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# Chapter Notes - Chapter 6 - Database and SQL, Computer Science, Class 12 Class 12 Notes | EduRev

## COMPUTER SCIENCE for Class 12(XII) - CBSE & NCERT Curriculum

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## Class 12 : Chapter Notes - Chapter 6 - Database and SQL, Computer Science, Class 12 Class 12 Notes | EduRev

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Chapter 6 - DATABASE AND SQL, Chapter Notes, Class 12,  Computer Science

UNIT-3 DATABASE AND SQL

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.
File: Collection of similar types of records is called a file.
Table: Collection of rows and columns that contains useful data/information is called a
table. A table generally refers to the passive entity which is kept in secondary
storage device.
Relation: Relation (collection of rows and columns) generally refers to an active entity on
which we can perform various operations.
Database: Collection of logically related data along with its description is termed as database.
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.
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.
Relational Algebra: Relation algebra is a set operation such as select, project, union, &
Cartesian product etc.
Select operation : Yields set of set of rows depending upon certain condition. Mathematically it
is denoted as σ
e.g σ roll no>10(student) -means show those rows of student table whose roll no.’s
are >10.
Project Operation : yields set of columns as result which are specified. Mathematically it is
denoted as π
e.g. π, roll no. name (student) - means show only rollno and name column only.
Union Operation : Two relation are said to be union compatible if their degree and column are
same.

Cartesian product: Cartesian Product of two relation A and B gives resultant relation whose no. of
column are sum of degrees of two relation and no. of rows are product of cardinality of two relations.

Structured Query Language
SQL is a non procedural language that is used to create, manipulate and process the databases(relations).

Characteristics of SQL
1. It is very easy to learn and use.
2. Large volume of databases can be handled quite easily.
3. 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.
4. SQL can be linked to most of other high level languages that makes it first choice for the database
programmers.
Processing Capabilities of SQL
The following are the processing capabilities of SQL
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 etc.

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 etc.

3. Data Control Language:
This language is used for controlling the access to the data. Various commands like GRANT,
REVOKE etc are available in DCL.

4. Transaction Control Language (TCL)
TCL include commands to control the transactions in a data base system. The commonly used
commands in TCL are COMMIT, ROLLBACK etc.

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:
Create table command is used to create a table in SQL. It is a DDL type of command.

Syntax :

CREATE TABLE <table name>
( <column name> <data types>[(size)] [,<column name> <data types>[(size)]….);
e.g.

Create table student
(rollno number(2), name
varchar2(20), dob date);

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.They are used to ensure integrity of
a relation, hence named as integrity constraints.
1. NOT NULL
2. UNIQUE
3. PRIMARY KEY
4. FOREIGN KEY
5. CHECK
6. DEFAULT

i. Not Null constraint : It ensures that the column cannot contain a NULL value.

ii. Unique constraint : A candidate key is a combination of one or more columns, the value of
which uniquely identifies each row of a table.

iii. Primary Key : It ensures two things : (i) Unique identification of each row in the table. (ii)
No column that is part of the Primary Key constraint can contain a NULL value.

iv. Foreign Key : The foreign key designates a column or combination of columns as a foreign
key and establishes its relationship with a primary key in different table.

Create table Fee
(RollNo number(2) Foreign key (Rollno) references Student (Rollno),
Name varchar2(20) Not null, Amount
number(4), Fee_Date date);

v. Check Constraint : Sometimes we may require that values in some of the
columns of our table are to be within a certain range or they must satisfy
cceorntdaiintions.
Example:
Create table Employee
(EmpNo number(4) Primary Key,
Name varchar2(20) Not Null,
Salary number(6,2) check (salary > 0),
DeptNo number(3)
);

Data Manipulation in SQL
DML Commands are as under:
SELECT - Used for making queries
INSERT - Used for adding new row or record into table
UPDATE- used for modification in existing data in a table
DELETE – used for deletion of records.
INSERT Statement
To insert a new 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','XI',400,'Jammu');
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.
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.

Operators in SQL:

The following are the commonly used operators in SQL
1. Arithmetic Operators +, -, *, /
2. Relational Operators =, <, >, <=, >=, <>
3. 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.
Other operators :
4. Range check – between low and high
5. List check – in
6. Pattern check – like , not like ( % and _ ‘under score’ is used)

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.
e.g. SELECT name, class FROM student;
The above command displays only name and class attributes from student table.

Projection: This is the capability of SQL to return only specific attributes in the relation. Use of where
clause is required when specific tuples are to be fetched or manipulated.
e.g 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 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;
92
The above command returns
Name
Rohan
Aneeta Chopra
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 (‘Jammu’,’Amritsar’,’Gurdaspur’);
The above command displays all those records whose city is either Jammu or Amritsar or Gurdaspur.

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;
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.
SQL Functions :
SQL supports functions which can be used to compute and select numeric, character and date
columns of a relations. These functions can be applied on a group of rows. The rows are grouped on a
common value of a column in the table. These functions return only one value for a group and
therefore, they are called aggregate or group functions.

1. SUM() :
It returns the sum of values of numeric type of a column.
Eg. Select sum(salary) from employee;

2. AVG() :
It returns the average of values of numeric type of a column.
Eg. Select avg(salary) from employee;

3. MIN() :
It returns the minimum of the values of a column of a given relation.
Eg. Select min(salary) from employee;

4. MAX() :
It returns the maximum of the values of a column of a given relation.
Eg. Select max(salary) from employee;

5. COUNT():
It returns the number of rows in a relation.
Eg. Select count(*) from employee;

Group By Clause :
The rows of a table can be grouped together based on a common value by using the Group By clause of
SQL in a select statement.
Syntax :
SELECT <attribute name>, <attribute name> ---- [functions]
FROM <relation name>
GROUP BY <group by column>;
Eg. Select age, count (rollno)
From students Group by age;

Output :

Age Count(rollno)
15          2
14.5       2
14          5

Group-By-Having Clause :
It is used to apply some condition to the Group By clause.
Eg.
Select class
From students Group by class
Having count(*) > 5;

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 = ‘Jammu’;
This command deletes all those records whose city is Jammu.
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, UPDATE command is used.
e. g. UPDATE student SET marks = marks + 100;
Increase marks of all the students by 100.
94
e. g. UPDATE student SET City = ‘Udhampur’ WHERE city = ‘Jammu’;
changes the city of those students to Udhampur whose city is Jammu.
We can also update multiple columns with update command, like
e. g. UPDATE student set marks = marks + 20, city = ‘Jalandhar’
WHERE city NOT IN (‘Jammu’,’Udhampur’);

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 <>’Jammu’;
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.
ALTER TABLE table_name
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 Jammu
102 Aneeta Chopra XII 390 Udhampur
103 Pawan Kumar IX 298 Amritsar
104 Rohan IX 376 Jammu
105 Sanjay VII 240 Gurdaspur
113 Anju MAhajan VIII 432 Pathankot

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.

Removing a column
ALTER TABLE table_name
DROP COLUMN column_name;
e.g ALTER TABLE Student
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

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