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Home Data Science Data Science Tutorials SQL Tutorial Join Query in SQL

Join Query in SQL

By Roja MetlaRoja Metla

Join Query in SQL

Introduction to Join Query in SQL

In the below section let us see what are joins and different types of joins with examples.

What are Joins?

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Joins are used to get data from more than one table. To join more than one table we need at least one column common in both tables. Tables get joined based on the condition specified. “ON” keyword is used. There are different types of joins in SQL as below.

Different Types of Joins

  • INNER Join
  • LEFT Join
  • RIGHT Join
  • FULL Join

1. INNER Join

Inner join gets all the rows that are common in both tables based on the condition specified. Let us take an example of the inner join.

Example:

Below represents the Venn diagram of the inner join.

Join Query in SQL-1.1

Here let’s consider table A and table B. As Inner join considers the common rows of both the tables. The shaded area represents the common rows of both table.

Syntax:

SELECT * FROM TABLE_A A
INNER JOIN TABLE_B B
ON A. Common_COLUMN =B. Common_COLUMN

2. LEFT Join

Left Join gets all the rows from the Left table and common rows of the both table. Let us take an example of the left join.

Example:

Below represents the Venn diagram of the left join.

In below diagram Table A is left join to the table B. Here all the rows from the table A gets considered and common rows from both table.

Join Query in SQL-1.2

Syntax:

SELECT * FROM TABLE_A A
LEFT JOIN TABLE_B B
ON A. Common_COLUMN =B. Common_COLUMN

3. RIGHT Join

Right Join gets all the rows from the Right table and common rows of the both table. Let us take an example of the right join.

Example:

Below represents the Venn diagram of the right join.

Join Query in SQL-1.3

In below diagram Table A is right join to the table B. Here all the rows from the table B gets considered and common rows from both table.

Syntax:

SELECT * FROM TABLE_A A
RIGHT JOIN TABLE_B B
ON A.Common_COLUMN=B.Common_COLUMN

4. FULL Join

Full Join gets all the rows from the both tables. Let us take an example of the right join.

Example:

Below represents the Venn diagram of the FULL join.

Join Query in SQL-1.4

Syntax:

SELECT * FROM TABLE_A A
FULL JOIN TABLE_B B
ON A. Common_COLUMN =B. Common_COLUMN

The result set contains NULL set values. Below syntax can be used to neglect the NULL values: –

SELECT * FROM TABLE_A A
FULL JOIN TABLE B B
ON A. Common_COLUMN =B. Common_COLUMN
WHERE A.Common_COLUMN IS NULL
AND A.Common_COLUMN IS NULL

How to Use Join Query in SQL with Examples

Here we discuss the uses of join query with examples:

1. Left Join

Left Join = All rows from left table + INNER Join

Example:

Let us consider two tables and apply Left join on the tables: –

Loan Table:

Loan_no Loan_status State Loan_Amount Ap_Date
12 Open Ranchi 30000 01-02-2015
23 Close Patna 50000 03-04-2017
31 Pending Kolkata 80000 09-07-2018
43 Approval Electronic city 54000 10-11-2019
11 Rejected Bangalore 43000 03-04-2017
33 Close Patna 90000 09-07-2018
44 Open Kolkata 67000 01-01-2020

Borrower Table:

Loan_no Borrower_Name Borrower_Date Bank_id
12 Ram 09-07-2014 A1
27 Sundar 19-06-2016 A2
43 Browny 16-07-2019 A4
31 Blacky 09-07-2015 A2

Query to get the loan_no, status and borrower date from two tables: –

Query:

SELECT L.LOAN_NO, L.LOAN_STATUS,B.BORROWER_DATE
FROM LOAN L LEFT JOIN BORROWER B
ON L.LOAN_NO=B.LOAN_NO

Let’s check the output of the above table after applying the Left join on them.

Output:

Join Query in SQL-1.5

2. RIGHT Join

RIGHT Join = All rows from RIGHT table + INNER Join

Example:

Let us consider two tables and apply RIGHT join on the tables: –

Query to get the loan_no, status and borrower date from two tables: –

Query: 

SELECT L.LOAN_NO, L.LOAN_STATUS,B.BORROWER_DATE
FROM LOAN L RIGHT JOIN BORROWER B
ON L.LOAN_NO=B.LOAN_NO

Let’s check the output of the above table after applying the right join on them.

Output:

Join Query in SQL-1.6

3. INNER Join

Inner Join = All common rows from both table. While joining at least one column should be of same data type and common among tables.

Example:

Let us consider two tables and apply INNER join on the tables: –

Let us build a query to get the loan_no, status and borrower date from two tables: –

Query:

SELECT L.LOAN_NO,L.LOAN_STATUS,B.BORROWER_DATE
FROM LOAN L INNER JOIN BORROWER B
ON L.LOAN_NO=B.LOAN_NO

Let’s check the output of the above table after applying the inner join on them.

Output:

Here in the above output we got the common rows of both table based on the condition “L.LOAN_NO=B.LOAN_NO”.

Join Query in SQL-1.7

4. FULL OUTER Join

FULL OUTER Join = All rows from both table. While joining at least one column should be of same data type and common among tables.

Example:

Let us consider two tables and apply FULL OUTER join on the tables: –

Let us build a query to get the ap_date and borrower date from two tables: –

Query: 

SELECT L.LOAN_NO,L.LOAN_STATUS,B.BORROWER_DATE
FROM LOAN L FULL OUTER JOIN BORROWER B
ON L.LOAN_NO=B.LOAN_NO

Let’s check the output of the above table after applying the inner join on them.

Output:

Here in the above output we got the common rows of both table based on the condition “L.LOAN_NO=B.LOAN_NO”.

Join Query in SQL-1.8

Conclusion

To fetch data relevant to the customer requirement we might need to join tables which will be fulfilled by joins. As mentioned earlier joins are used to get data from more than one table. To join more than one table we need at least one column common in both tables. Tables get joined based on the condition specified.

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