SQL Left Join

The SQL Left Join query displays all the records from the table and displays similar records from the right table. The query displays zero records if it doesn’t find any similar records. If similar records are not found in the right table, the NULL keyword is returned in the row of the left table.

Syntax of the SQL Left Join

SELECT Column_Name1, Column_Name2, Column_Name3, Column_Name4 FROM Table_1 LEFT JOIN Table_2 ON Expression;  

Column_Name1, Column_Name2, Column_Name3, Column_Name4 columns from both the table.

Left Table: Table_1, Right Table: Table_2.

Expression: Condition expression with ON clause.

This conditional expression is a comparison condition of comparison operators and logical operators of SQL. Remember that Comparison Operators are >, <, >=, <=, =, !=, LIKE, BETWEEN, etc., and Logical Operators are AND, OR, and NOT.

There is one too many relationships between the Student and the Course table. One to Many relationships means that one Student can apply for one course or more than one course. When we execute the SQL Left Join query on these tables on Students_Id and Course_Id, all the records of the student table are displayed in the result set with similar records in the Course records.

Students who have not applied for any courses will be in the result set with the NULL keyword from the right side table.  

Example of Left Join in Structured Query Language:

Table Number 1: Student

Student_Id	Student_Name	City	Age
1	Pratik Srivastav	Pune	23
2	Utkarsh Rokade	Mumbai	22
3	Sourabh Chougale	Nashik	23
4	Prateek Zimbre	Pune	24
5	Sakshi Patil	Aurangabad	22
6	Shruti Sharma	Mumbai	21
7	Pranoti Shende	Aurangabad	23
8	Harshada Dhanwat	Nashik	24
9	Tejas Bairagi	Nashik	21
10	Nikhil Patil	Pune	24
11	Samaira Sharma	Mumbai	22
12	Anushka Sen	Aurangabad	23

Table Number 2: Course

Course_Id	Course_Name	Student_Id	Duration
11	Cloud Computing	2	3
12	SQL Database	1	1
13	Advance Java	4	3
14	Data Structures	7	6
15	AWS	8	2
16	Angular Js	10	3
17	Oracle Integration Cloud	11	6
18	Python	6	1
19	ReactJs	1	3
20	Computing	6	4

Example 1: Execute a left joins query on the Student and Course table name.

SELECT S.Student_Id, S.Student_Name, S.City, S.Age, C.Course_Id, C.Course_Name, C.Duration FROM Student S LEFT JOIN Course C ON S.Student_Id = C.Student_Id;

The above left join query joins the employees' table and course table and retrieve the data from the tables where S.Student_Id = C.Student_Id.

The output of the above query is as follows:

Student_Id	Student_Name	City	Age	Course_Id	Course_Name	Duration
1	Pratik Srivastav	Pune	23	12	SQL Database	1
1	Pratik Srivastav	Pune	23	19	ReactJs	3
2	Utkarsh Rokade	Mumbai	22	11	Cloud Computing	3
3	Sourabh Chougale	Nashik	23	NULL	NULL	NULL
4	Prateek Zimbre	Pune	24	13	Advance Java	3
5	Sakshi Patil	Aurangabad	22	NULL	NULL	NULL
6	Shruti Sharma	Mumbai	21	18	Python	1
6	Shruti Sharma	Mumbai	21	20	Computing	4
7	Pranoti Shende	Aurangabad	23	14	Data Structures	6
8	Harshada Dhanwat	Nashik	24	15	AWS	2
9	Tejas Bairagi	Nashik	21	NULL	NULL	NULL
10	Nikhil Patil	Pune	24	16	Angular Js	3
11	Samaira Sharma	Mumbai	22	17	Oracle Integration Cloud	6
12	Anushka Sen	Aurangabad	23	NULL	NULL	NULL

Example 2: Execute a left join query on the Student and Course table name using the WHERE Clause.

SELECT S.Student_Id, S.Student_Name, S.City, S.Age, C.Course_Id, C.Course_Name, C.Duration FROM Student S LEFT JOIN Course C ON S.Student_Id = C.Student_Id WHERE City IN ('Pune', 'Mumbai', 'Aurangabad');

The above left join query joins the employees' table and manager table and retrieve the data from the tables where S.Student_Id = C.Student_Id. The Student records show students who reside in the 'Pune', 'Mumbai' and 'Aurangabad' cities. We have applied the WHERE clause condition on the City of the Student table.

The output of the above query is as follows:

Student_Id	Student_Name	City	Age	Course_Id	Course_Name	Duration
1	Pratik Srivastav	Pune	23	12	SQL Database	1
1	Pratik Srivastav	Pune	23	19	ReactJs	3
2	Utkarsh Rokade	Mumbai	22	11	Cloud Computing	3
4	Prateek Zimbre	Pune	24	13	Advance Java	3
5	Sakshi Patil	Aurangabad	22	NULL	NULL	NULL
6	Shruti Sharma	Mumbai	21	18	Python	1
6	Shruti Sharma	Mumbai	21	20	Computing	4
9	Tejas Bairagi	Nashik	21	NULL	NULL	NULL
10	Nikhil Patil	Pune	24	16	Angular Js	3
11	Samaira Sharma	Mumbai	22	17	Oracle Integration Cloud	6
12	Anushka Sen	Aurangabad	23	NULL	NULL	NULL

Example 3: Execute a left join query on the Student and Course table name using the ORDER BY Clause.

SELECT S.Student_Id, S.Student_Name, S.City, S.Age, C.Course_Id, C.Course_Name, C.Duration FROM Student S LEFT JOIN Course C ON S.Student_Id = C.Student_Id ORDER BY Student_Name;

The above left join query joins the employees' table and manager table and retrieves the data from the tables where S.Student_Id = C.Student_Id and the Student's records will display in the ascending order as we used the ORDER BY clause in the query.

The output of the above query is as follows:

Student_Id	Student_Name	City	Age	Course_Id	Course_Name	Duration
12	Anushka Sen	Aurangabad	23	NULL	NULL	NULL
8	Harshada Dhanwat	Nashik	24	15	AWS	2
10	Nikhil Patil	Pune	24	16	Angular Js	3
7	Pranoti Shende	Aurangabad	23	14	Data Structures	6
4	Prateek Zimbre	Pune	24	13	Advance Java	3
1	Pratik Srivastav	Pune	23	12	SQL Database	1
1	Pratik Srivastav	Pune	23	19	ReactJs	3
5	Sakshi Patil	Aurangabad	22	NULL	NULL	NULL
11	Samaira Sharma	Mumbai	22	17	Oracle Integration Cloud	6
6	Shruti Sharma	Mumbai	21	18	Python	1
6	Shruti Sharma	Mumbai	21	20	Computing	4
3	Sourabh Chougale	Nashik	23	NULL	NULL	NULL
9	Tejas Bairagi	Nashik	21	NULL	NULL	NULL
2	Utkarsh Rokade	Mumbai	22	11	Cloud Computing	3