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Difference Between Inner Join And Left Join In SQL Server

Introduction

 
In this article, I am going to explain  what is the difference between Inner join and Left join with examples. This is one of the very common SQL server interview questions.
 
Here we will be using SQL Server 2017 or you can use SQL Server 2008 or above.
 
Read my previous Joins in SQL Server 2017 part of this article using the below links before reading this article,

Definition of Joins

 
It is used to fetch/retrieve data from two or more related tables from data base. In general, tables are related to each other using foreign key constraints.
 
Prerequisites
 
SQL Server 2017 or you can use SQL server 2008 or above version.
 
Now, first we will create a Database and two tables to apply the joins for understanding.
 

Creating Database and Two Tables

 
Step 1 - Create a Database
 
Open your SQL Server and use the following script to create the “chittadb” Database.
  1. Create database chittadb 
Now, select the script query then press F5 or click on Execute button to execute the above script.
 
You should see a message, “Command(s) completed successfully.” This means your new database created.
 
Step 2 - Create first table
 
Open your SQL Server and use the following script to create table “tbl_Department”.
  1. create table tbl_Department  
  2. (  
  3. DeptId int primary key not null identity(1,1),  
  4. DeptName nvarchar(50),  
  5. DeptHead nvarchar(50),  
  6. Location nvarchar(100)  
  7. ) 
Execute the above query to create “tbl_Department “.
 
You should see a message, “Command(s) completed successfully.”
 
Now, data is inserted into the table.
  1. Insert into tbl_Department values ( 'IT', 'Chitta', 'Chennai')  
  2. Insert into tbl_Department values ( 'Payroll', 'Akhil', 'Odisha')  
  3. Insert into tbl_Department values ( 'HR', 'Ram', 'Pune')  
  4. Insert into tbl_Department values ( 'Timesheet', 'Kannan', 'chennai') 
Execute the above query, you should see a message, “Command(s) completed successfully.”
 
Now retrieve all data from “tbl_Department” table.
  1. select * from tbl_Department 
output
 
Difference Between Inner Join And Left Join In SQL Server 
Step 3 - Create second table
 
Open your SQL Server and use the following script to create table “tbl_Employee”.
  1. Create table tbl_Employee  
  2. (  
  3.      EmpID int primary key not null identity(1,1),  
  4.      Name nvarchar(50),  
  5.      Gender nvarchar(50),  
  6.      country nvarchar(20),  
  7.      Salary int,  
  8.      DepartmentId int foreign key references tbl_Department(DeptId)  
  9. ) 
Execute the above query to create “tbl_Employee “.
 
You should see a message, “Command(s) completed successfully.”
 
Now, data is inserted into the table.
  1. Insert into tbl_Employee values ( 'Jitu', 'Male','India',4000, 1)  
  2. Insert into tbl_Employee values ( 'Rani', 'Female','India', 5000, 3)  
  3. Insert into tbl_Employee values ( 'Rohit', 'Male','India', 5500, 1)  
  4. Insert into tbl_Employee values ( 'Dibas', 'Male','India', 6500, 2)  
  5. Insert into tbl_Employee values ( 'Gajendra', 'Male','India', 3800, 2)  
  6. Insert into tbl_Employee values ( 'Raja', 'Male','India', 9000, 1)  
  7. Insert into tbl_Employee values ( 'Jeni', 'Female','India', 5800, 3)  
  8. Insert into tbl_Employee values ( 'Chandin', 'Female','India', 8500, 1)  
  9. Insert into tbl_Employee values ( 'pintu', 'Male','India', 9500, NULL)  
  10. Insert into tbl_Employee values ( 'Subrat', 'Male','India', 9800, NULL) 
 Execute the above query, you should see a message, “Command(s) completed successfully.”
 
Now retrieve all data from “tbl_Employee” table.
  1. select * from tbl_Employee 
Output
 
Difference Between Inner Join And Left Join In SQL Server
 
General Formula for Joins
  1. SELECT      ColumnList (whatever column you want to display)   
  2. FROM           LeftTableName   
  3. JOIN_TYPE  RightTableName   
  4. ON                 JoinCondition 

 INNER JOIN

 
Inner join returns only the matching rows between both the tables, non-matching rows are eliminated.
 
Example
 
Write a query to retrieve Name, Gender, Country, Salary and DeptName from tbl_Employee and tbl_Department table.
 
INNER JOIN Query
  1. SELECT Name, Gender,country, Salary, DeptName   
  2. FROM tbl_Employee  
  3. INNER JOIN tbl_Department  
  4. ON tbl_Employee.DepartmentId = tbl_Department.DeptId 
OR
  1. SELECT Name, Gender,country, Salary, DeptName   
  2. FROM tbl_Employee  
  3.  JOIN tbl_Department  
  4. ON tbl_Employee.DepartmentId = tbl_Department.DeptId 
Note
JOIN or INNER JOIN  are the same. It's always better to use INNER JOIN.
 
OR
  1. select emp.Name,emp.Gender,emp.country,emp.Salary,dept.DeptName   
  2.     from tbl_Employee emp   
  3.     inner join tbl_Department dept  
  4.     on emp.DepartmentId=dept.DeptId 
Output
 
Difference Between Inner Join And Left Join In SQL Server 
 

LEFT JOIN or LEFT OUTER JOIN

 
Left Join or Left Outer Join returns only the matching rows between both the tables, plus non matching rows from the left table.
 
Example
 
Write a query to retrieve Name, Gender, Country, Salary and DeptName from tbl_Employee and tbl_Department table.
 
LEFT JOIN or LEFT OUTER JOINQuery
  1. SELECT Name, Gender,country, Salary, DeptName   
  2. FROM tbl_Employee  
  3. LEFT OUTER JOIN tbl_Department  
  4. ON tbl_Employee.DepartmentId = tbl_Department.DeptId 
OR
  1. SELECT Name, Gender,country, Salary, DeptName   
  2. FROM tbl_Employee  
  3. LEFT JOIN tbl_Department  
  4. ON tbl_Employee.DepartmentId = tbl_Department.DeptId 
Note
You can use, LEFT JOIN or LEFT OUTER JOIN. OUTER keyword is optional.
 
OR
  1. select emp.Name,emp.Gender,emp.country,emp.Salary,dept.DeptName   
  2.     from tbl_Employee emp   
  3.     LEFT JOIN tbl_Department dept  
  4.     on emp.DepartmentId=dept.DeptId 
OutPut
 
Difference Between Inner Join And Left Join In SQL Server 
 

What is the difference between INNER JOIN and LEFT JOIN

 
INNER JOIN
 
Inner join returns only the matching rows between both the tables, non matching rows are eliminated.
 
Difference Between Inner Join And Left Join In SQL Server 
 
LEFT JOIN
 
 Left Join or Left Outer Join returns only the matching rows between both the tables, plus non matching rows from the left table.
 
Difference Between Inner Join And Left Join In SQL Server 

What is the Difference between INNER JOIN and JOIN


There is no difference between inner join and join, they are exactly the same. 
  1. LEFT JOIN and LEFT OUTER JOIN  
  2. RIGHT JOIN and RIGHT OUTER JOIN  
  3. FULL JOIN and FULL OUTER JOIN 

Conclusion

 
In this article, I explained  the difference between inner join and left join in SQL Server with examples. I hope this article has helped you to understanding this topic. Post your valuable feedback in the comments section.


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