SQL Full Join

Updated March 27, 2023

Overview of SQL Full Join

A SQL FULL join is a structured query language (SQL) statement in which when two tables are joined together, the statement returns all the rows from the joined tables even if all the rows do not meet the specified conditions, however, the non-matched rows in any of the tables are displayed as NULL, that is a FULL JOIN can be considered something similar to a combination of LEFT AND RIGHT JOINS in SQL.

Syntax:

SELECT column_name(s)
FROM table_name1
FULL JOIN table_name2
ON table_name1.column_name = table_name2.column_name
WHERE condition;</code

Parameters:

The different parameters used in the syntax are :

• SELECT column_name(s): It is used to select the required data from the database. Here, column_name(s) is the name of the columns from the joined tables.
• FROM table_name1 FULL JOIN table_name2: It is used to specify the source from which data has to be fetched. Here, table_name1 is the name of the left table and table_name2 is the name of the right table. FULL JOIN will fetch all records when there is a match in the left or right table records.
• ON table_name1.column_name = table_name2.column_name: It is used to specify the common conditions on which the two tables will be joined. It can be a pair of primary and foreign keys.
• WHERE condition: It is used to specify any other conditions that the records should satisfy to make to the resultant records.

Of the above-mentioned parameters, all the parameters except the WHERE clause are mandatory. You may use GROUP BY, ORDER BY and HAVING clauses based on your requirement.

How does SQL Full Join work?

A SQL FULL join is typically used in visualizing exception reports or ETL or any other peculiar situations where both sides have data you are trying to combine.

The following Venn diagram explains how a SQL full join works.

Going ahead we will be discussing the above-mentioned self join in great detail.

In order to demonstrate and explain the FULL join in SQL effectively, we will be using the following tables. These tables are made for an e-commerce website. The first table “customers ”contains customer id, names, city to which they belong. The second table “cities” contains the id, city, and country to which they belong.

The schema for the above mentioned “customers” table is:

Number of records: 15

Customers

ID(primary key)

Customer

City

Let’s have a look at the records in the customer’s table. So that later, we can understand how self-join is helpful:

ID	Customer	City	Items_purchased	Amount_paid
1	Peter King	Manchester	Books	120
2	Priya Krishna	New Delhi	pen	50
3	Jim Halpert	Manchester	pencil	43
4	Michael Scott	New York	Books	250
5	Harvey Spector	Birmingham	pencil	100
6	Deepa Kamat	Mumbai	Books	370
7	Anita Desai	London	pencil	50
8	Rachel Zane	Michigan	pen	70
9	Pretoria John	Canberra	pen	190
10	John L	Budapest	Books	540
11	Justin Green	Ottawa City	pen	65
12	Babita Ghosh	Kolkata	pencil	75
13	Krish Pratt	London	eraser	30
14	Elizabeth Blunt	London	pencil	340
15	Nina Debrov	Amsterdam	Books	452

The schema for “cities” table is:

Number of Records: 10

Customers

ID(primary key)

city_name

country

Let’s have a look at the records in the cities table.

ID	City_Name	Country
1	New Delhi	India
2	Mumbai	India
3	Kolkata	India
4	London	England
5	Manchester	England
6	Ottawa City	Canada
7	Ontario	Canada
8	Pune	India

Examples to Implement SQL Full Join

Some of the examples are given below:

Example #1

Find the names of customers whose resident cities are not present in the cities table.

Code:

SELECT t1.Customer, t1.City, t2.city_name
FROM customers as t1 FULL JOIN cities as t2
ON t1.City = t2.city_name;

Output:

In the above example, we can notice that cities like Birmingham, Michigan, Canberra, Budapest, and Amsterdam are not present in the cities table but they are still shown on FULL join with NULL values. Similarly, cities like Ontario, Pune and Washington D.C are not present in customers but were present in the cities table are also displayed with NULL values for not matching records.

Example #2

Find the names of customers along with the country to which they belong.

Code:

SELECT t1.Customer, t2.country
FROM customers as t1 FULL JOIN cities as t2
ON t1.City = t2.city_name;

Output:

Example #3

Find the names of customers who purchased items in the books category, along with the country to which they belong.

Code:

SELECT t1.Customer, t2.country
FROM customers as t1 FULL JOIN cities as t2
ON t1.City = t2.city_name
WHERE Items_purchased = 'Books';

Output:

Example #4

Find the number of customers per country ordered in descending order.

Code:

SELECT t2.country, count(t1.Customer) as No_of_Customers
FROM customers as t1 FULL JOIN cities as t2
ON t1.City = t2.city_name
GROUP BY t2.country
ORDER BY 2 DESC;

Output:

Example #5

Find the total revenue generated by the e-commerce company across different countries ordered from highest to lowest.

Code:

SELECT t2.country, sum(t1.Amount_paid ) as "Total Revenue Generated"
FROM customers as t1 FULL JOIN cities as t2
ON t1.City = t2.city_name
GROUP BY t2.country
ORDER BY 2 DESC;

Output:

Example #6

Find the total revenue generated by the e-commerce company in the Books and pencils category across different countries ordered from highest to lowest.

Code:

SELECT t2.country, sum(t1.Amount_paid ) as "Total Revenue Generated", t1.Items_purchased
FROM customers as t1 FULL JOIN cities as t2
ON t1.City = t2.city_name
WHERE t1.Items_purchased = 'Books' OR t1.Items_purchased = 'pencil'
GROUP BY t2.country, t1.Items_purchased
ORDER BY 1 DESC;

Output:

When performing joins in SQL, we should always try to use table aliases which are abbreviations of the given tables. This helps in writing beautiful pieces of code.

Conclusion

SQL full join is a statement that returns all the records from both the joined tables. We should use it when we want to check for missing information in the database.

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