SQL Revision – Database Concepts & SQL Commands

Before learning advanced SQL functions in Class 12, it is important to revise the database concepts and SQL commands studied in Class XI. These concepts form the foundation for writing queries, retrieving data, and managing databases efficiently.

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What is Data?

Data refers to raw facts and figures that can be processed to produce meaningful information.

Examples: Student names, marks, employee IDs, salaries.

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What is a Database?

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What is DBMS?

A Database Management System (DBMS) is software used to create, store, retrieve, update, and manage databases.

Examples: MySQL, Oracle, Microsoft Access, PostgreSQL.

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What is RDBMS?

A Relational Database Management System (RDBMS) stores data in the form of related tables and supports relationships between them.

Examples: MySQL, Oracle, SQL Server.

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DBMS vs RDBMS

DBMS	RDBMS
Stores data in files or tables.	Stores data in related tables.
Relationships are limited.	Supports relationships using keys.
Less secure.	More secure.
Suitable for small applications.	Suitable for large applications.

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Basic Database Terminology

Term	Description
Table	Collection of rows and columns.
Record (Row)	A single entry in a table.
Field (Column)	A single attribute of a record.
Tuple	Another name for a row.
Attribute	Another name for a column.

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Keys in a Database

Primary Key

A Primary Key uniquely identifies each record in a table.

Example: RollNo, EmployeeID.

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Candidate Key

A Candidate Key is any column that can uniquely identify a record.

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Alternate Key

A Candidate Key that is not selected as the Primary Key becomes an Alternate Key.

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Foreign Key

A Foreign Key is a field in one table that refers to the Primary Key of another table.

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What is SQL?

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Categories of SQL Commands

Category	Purpose	Examples
DDL	Define database objects	CREATE, ALTER, DROP
DML	Manipulate data	INSERT, UPDATE, DELETE
DQL	Retrieve data	SELECT
DCL	Control permissions	GRANT, REVOKE
TCL	Transaction control	COMMIT, ROLLBACK

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Creating a Table

CREATE TABLE Student
(
    RollNo INT PRIMARY KEY,
    Name VARCHAR(30),
    Marks INT,
    City VARCHAR(20)
);

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Inserting Records

INSERT INTO Student
VALUES
(101,'Amit',85,'Jaipur');

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Viewing Records

SELECT * FROM Student;

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Selecting Specific Columns

SELECT Name, Marks
FROM Student;

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WHERE Clause

The WHERE clause filters records based on a condition.

SELECT *
FROM Student
WHERE Marks > 80;

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DISTINCT Keyword

Returns only unique values.

SELECT DISTINCT City
FROM Student;

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LIKE Operator

Used for pattern matching.

SELECT *
FROM Student
WHERE Name LIKE 'A%';

Returns all names starting with A.

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BETWEEN Operator

SELECT *
FROM Student
WHERE Marks BETWEEN 70 AND 90;

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IN Operator

SELECT *
FROM Student
WHERE City IN
('Jaipur','Delhi');

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Updating Records

UPDATE Student
SET Marks=90
WHERE RollNo=101;

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Deleting Records

DELETE FROM Student
WHERE RollNo=101;

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NULL Values

A NULL value means that data is missing or unknown.

Finding NULL Values

SELECT *
FROM Student
WHERE City IS NULL;

Finding Non-NULL Values

SELECT *
FROM Student
WHERE City IS NOT NULL;

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Common SQL Operators

Operator	Purpose
=	Equal to
>	Greater than
<	Less than
>=	Greater than or equal to
<=	Less than or equal to
<> or !=	Not equal to

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Real-Life Applications

Field	Database Example
School	Student Management System
Hospital	Patient Records
Bank	Customer Accounts
Library	Book Management
E-Commerce	Orders and Products

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Common Errors

Error	Reason
Syntax Error	Incorrect SQL syntax.
Unknown Column	Column name does not exist.
Duplicate Entry	Duplicate value in Primary Key.
Table Doesn't Exist	Incorrect table name.

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Quick Revision

Concept	Remember
Database	Collection of related data
DBMS	Database management software
RDBMS	Stores related tables
Primary Key	Uniquely identifies a record
Foreign Key	Creates relationship between tables
SELECT	Retrieve data
INSERT	Add records
UPDATE	Modify records
DELETE	Remove records

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CBSE Exam Tips

• Revise all SQL commands from Class XI before learning SQL functions.
• Remember the categories of SQL commands (DDL, DML, DQL, DCL, TCL).
• Understand the difference between Primary Key and Foreign Key.
• Practice writing SQL queries using WHERE, LIKE, BETWEEN, and IN.
• Always use IS NULL and IS NOT NULL for NULL values.

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Summary

SQL is the standard language for managing relational databases. Before studying advanced SQL functions in Class 12, students should have a clear understanding of database concepts, keys, SQL command categories, and basic SQL statements such as CREATE, INSERT, SELECT, UPDATE, and DELETE. These concepts form the foundation for writing efficient database queries.