SQL Equi Join (Working with Two Tables)

In a relational database, information is often stored in multiple related tables instead of one large table. To retrieve related information from two or more tables, SQL provides the concept of Joins.

According to the CBSE Class 12 Informatics Practices syllabus, students are required to learn the Equi Join, which joins two tables using matching values in common columns.

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Why Do We Need Multiple Tables?

Storing all information in a single table creates data redundancy and makes database management difficult. Splitting data into related tables improves organization, reduces duplication, and increases efficiency.

Example:

• The Student table stores student details.
• The Course table stores course details.
• A common column such as CourseID links both tables.

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Primary Key and Foreign Key

Primary Key

A Primary Key uniquely identifies each record in a table.

Example: StudentID, CourseID

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Foreign Key

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

It establishes a relationship between the tables.

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Sample Tables

Student Table

StudentID	Name	CourseID
101	Amit	C01
102	Neha	C02
103	Rohan	C01
104	Priya	C03

Course Table

CourseID	CourseName	Fee
C01	Python	5000
C02	SQL	4500
C03	AI	6000

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What is an Equi Join?

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Syntax of Equi Join

SELECT column_list
FROM Table1, Table2
WHERE Table1.CommonColumn =
      Table2.CommonColumn;

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Example 1: Display Student Name and Course Name

SELECT Student.Name,
       Course.CourseName
FROM Student, Course
WHERE Student.CourseID =
      Course.CourseID;

Output

Name	CourseName
Amit	Python
Neha	SQL
Rohan	Python
Priya	AI

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Example 2: Display Student Name and Course Fee

SELECT Student.Name,
       Course.Fee
FROM Student, Course
WHERE Student.CourseID =
      Course.CourseID;

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Example 3: Display All Student and Course Details

SELECT *
FROM Student, Course
WHERE Student.CourseID =
      Course.CourseID;

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Using Table Aliases

Table aliases make SQL queries shorter and easier to read.

SELECT S.Name,
       C.CourseName
FROM Student S,
     Course C
WHERE S.CourseID =
      C.CourseID;

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Another Example

Employee Table

EmpID	Name	DeptID
1	Amit	D01
2	Neha	D02
3	Rohan	D01

Department Table

DeptID	Department
D01	Sales
D02	Finance

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Example Query

SELECT Employee.Name,
       Department.Department
FROM Employee,
     Department
WHERE Employee.DeptID =
      Department.DeptID;

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How an Equi Join Works

Student Table
     │
     │ CourseID
     ▼
Course Table
     │
Matching Values
     ▼
Combined Result

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Advantages of Equi Join

• Combines related information from different tables.
• Reduces duplicate data.
• Maintains database normalization.
• Produces meaningful reports.
• Improves database organization.

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Real-Life Applications

Field	Example
School	Students and Classes
Hospital	Patients and Doctors
Bank	Customers and Accounts
Library	Books and Publishers
E-Commerce	Products and Categories

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Common Errors

Error	Reason
Unknown Column	Incorrect column name.
Ambiguous Column	Same column name exists in both tables. Use table names or aliases.
Missing Join Condition	Produces a Cartesian Product instead of an Equi Join.
Syntax Error	Incorrect SQL syntax.

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Quick Revision

Concept	Remember
Primary Key	Uniquely identifies each record.
Foreign Key	Links two tables.
Equi Join	Uses the = operator.
Table Alias	Short name for a table.
Join Condition	Written in the WHERE clause.

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CBSE Exam Tips

• Always identify the common column between the tables.
• Use the equality operator (=) in an Equi Join.
• Remember that the join condition is written in the WHERE clause in the CBSE syllabus style.
• Use table aliases to make queries shorter and easier to understand.
• Do not forget the join condition; otherwise, a Cartesian Product may be generated.

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Summary

An Equi Join combines related records from two tables using matching values in a common column. It is one of the most frequently used SQL operations in relational databases and plays an important role in retrieving meaningful information stored across multiple tables. Understanding Primary Keys, Foreign Keys, and join conditions is essential for solving CBSE Class 12 Informatics Practices SQL queries and practical problems.