is
denoted
by
X
Y
between two sets of attributes 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.
3. When
is a functional dependency F said to be minimal?
Ø
Every dependency in F has a single attribute for its right hand side.
Ø 
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.
Ø We cannot
remove any dependency from F and still have set of dependency that is
equivalent to F.
4. 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
Ø t3[x] = t4[X] = t1[X] = t2[X]
Ø
t3[Y] = t1[Y] and t4[Y] = t2[Y]
Ø
t3[Z] = t2[Z] and t4[Z] = t1[Z]
where [Z = (R-(X U Y)) ]
5. What
is Lossless join property?
It guarantees that the spurious tuple generation does not occur with
respect to relation schemas after decomposition.
1. What
is normalization?
It is a process of analyzing the given relation
schemas based on their Functional Dependencies (FDs) and primary
key to achieve the properties
Ø
Minimizing redundancy
Ø
Minimizing insertion, deletion and update anomalies.
2. What
is Functional Dependency?
A Functional dependency is
denoted
by
X
Y
between two sets of attributes 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.
3. When
is a functional dependency F said to be minimal?
Ø
Every dependency in F has a single attribute for its right hand side.
Ø 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.
Ø We cannot
remove any dependency from F and still have set of dependency that is
equivalent to F.
4. 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
Ø t3[x] = t4[X] = t1[X] = t2[X]
Ø
t3[Y] = t1[Y] and t4[Y] = t2[Y]
Ø
t3[Z] = t2[Z] and t4[Z] = t1[Z]
where [Z = (R-(X U Y)) ]
5. What
is Lossless join property?
It guarantees that the spurious tuple generation does not occur with
respect to relation schemas after decomposition.
1. What
is 1 NF (Normal Form)?
The domain of attribute must
include only atomic (simple, indivisible) values.
2. What
is Fully Functional dependency?
It is based on concept of full
functional dependency. A functional dependency X Y is fully functional
dependency if removal of any attribute A from X means that the dependency does
not hold any
more.
3. 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.
4. 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
Ø X is a Super-key of R.
Ø
A is a prime attribute of R.
In other words, if every non
prime attribute is non-transitively dependent on primary key.
5. What
is BCNF (Boyce-Codd Normal Form)?
A relation schema R is in BCNF if it
is in 3NF and satisfies additional constraints that for every FD X A, X must be
a candidate key.
6. 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
Ø X is subset or equal to (or)
XY = R.
Ø
X is a super key.
7. 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
Ø
Ri = R for some i.
Ø
The join dependency is implied by the set of FD, over R in which the
left side is key of R.
1. What
is Domain-Key Normal Form?
A relation is said to be in DKNF if all constraints and
dependencies that should hold on the constraint can be enforced by simply
enforcing the domain constraint and key constraint on the relation.
2. 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:
ArtificialKey:
All Candidate Keys excluding the
Primary Key are known as Alternate Keys.
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.
CompoundKey:
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.
NaturalKey:
When one of the data elements stored within a construct is utilized as
the primary key, then it is called the natural key.
3. What
is indexing and what are the different kinds of indexing?
Indexing is a technique for
determining how quickly specific data can be found.
Ø
Binary search style indexing
Ø
B-Tree indexing
Ø
Inverted list indexing
Ø
Memory resident table
Table indexing
1. 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.
2. 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.
3. What
is join dependency and inclusion dependency?
JoinDependency:
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. InclusionDependency:
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.
4. 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.
5. 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.
1. 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.
2. 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.
3. What
are the different phases of transaction?
Different phases are
Ø
Analysis phase
Ø
Redo Phase
Ø
Undo phase
4. 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.
5. What
is "transparent DBMS"?
It is one, which keeps its
Physical Structure hidden from user.
6. 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.
7. 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.
8. 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)
1. What
are the primitive operations common to all record management systems?
Addition, deletion and
modification.
2.
Name the buffer in which all the commands that
are typed in are stored ‘Edit’ Buffer
3.
What are the unary
operations in Relational Algebra?
PROJECTION and SELECTION.
4. 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.
5. 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-level 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.
6. 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
1. 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.
2. 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.
3. Not only RDBMS takes care of locating
data it also
Determines an optimal access path
to store or retrieve the data
4. How
do you communicate with an RDBMS?
You communicate with an RDBMS
using Structured Query Language (SQL)
5. 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.
6. 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
Ø
Database files
Ø
Control files
Ø
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.
7. 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
1.
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).
2.
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
ControlFiles
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 redoes 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.
Entity-Relationship Diagram
Schema Diagram
