CBSE Revision Notes Class-11 Computer Science (New Syllabus) Unit 3: Data Management (DM-1) Database

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1 CBSE Revision Notes Class-11 Computer Science (New Syllabus) Unit 3: Data Management (DM-1) Database Database: A Database is an organized collection of facts. In other words we can say that it is a collection of information arranged and presented to serve an assigned purpose. An example of a database is a dictionary where words are arranged alphabetical order. In order to keep database updated we may need to perform operations like adding information, removing information, editing existing information etc. Take a case of phone contacts, in which you save name phone number of friends. This is also a type of database. If you make new friend you save his phone number name it s like adding record to the database. Why Database: Any organization be it Bank, Manufacturing Company, Hospital, University, or Government department requires huge amount of data in some form or other. All such organizations need to collect data, manipulate them and store them for future use. All such type of organization requires data of a number of purpose say: 1. Preparing sales report. 2. Forecasting sales. 3. Preparing accounts reports. 4. Getting medical histories of patients. Thus we can say that data are very vital corporate resources. The amount of data used these days in organization can be measured in trillions of bytes. The financial investment involved is very high. Many organizations have become critically dependent on the continued and successful operations of database. Relational databases: A Relational database is defined as a database that allows you to group its data items into one or more independent tables that can be related to one another by using fields common to each related table. In Relational Database model, data is organized in the form of rows and columns similar to a table. The tables are referred to as relation in a relational data model. Rows of a table are referred to as tuples and the columns of a table are referred to as attribute. Tables in a relational database is similar to the traditional file system Material downloaded from mycbseguide.com. 1 / 3

2 with its records, fields and files. The tables rows contain records and columns contains fields. Each tuple or row has values for the attributes. The attributes describes some property or features of the objects. The full two dimensional table represents a file or relation. For example there is a Employee-service relation that describes the entity Employee by the attribute Emp_code, Name and years of service (years). EMP_CODE Name Years 1234 Harsh Yash Amit Wasim 8 Employee-service relation Here EMP_CODE, Name and years are called Attributes. 1234, harsh and 2 are tuples. Relation; All database items and relationships among them are represented in a two dimensional table called a relation. For example Employee-service table shown above is called relation. A database constructed using relations referred to as a relational database. Relational database is constructed from the arrangement of data items. All relations have three components: 1. Name: Name is represented by the title or the entity-identifier. For example Employee name and years of services in above table. 2. Degree: The number of columns associated with a table or relation among them is called the degree. In Employee-service relation table degree is 3 i.e. there are 3 attributes or fields associated with the table. 3. Cardinality: The number of rows in a table is called cardinality. In our example there are 4 records so its cardinality is 4. Keys: The key is the data item that exclusively identifies a record. For example Account_number, Product_code, Employee_number are used as key fields because they specifically identify a record stored in a database. Primary Key: The primary key uniquely identifies each record in a table and must never be Material downloaded from mycbseguide.com. 2 / 3

3 the same for two records. For example Emp-code can be primary key for the entity set Employees. The primary key should be chosen such that its attributes are never very rarely changed. For instance the address field of a person should not be part of the primary key since it s likely to change. EMP_CODE on the other hand is not changed till he is working in the organization. Foreign key: In a relation the column whose data values correspond to the values of a key column in another relation is called a foreign key. Foreign key of a relation may be the primary key of another relation. For example we have 2 tables Product and Supplier Material downloaded from mycbseguide.com. 3 / 3

4 CBSE Revision Notes Class-11 Computer Science (New Syllabus) Unit 3: Data Management (DM-1) SQL Commands CREATE TABLE: Tables are defined with the create table command. Create table command basically defines a table name describing a set of named columns in a specified order. It also defines data type and size of the columns. Each table must have 1 column. Syntax for create table command is as follows CREATE TABLE Products ( Product_code int PRIMARY KEY, Product_Name varchar(10) NOT NULL, Supp_CODE int NOT NULL ); In the table product product_code is of type integer and it s a primary key. Product_name is of type varchar which means it can hold characters and NOT NULL means it s value cannot be NULL. Supp_CODE is of type integer. CREATE TABLE Supplier ( Supp_CODE int PRIMARY KEY, Sup_Name varchar(10) NOT NULL, FOREIGN KEY (Supp_CODE) REFERENCES Products (Supp_CODE) ); Material downloaded from mycbseguide.com. 1 / 4

5 Here we are creating a table called supplier here Supp_code is primary key and it s also a foreign key. INSERT: The INSERT INTO statement is used to insert new records in a table. It s syntax is as follows INSERT INTO table_name VALUES (value1, Value2, Value3,.); INSERT INTO product VALUES (1234, Jeans, 54321); This line will add values in table called product and will put value in their respective columns. DELETE: The DELETE statement is used to delete existing records in a table. Syntax of DELETE command is as follows. DELETE FROM table-name where condition; DELETE FROM product where product_code=1234; This will delete the record whose product code is 1234 DELETE FROM product where Product_name= Jeans ; This command will delete all the records whose product name is jeans. If we ll omit where clause DELETE FROM product; It will delete all the records in the table DROP TABLE: We can also delete the structure of the table completely by drop command DROP TABLE product; It will delete the table itself. NOTE: DELETE command only deletes the records and DROP command deletes whole table. SELECT : SELECT command retrieves data from the table. This means that by executing Material downloaded from mycbseguide.com. 2 / 4

6 SELECT statement information currently in table will be shown on the screen. Syntax of SELECT command SELECT * FROM product; This will display all the records from product table SELECT Product_name FROM product; This will display all the product names from product table SELECT product_name, sup_code FROM product WHERE product_code=1234 It will display the name and supplier code whose product code is JOINS: A JOIN clause is used to combine rows from two or more tables, based on a related column between them. This can be done using SELECT command. Let s understand this with an example suppose we have 2 table s products and supplier Product table Product_code Product_Name Supp_code 123 Jeans Shirts Tshirts 333 Supplier table Supp_code Supp_name 321 DENIM 322 Adidas 333 Reebok 334 Jockey SELECT product.product_name, Supplier.Supp_name FROM product, supplier WHERE Material downloaded from mycbseguide.com. 3 / 4

7 product.supp_code=supplier.supp_code; Output of this query will be Product_name Jeans Shirts Tshirts Supp_name DENIM Adidas Reebok This command will select product name from product table and supplier table and will display those records where sup_code of both the records are matching. INDEX: The CREATE INDEX statement is used to create indexes in tables. Indexes are used to retrieve data from the database very fast. The users cannot see the indexes, they are just used to speed up searches/queries. Updating a table with indexes takes more time than updating a table without indexes because indexes also need an update. So create indexes on only those columns that will be frequently searched against. Syntax to create indexes are as follows. CREATE INDEX index_name ON table_name (column1, column2..); This command creates and indexes and duplicates are allowed in this. CREATE UNIQUE INDEX index_name ON table_name (column1, column2,...); This command creates indexes which are unique and does not allow duplicates. CREATE INDEX lname ON person(last_name) Material downloaded from mycbseguide.com. 4 / 4