Dbms Basics

DR .Ram Pathak
Systems Analyst
Department of Commerce
Dr H S G University, Sagar
Introduction of Database and SQL

Paradigm Shift from File System to DBMS
File System manages data using files in hard disk. Users are allowed to create, delete, and update the files according to their requirement.
• Sequential File
•Direct File
•Index Sequential file
Example : File based University Management System. Data of students is available to their respective Departments, Academics
Section, Result Section, Accounts Section, Hostel Office etc. Some of the data is common for all sections like Roll No, Name, Father Name, Address
and Phone number of students but some data is available to a particular section only like Hostel allotment number which is a part of hostel office. Let
us discuss the issues with this system:
•Redundancy of data: Data is said to be redundant if same data is copied at many places. If a student wants to change Phone number, he has to
get it updated at various sections. Similarly, old records must be deleted from all sections representing that student.
•Inconsistency of Data: Data is said to be inconsistent if multiple copies of same data does not match with each other. If Phone number
is different in Accounts Section and Academics Section, it will be inconsistent. Inconsistency may be because of typing
errors or not updating all copies of same data.
•Difficult Data Access: A user should know the exact location of file to access data, so the process is very cumbersome and tedious. If user wants
to search student hostel allotment number of a student from 10000 unsorted students’ records, how difficult it can be.
•Unauthorized Access: File System may lead to unauthorized access to data. If a student gets access to file having his marks, he can change it in
unauthorized way.
•No Concurrent Access: The access of same data by multiple users at same time is known as concurrency. File system does not allow concurrency
as data can be accessed by only one user at a time.
•No Backup and Recovery: File system does not incorporate any backup and recovery of data if a file is lost or corrupted.
These are the main reasons which made a shift from file system to DBMS.

Important Terminology
• Database ( Data and Base two terms : Database is base for Data)
Database is a collection of inter-related data which helps in efficient retrieval, insertion and deletion of data from
database and organizes the data in the form of tables, views, schemas, reports etc. For Example, university database
organizes the data about students, faculty, and admin staff etc. which helps in efficient retrieval, insertion and deletion of
data from it.
• Database Management System:
The software which is used to manage database is called Database Management System (DBMS). For Example, MySQL,
Oracle etc. are popular commercial DBMS used in different applications.
DBMS allows users the following tasks:
– Data Definition: It helps in creation, modification and removal of definitions that define the organization of data in
database.
– Data Updation: It helps in insertion, modification and deletion of the actual data in the database.
– Data Retrieval: It helps in retrieval of data from the database which can be used by applications for various purposes.
– User Administration: It helps in registering and monitoring users, enforcing data security, monitoring performance,
maintaining data integrity, dealing with concurrency control and recovering information corrupted by unexpected
failure.

File System
Data :- Raw facts and figures which are useful to an
organization. We cannot take decisions on the basis of data.
Information:- Well processed data is called information. We
can take decisions on the basis of information
Field: Set of characters that represents specific data
element.
Record: Collection of fields is called a record. A record
can have fields of different data types.
 Reduction of data Redundancy
 Reduction of Inconsistency
 Sharing of data
 Better Interaction with Users
 Enforcement of Standards
 Data Security
 Maintenance of Data integrity
 Efficient System
Benefits of DBMS and Terminology of File & DBMS
Data Database:
Table : Table is alogically related multiple records
Tuple: A row in a relation is called a tuple.
Attribute: A column in a relation is called an attribute. It is
also termed as field or data item.
Degree: Number of attributes in a relation is called degree
of a relation.
Cardinality: Number of tuples in a relation is called cardinality
of a relation.
Benefits

• Primary Key: Primary key is a key that can uniquely identifies the records/tuples in a
relation. This key can never be duplicated and NULL.
• Foreign Key: Foreign Key is a key that is defined as a primary key in some other relation.
This key is used to enforce referential integrity in RDBMS.
• Candidate Key: Set of all attributes which can serve as a primary key in a relation.
• Alternate Key: All the candidate keys other than the primary keys of a relation are
alternate keys for a relation.
• DBA: Data Base Administrator is a person (manager) that is responsible for
defining the data base schema, setting security features in database, ensuring proper
functioning of the data bases etc.
Primary Key
Purch_Id Purch_date Item_code
Item_code Item_cost Item_Qty
Foreign Key
Purchase Database
Item DatabasePrimary Key

Structured Query Language(SQL)
• SQL is a non procedural language that is used to create, manipulate and process the
databases(relations).
• Characteristics of SQL
 It is very easy to learn and use.
 Large volume of databases can be handled quite easily.
 It is non procedural language. It means that we do not need to specify the procedures to
accomplish a task but just to give a command to perform the activity.
 SQL can be linked to most of other high level languages that makes it first choice for the database
programmers.

Structured Query Language
1.Data Definition Language (DDL) : DDL contains commands that are used to create the tables, databases, indexes,
views, sequences and synonyms etc.
e.g: Create table, create view, create index, alter table
2. Data Manipulation Language (DML) : DML contains command that can be used to manipulate the data base objects
and to query the databases for information retrieval.
e.g: Select, Insert, Delete, Update.
3. View Definition: DDL contains set of command to create a view of a relation.
e.g : create view
4. Data Control Language(DCL) This language is used for controlling the access to the data. The commonly used
commands DCL are,
e.g. GRANT, REVOKE
5. Transaction Control Language (TCL) TCL include commands to control the transactions in a data base system. The
commonly used commands in TCL are
e.g. COMMIT, ROLLBACK

Data types of SQL
Just like any other programming language, the facility of defining data of various types is available in SQL also.
Following are the most common data types of SQL.
1. NUMBER
2. CHAR
3. VARCHAR / VARCHAR2
4. DATE
5. LONG
6. RAW/LONG RAW
1. NUMBER Used to store a numeric value in a field/column. It may be decimal, integer or a
real value.
General syntax is: Number(n,d)
Where n specifies the number of digits and d specifies the number of digits to
the right of the decimal point.
e.g marks number(3) declares marks to be of type number with maximum value 999.
pct number(5,2) declares pct to be of type number of 5 digits with two digits to the
right of decimal point.

2. CHAR Used to store character type data in a column.
General syntax is Char (size)
where size represents the maximum number of characters in a column. The CHAR type data can hold at most 255
characters.
e.g name char(25) declares a data item name of type character of upto 25 size long.
3. VARCHAR/VARCHAR2 This data type is used to store variable length alphanumeric data.
General syntax is, varchar(size) / varchar2(size)
where size represents the maximum number of characters in a column. The maximum allowed size in this data type is
2000 characters.
e.g address varchar(50);
address is of type varchar of upto 50 characters long.

4. DATE Date data type is used to store dates in columns. SQL supports the various date formats other that the standard DD-
MON-YY.
e.g.: dob date; declares dob to be of type date.
5. LONG This data type is used to store variable length strings of upto 2 GB size.
e.g.: description long;
6. RAW/LONG RAW To store binary data (images/pictures/animation/clips etc.) RAW or LONG RAW data type is used. A
column LONG RAW type can hold upto 2 GB of binary data.
e.g image raw(2000);

SQL Commands
Create table command is used to create a table in SQL. It is a DDL type of command. The general syntax of creating a table is
create table <table> (
<column 1> <data type> [not null] [unique] [<column constraint>],
. . . . . . . . .
<column n> <data type> [not null] [unique] [<column constraint>],
[<table constraint(s)>]
);
CREATE TABLE Command:
For each column, a name and a data type must be specified and the column name must be unique within the table definition.
Column definitions are separated by comma. Uppercase and lowercase letters makes no difference in column names, the only place
where upper and lower case letters matter are strings comparisons. A not null Constraint means that the column cannot have null
value, that is a value needs to be supplied for that column. The keyword unique specifies that no two tuples can have the same
attribute value for this column.

Constraints: Constraints are the conditions that can be enforced on the attributes of a relation. The constraints come in play when
ever we try to insert, delete or update a record in a relation.
1. NOT NULL
2. UNIQUE
3. PRIMARY KEY
4. FOREIGN KEY
5. CHECK
6. DEFAULT
1. NOT NULL: Ensures that we cannot leave a column as null. That is a value has to be supplied for that column.
e.g name varchar(25) not null;
2. UNIQUE: Constraint means that the values under that column are always unique.
e.g Roll_no number(3) unique;

3. PRIMARY KEY: Constraint means that a column can not have duplicate values and not even a null value.
e.g. Roll_no number(3) primary key;
The main difference between unique and primary key constraint is that a column specified as unique may have null value but primary
key constraint does not allow null values in the column.
4. FOREIGN KEY: Is used to enforce referential integrity and is declared as a primary key in some other table.
e.g cust_id varchar(5) references master(cust_id);
it declares cust_id column as a foreign key that refers to cust_id field of table master. That means we cannot insert that value in cust_id
filed whose corresponding value is not present in cust_id field of master table.
5. CHECK: Constraint limits the values that can be inserted into a column of a table.
e.g marks number(3) check(marks>=0);
The above statement declares marks to be of type number and while inserting or updating the value in marks it is ensured that its value
is always greater than or equal to zero.

6. DEFAULT: Constraint is used to specify a default value to a column of a table automatically. This default value
will be used when user does not enter any value for that column.
e.g balance number(5) default = 0;
CREATE TABLE student (
Roll_no number(3) primary key,
Name varchar(25) not null,
Class varchar(10),
Marks number(3) check(marks>0),
City varchar(25) );

Operators in SQL:
The following are the commonly used operators in SQL
Arithmetic Operators +, -, *, /
Relational Operators =, <, >, <=, >=, <>
Logical Operators OR, AND, NOT
Arithmetic operators are used to perform simple arithmetic operations.
Relational Operators are used when two values are to be compared and
Logical operators are used to connect search conditions in the WHERE Clause in SQL.

Data Modifications in SQL
After a table has been created using the create table command, tuples can be
inserted into the table, or tuples can be deleted or modified.
INSERT Statement
The simplest way to insert a tuple into a table is to use the insert statement
insert into <table> [(<column i, . . . , column j>)] values (<value i, . . . , value j>);
INSERT INTO student VALUES(101,'Rohan',‘MCA I',400,‘Sagar');
While inserting the record it should be checked that the values passed are of same data types as
the one which is specified for that particular column.

Data Modifications in SQL
INSERT Statement For inserting a row interactively (from keyboard) & operator can be used.
e.g INSERT INTO student VALUES(‘&Roll_no’,’&Name’,’&Class’,’&Marks’,’&City’);
In the above command the values for all the columns are read from keyboard and inserted into the table student.
NOTE:- In SQL we can repeat or re-execute the last command typed at SQL prompt by typing “/” key and
pressing enter.
Roll_no Name Class Marks City
101 Rohan XI 400 Sagar
102 Aneeta XII 390 Jablpur
103 Pawan Kumar IX 298 Damoh
104 Rohan IX 376 Delhi
105 Sanjay VII 240 Mumbai
113 Anju VIII 432 Delhi
Table:
Student

Queries:
To retrieve information from a database we can query the databases. SQL SELECT statement is used to select rows and
columns from a database/relation
SELECT Command This command can perform selection as well as projection.
Selection: This capability of SQL can return you the tuples form a relation with all the attributes.
Projection: This is the capability of SQL to return only specific attributes in the relation.
SELECT Command
 SELECT * FROM student; command will display all the tuples in the relation student
 SELECT * FROM student WHERE Roll_no <=102;
The above command display only those records whose Roll_no less than or equal to 102.
Select command can also display specific attributes from a relation.
SELECT name, class FROM student; =: The command displays only name and class attributes from student table.

SELECT Command
 SELECT count(*) AS “Total Number of Records” FROM student;
Display the total number of records with title as “Total Number of Records” i.e an alias
We can also use arithmetic operators in select statement, like
 SELECT Roll_no, name, marks+20 FROM student;
 SELECT name, (marks/500)*100 FROM student WHERE Roll_no > 103;
Eliminating Duplicate/Redundant data
DISTINCT keyword is used to restrict the duplicate rows from the results of a SELECT statement.
e.g. SELECT DISTINCT name FROM student;
The above command returns,
Name
Rohan
Aneeta
Pawan Kumar

Conditions based on a range
SQL provides a BETWEEN operator that defines a range of values that the column value must fall for the
condition to become true.
e.g. SELECT Roll_no, name FROM student WHERE Roll_no BETWENN 100 AND 103;
The above command displays Roll_no and name of those students whose Roll_no lies in the range 100 to 103
(both 100 and 103 are included in the range).
Conditions based on a list
To specify a list of values, IN operator is used. This operator select values that match any value in the given list.
e.g. SELECT * FROM student WHERE city IN (‘Sagar’,’Delhi’,’Damoh’);
The above command displays all those records whose city is either Sagar or Delhi or Damoh

Conditions based on Pattern
SQL provides two wild card characters that are used while comparing the strings with LIKE operator.
a. percent ( % ) Matches any string
b. Underscore ( _ ) Matches any one character
e.g SELECT Roll_no, name, city FROM student WHERE Roll_no LIKE “%3”;
displays those records where last digit of Roll_no is 3 and may have any number of characters in front.
e.g SELECT Roll_no, name, city FROM student WHERE Roll_no LIKE “1_3”;
displays those records whose Roll_no starts with 1 and second letter may be any letter but ends with digit 3.

ORDER BY Clause
ORDER BY clause is used to display the result of a query in a specific order(sorted order).
The sorting can be done in ascending or in descending order. It should be kept in mind that the actual data in the
database is not sorted but only the results of the query are displayed in sorted order.
e.g. SELECT name, city FROM student ORDER BY name;
The above query returns name and city columns of table student sorted by name in increasing/ascending order.
e.g. SELECT * FROM student ORDER BY city DESC; DESC - Descending
It displays all the records of table student ordered by city in descending order.
Note:- If order is not specifies that by default the sorting will be performed in ascending order.

GROUP BY Clause
The GROUP BY clause can be used in a SELECT statement to collect data across multiple records and group the
results by one or more columns.
The syntax for the GROUP BY clause is:
SELECT column1, column2, ... column_n, aggregate_function (expression)
FROM tables
WHERE conditions
GROUP BY column1, column2, ... column_n;
aggregate_function can be a function such as SUM, COUNT, MAX, MIN, AVG etc.
e.g SELECT name, COUNT(*) as "Number of
employees“ FROM student WHERE marks>350
GROUP BY city;

HAVING Clause
The HAVING clause is used in combination with the GROUP BY clause. It can be used in a SELECT statement to
filter the records that a GROUP BY returns.
The syntax for the HAVING clause is:
SELECT column1, column2, ... column_n, aggregate_function (expression)
FROM tables
WHERE predicates
GROUP BY column1, column2, ... column_n
HAVING condition1 ... condition_n;
e.g SELECT SUM(marks) as "Total marks"
FROM student
GROUP BY department
HAVING SUM(sales) > 1000;
Note: select statement can contain only those attribute which are already present in the group by clause.

Functions available in SQL
SQL provide large collection of inbuilt functions also called library functions that can be used directly in SQL
statements.
1. Mathematical functions
2. String functions
3. Date & Time functions
1. Mathematical functions Some of the commonly used mathematical functions are sum() avg(), count(), min(),
max() etc.
e.g. SELECT sum(marks) FROM student;
displays the sum of all the marks in the table student.
e.g. SELECT min(Roll_no), max(marks) FROM student;
displays smallest Roll_no and highest marks in the table student.

2. String functions
These functions are used to deal with the string type values like
ASCII, LOWEWR, UPPER, LEN, LEFT, RIGHT, TRIM, LTRIM, RTRIM etc.
ASCII : Returns the ASCII code value of a character (leftmost character of string). Syntax: ASCII(character)
2. String functions
SELECT ASCII('a') returns 97
SELECT ASCII('A') returns 65
SELECT ASCII('1') returns 49
SELECT ASCII('ABC') returns 65
For Upper character 'A' to 'Z' ASCII value 65 to 90
For Lower character 'A' to 'Z' ASCII value 97 to 122
For digit '0' to '9' ASCII value 48 to 57

2. String functions
LOWER : Convert character strings data into lowercase.
Syntax: LOWER(string)
SELECT LOWER('STRING FUNCTION')
returns string function
UPPER : Convert character strings data into Uppercase.
Syntax: UPPER(string)
SELECT UPPER('string function')
returns STRING FUNCTION
LEN : Returns the length of the character string.
Syntax: LEN(string)
SELECT LEN('STRING FUNCTION')
returns 15

REPLACE : Replaces all occurrences of the second string(string2) in the first string(string1) with a third string(string3).
Syntax: REPLACE('string1','string2','string3')
SELECT REPLACE('STRING FUNCTION','STRING','SQL')
returns SQL Function
Returns NULL if any one of the arguments is NULL.
LEFT : Returns left part of a string with the specified number of characters counting from left.LEFT function is used to
retrieve portions of the string.
Syntax: LEFT(string,integer)
SELECT LEFT('STRING FUNCTION', 6)
returns STRING
RIGHT : Returns right part of a string with the specified number of characters counting from right.RIGHT function is used to
retrieve portions of the string.
Syntax: RIGHT(string,integer)
SELECT RIGHT('STRING FUNCTION', 8)
returns FUNCTION

LTRIM : Returns a string after removing leading blanks on Left side.(Remove left side space or blanks)
Syntax: LTRIM(string)
SELECT LTRIM(' STRING FUNCTION')
RTRIM : Returns a string after removing leading blanks on Right side.(Remove right side space or blanks)
Syntax: RTRIM( string )
SELECT RTRIM('STRING FUNCTION ')
REVERSE : Returns reverse of a input string.
Syntax: REVERSE(string)
SELECT REVERSE('STRING FUNCTION')
returns NOITCNUF GNIRTS

REPLICATE : Repeats a input string for a specified number of times.
Syntax: REPLICATE (string, integer)
SELECT REPLICATE('FUNCTION', 3)
returns FUNCTIONFUNCTIONFUNCTION
SPACE : Returns a string of repeated spaces. The SPACE function is an equivalent of using REPLICATE function to repeat spaces.
Syntax: SPACE ( integer) (If integer is negative, a null string is returned.)
SELECT ('STRING') + SPACE(1) + ('FUNCTION')
SUBSTRING : Returns part of a given string.
SUBSTRING function retrieves a portion of the given string starting at the specified character(startindex) to the number of characters
specified(length).
Syntax: SUBSTRING (string,startindex,length)

SUBSTRING : Returns part of a given string.
SELECT SUBSTRING('STRING FUNCTION', 1, 6)
returns STRING
SELECT SUBSTRING('STRING FUNCTION', 8, 8)
returns FUNCTION
SQL Commands

DELETE Command
To delete the record fro a table SQL provides a delete statement. General syntax is:-
DELETE FROM <table_name> [WHERE <condition>];
e.g. DELETE FROM student WHERE city = ‘Sagar’;
This command deletes all those records whose city is Sagar.
NOTE: It should be kept in mind that while comparing with the string type values lowercase and uppercase letters are treated as
different. That is ‘Jammu’ and ‘jammu’ is different while comparing.
UPDATE COMMAND
To update the data stored in the data base, UOPDATE command is used.
e. g. UPDATE student SET marks = marks + 100;
Increase marks of all the students by 100.
e. g. UPDATE student SET City = ‘Sagar’ WHERE city = ‘Bhopal’;
changes the city of those students to Sagar whose city is Bhopal.

UPDATE Command
We can also update multiple columns with update command, like
e. g. UPDATE student set marks = marks + 20, city = ‘Mangalore’
WHERE city NOT IN (‘Delhi’,’Mysore’);
CREATE VIEW Command
In SQL we can create a view of the already existing table that contains specific attributes of the table.
e. g. the table student that we created contains following fields:
Student (Roll_no, Name, Marks, Class, City)
Suppose we need to create a view v_student that contains Roll_no,name and class of student table, then Create View command can
be used:
CREATE VIEW v_student AS SELECT Roll_no, Name, Class FROM student;
The above command create a virtual table (view) named v_student that has three attributes as mentioned and all the rows under
those attributes as in student table.
We can also create a view from an existing table based on some specific conditions, like
CREATE VIEW v_student AS SELECT Roll_no, Name, Class FROM student WHERE City <>’Delhi’;

Difference between VIEW and Table
The main difference between a Table and view is that:
A Table is a repository of data. The table resides physically in the database.
A View is not a part of the database's physical representation. It is created on a table or another view. It is precompiled, so that data
retrieval behaves faster, and also provides a secure accessibility mechanism.
ALTER TABLE Command
In SQL if we ever need to change the structure of the database then ALTER TABLE command is used. By using this command we can
add a column in the existing table, delete a column from a table or modify columns in a table.
Adding a column
The syntax to add a column is:-
ALTER TABLE table_name
ADD column_name datatype;

ALTER TABLE Command
e.g ALTER TABLE student ADD(Address varchar(30));
The above command add a column Address to the table student.
If we give command
SELECT * FROM student;
The following data gets displayed on screen:
Roll_no Name Class Marks City Address
101 Rohan XI 400 Sagar
102 Aneeta XII 390 Jablpur
103 Pawan
Kumar
IX 298 Damoh
104 Rohan IX 376 Delhi
105 Sanjay VII 240 Mumbai
113 Anju VIII 432 Delhi
Note that we have just added a column and there will be no data under this attribute. UPDATE command can be used to
supply values / data to this column.
Student Table

ALTER TABLE Command
Removing a column
ALTER TABLE table_name
DROP COLUMN column_name;
e.g ALTER TABLE Student
DROP COLUMN Address;
The column Address will be removed from the table student
DROP TABLE Command
Sometimes you may need to drop a table which is not in use. DROP TABLE command is used to Delete / drop a table permanently. It
should be kept in mind that we can not drop a table if it contains records. That is first all the rows of the table have to be deleted and
only then the table can be
dropped.
The general syntax of this command is:-
DROP TABLE <table_name>;
e.g DROP TABLE student;
This command will remove the table student

It is the collections of rules and operations on relations(tables). The various operations are selection, projection, Cartesian product,
union, set difference and intersection, and joining of relations.
Define the terms:
i. Database Abstraction
Ans: Database system provides the users only that much information that is required by them, and hides certain details like, how the
data is stored and maintained in database at hardware level. This concept/process is Database abstraction.
ii. Data inconsistency
Ans: When two or more entries about the same data do not agree i.e. when one of them stores the updated information and the
other does not, it results in data inconsistency in the database.
iii. Conceptual level of database implementation/abstraction
Ans: It describes what data are actually stored in the database. It also describes the relationships existing among data. At this level
the database is described logically in terms of simple data-structures.
iv. Primary Key
Ans : It is a key/attribute or a set of attributes that can uniquely identify tuples within the relation.
Relational Algebra

Define the terms:
v. Candidate Key
Ans : All attributes combinations inside a relation that can serve as primary key are candidate key as they are candidates for being as
a primary key or a part of it.
vi. Relational Algebra
Ans : It is the collections of rules and operations on relations(tables). The various operations are selection, projection, Cartesian
product, union, set difference and intersection, and joining of relations.
vii. Domain
Ans : it is the pool or collection of data from which the actual values appearing in a given column are drawn.

Ram K. Pathak
ramkumar.pathak@gmail.com

Dbms Basics

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