SQL Interview Questions

CROSS JOIN and FULL JOIN are both types of joins in SQL, but they serve very different purposes and produce different types of results. Here is an explanation of each, along with their differences:

CROSS JOIN

Purpose: The CROSS JOIN produces a Cartesian product of the two tables involved. This means it returns all possible combinations of rows from the two tables.

Characteristics

• No Join Condition: The CROSS JOIN does not require a join condition (ON clause).
• Result Size: The number of rows in the result set is the product of the number of rows in the first table and the number of rows in the second table.
• Use Case: It is useful when you need to combine every row of one table with every row of another table.

Syntax

SELECT *
FROM table1
CROSS JOIN table2;

Example

Suppose you have two tables:

Products Table

ProductID	ProductName
1	Apple
2	Banana

Suppliers Table

SupplierID	SupplierName
A	Supplier1
B	Supplier2

CROSS JOIN Result

SELECT *
FROM Products
CROSS JOIN Suppliers;

ProductID	ProductName	SupplierID	SupplierName
1	Apple	A	Supplier1
1	Apple	B	Supplier2
2	Banana	A	Supplier1
2	Banana	B	Supplier2

FULL JOIN

Purpose: The FULL JOIN (or FULL OUTER JOIN) combines the results of both LEFT JOIN and RIGHT JOIN. It returns all rows when there is a match in one of the tables. This means it returns all rows from both tables, with NULL values in place where there is no match.

Characteristics

• Join Condition: The FULL JOIN requires a join condition (ON clause) to specify how the rows between the tables should be matched.
• Result Size: The result set includes all rows from both tables, with NULL in columns where there is no match.
• Use Case: It is useful when you need to retrieve all rows from both tables and identify matching and non-matching rows.

Syntax

SELECT *
FROM table1
FULL JOIN table2
ON table1.column_name = table2.column_name;

Example

Suppose you have two tables:

Customers Table

CustomerID	CustomerName
1	Alice
2	Bob

Orders Table

OrderID	CustomerID	OrderDate
101	1	2023-07-01
102	3	2023-07-02

FULL JOIN Result

SELECT *
FROM Customers
FULL JOIN Orders
ON Customers.CustomerID = Orders.CustomerID;

CustomerID	CustomerName	OrderID	OrderDate
1	Alice	101	2023-07-01
2	Bob	NULL	NULL
NULL	NULL	102	2023-07-02

Summary of Differences

Feature	CROSS JOIN	FULL JOIN
Purpose	Produces Cartesian product of two tables	Combines results of both LEFT JOIN and RIGHT JOIN
Join Condition	No join condition required	Requires a join condition (ON clause)
Result Size	Product of rows in both tables	All rows from both tables, with NULLs for non-matches
Use Case	Combine every row of one table with every row of another table	Retrieve all rows from both tables, with matching and non-matching rows
Example	Combine Products and Suppliers to see all combinations	Combine Customers and Orders to see all customers and orders

Conclusion

CROSS JOIN and FULL JOIN serve different purposes in SQL:

• CROSS JOIN is used to produce a Cartesian product, combining every row of one table with every row of another table, resulting in all possible combinations.
• FULL JOIN is used to combine the results of LEFT JOIN and RIGHT JOIN, returning all rows from both tables with NULL values where there is no match.

Understanding the differences between these joins allows you to use them appropriately based on the requirements of your data retrieval and manipulation tasks.