Top SQL Joins Interview Questions and Answers in 2024

SQL JOINS are used to join two sets of data. It is a type of programming language used to manipulate information and can be used to create, or retrieve information. This article is going to give you every possible guide you need to improve your concepts related to SQL SELF JOIN and will assist you with the top SQL JOINS interview questions and answers.

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Basic SQL JOINS Interview Questions and Answers for Freshers

1. What is SQL JOINS?

The SQL JOIN component joins rows from one or more tables in a relational database. Create sets that can be stored in tabular form or used routinely. JOIN is to combine columns from one table or multiple tables using the same value.

2. Different types of JOINS in SQL

There are different types of JOINS in SQL, which are the following:

INNER JOIN – An INNER JOIN is used to return records of the same value in two tables.
LEFT JOIN – LEFT JOIN is used to join all the rows in the left table with matching rows in the right table.
RIGHT JOIN – RIGHT JOIN is used to join all the rows in the right table with the corresponding rows in the left table.
FULL JOIN – A FULL JOIN is used to return all records from two tables if there are matching records in each table.
SELF JOIN – A SELF JOIN is a join used to join a table to itself. SELF JOINS treats one table as two tables.
CARTESIAN JOIN – CARTESIAN Integral is used to multiply the number of rows in the first table by the number of rows in the second table. It is also called CROSS JOIN.

3. What is the difference between INNER JOIN and SELF JOIN?

The most important difference between INNER and SELF JOIN is:

INNER JOIN is used to return the records which are present in both tables. Whereas, in SELF JOIN, a table is joined to itself.
A SELF JOIN is a type of INNER JOIN.

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4. What is the importance of SQL JOINS in database management?

The various importance of SQL JOINS in database management are as follows:

SQL JOINS is a method to integrate databases so that they are easy to read and use.
General data protection is also maintained. Data normalization helps reduce data loss so the application has fewer data gaps when records are deleted or updated.
The advantage of JOINS is that it is faster and therefore more efficient.
Retrieving data using summary queries is usually faster than using subqueries.
Using JOINS can reduce data usage and storage on the database. Here you can use one JOIN query instead of multiple queries. So you can use a large database to search, filter, organize, and more.

5. State the difference between the RIGHT JOIN and the LEFT JOIN.

Both LEFT JOIN and RIGHT JOIN do the same thing: they return the result of a query that contains all the records in the table. The only difference is that the left view shows all the records in the left table of the query, and the right view shows all the records in the right table.

6. Is SELF JOIN an INNER JOIN or OUTER JOIN?

The SELF JOIN can be an INNER JOIN, OUTER JOIN, or can also be CROSS JOIN. Tables are automatically linked based on columns that contain duplicate data in multiple rows.

7. What is the difference between FULL JOIN and CARTESIAN JOIN?

The combination of the LEFT and the RIGHT OUTER JOIN is called a FULL JOIN.

If the ON condition cannot be satisfied, it returns all rows in both tables that match the WHERE clause with a NULL value.

Whereas, a CARTESIAN or CROSS JOIN creates a cross-product between the two tables. For all rows, it returns a possible sequence.

8. What is NATURAL JOIN?

A NATURAL JOIN is used to create an absolute JOIN clause based on common attribute values in two tables. Shared variables are variables that are named in both tables. NATURAL JOINS do not need any equivalence operator like EQUI JOIN.

Example: In the below two tables the Customer and Shop tables have a common field ‘CustID’. So, in the case of NATURAL JOINS, we don’t need to mention the On clause over the ‘CustNo’ field of both tables.

CustID	Name	Area name
890	AVI	CP
678	NIO	MG

Customer table

ShopID	SName	CustID
23	ABY	678
31	BCY	890

Shop table

9. What is an EQUI JOIN?

An EQUI JOIN is a type of join operation in a database that combines rows from two or more tables based on a matching condition using the equality operator (=). It is used to retrieve data where values in specified columns are equal.

Here is an example of the syntax for an EQUI JOIN:

SELECT column_name(s)
FROM table1

JOIN table2

ON table1.column_name = table2.column_name;

10. Can you join a table to itself in SQL?

Yes, in SQL, it is possible to join a table to itself, which is known as a self-join.

By using table aliases, you can treat the same table as two separate entities and perform a join operation on them based on specified conditions. Self-joins are used when you need to retrieve information by comparing rows within the same table.

Intermediate SQL JOINS Interview Questions

11. Distinguish between nested subquery, correlated subquery, and join operation.

Subquery– Queries can be embedded in other queries. Therefore, an outer query is called the main query and the Internal queries are called subquery.
- Nested query– The inner query is first executed by nested queries and that also at once. An outer query returns the result of an inner query and Inner queries are used to manage outer queries.
- Correlated query– At first the outer query is executed and after that, an inner query is performed for each row of the outer query, which is the value from the outer query used in the inner query.
Join Operation– A join operation is an operation that combines data or rows from two or more tables into a common field. There are different types of joins that are INNER JOIN, FULL JOIN, LEFT JOIN, RIGHT JOIN, SELF JOIN, and CARTESIAN or CROSS JOIN.

12. How are JOINS different from the UNION clause?

A JOIN can be used if two tables share at least one attribute.

The length of the retrieved rows is greater than the length of the rows in the corresponding tables.

Whereas In the case of UNION, a JOIN can be used if the query has the same number of columns and the corresponding attributes are the same.

The number of rows returned is greater than the number of rows in each table in the query.

13. Is it required that the JOIN condition be based on equality?

No, because JOINS have the conditions of NON-EQUI. Sentence combinations can be done with common symbols such as <, <=, >, >=, !=, BETWEEN, for example, to represent data. Odd-pair indexing and identifying duplicate data are several cases where NON-EQUI JOINS performance can be demonstrated.

14. What is a HASH JOIN?

A HASH JOIN requires two inputs, an INNER table, and an OUTER table. HASH JOINS involve using a HASH table to identify matching rows between two tables. HASH JOINS are an option when other joins are not recommended. When joining large data sets that are unsorted or non-indexed HASH JOINS are better.

15. What is MERGE JOIN?

MERGE JOIN is the most important join in SQL Server. In MERGE JOIN, your query plan is an effective query plan and you don’t need to make many changes to improve query performance. As because the MERGE JOIN operator uses ordered data entry, it can use two large data sets.

16, Can you explain NESTED JOIN in SQL?

JOIN is one of the methods we use to join data from multiple tables into a relational database, and NESTED JOIN is one of the simplest methods to join two tables. Typically, one table is used as the OUTER JOIN table with NESTED JOINS and the other is used as the INNER JOIN table. Nested loop collections can be classified into indexed nesting and temporary Index Nested Loop Join.

17. Explain Common Table Expression SQL.

In general, a Common Table Expression (CTE) is a temporary, named result set that can be used to refer to an UPDATE, INSERT, SELECT, or DELETE statement. A CTE can be specified by adding WITH before an UPDATE, INSERT, DELETE, SELECT, or MERGE statement. Multiple CTEs can be used in the WITH clause by separating them with commas.

18. Is it necessary that the JOIN condition should be based on equality?

No, it is not based on column or row equality.

19. How will you structure data to perform a JOIN Operation in a one-to-many relationship situation?

To create a one-to-many, you need to add the primary key from one side to many sides as a column. To create many-to-many, you need a middle table that contains the primary keys from many-to-many tables.

20. Write an SQL syntax for joining 3 tables.

select tab1.col1, tab2.col2,tab3.col3 (columns to display) from table1
Join    ///Any type of join
table2 on tab1.col1=tab2.col1    //any matching columns
Join  ///Any type of join
table3 on tab 2.col1=tab 3.col1     //any matching columns

Advanced SQL JOINS Interview Questions for Experienced

21. Create tables- Customer detail and Product detail.

Fig. 1.1

Fig. 1.2

So, based on these two tables, let’s look into some of the questions related to SQL JOINS and queries.

22. Get customer name and product name order by first name from

SELECT a.first_name,b.Product_name
FROM [customer] A
INNER JOIN [product] B
ON A.customer_id = B.customer_id
ORDER BY a.first_name

23. Get customer name, and product name order by firstname from

SELECT a.first_name, b.Product_name FROM [customer] A
LEFT OUTER JOIN [Product] B
ON A.customer_id = B.customer_id
ORDER BY a.first_name

24. Get the Customer name and product name order by firstname from

SELECT a.First_Name, ISNULL(b.Product_name,'-No Project Assigned')
FROM customer A LEFT OUTER JOIN product B
ON A.customer_id = B.customer_id ORDER BY a.first_name

25. Get all product names even if they have not matched any Customer id, in the left table, order by firstname from

SELECT a.first_name,b.Product_name
FROM [customer] A RIGHT OUTER JOIN [product] B
ON a.customer_id = b.customer_id ORDER BY a.first_name

26. Get the complete record(Customer name, product name) from both tables([CustomerDetail],[ProductDetail]), if no match is found in any table then show NULL.

SELECT a.first_name,b.Product_name FROM [customer] A
FULL OUTER JOIN [product] B
ON a.customer_id = b.customer_id
ORDER BY a.first_name

27. Write a query to find out the Customer name who has not been assigned any product, and display

SELECT a.first_name, ISNULL(b.Product_name,'-No Project Assigned') AS [Product]
FROM [customer] A
LEFT OUTER JOIN [product] B
ON a.customer_id = b.customer_id
WHERE b.Product_name IS NULL

28. Write a query to find out the product name which is not assigned to any employee( tables:- [CustomerDetail],[ProductDetail]).

SELECT b.Product_name FROM [customer] A
RIGHT OUTER JOIN [product] B
ON a.customer_id = b.customer_id
WHERE a.first_name IS NULL

29. Write down the query to fetch CustomerName & Product who has assigned more than one project.

Select c.customer_id, c.first_name, p.Product_name from [customer] c INNER JOIN [product] P
ON c.customer_id = p.customer_id
WHERE c.customer_id IN
(SELECT customer_id FROM [product] GROUP BY customer_id HAVING COUNT(*)
>1 )

*The output will not come as there is no duplicate record in the product table.

30. Write down the query to fetch ProductName on which more than one customer is working along with CustomerName.

Select P.Product_name, c.first_name from product P INNER JOIN customer c
on p.customer_id = c.customer_id

where P.Product_name in(select Product_name from product group by Product_name having COUNT(1)>1)