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Table of contents
1.
Easy SQL Joins Interview Questions
1.1.
1. What are joins in SQL?
1.2.
2. Why are SQL joins essential in database management?
1.3.
3. Does SQL join help in maintaining a normalized database?
1.4.
4. What are the different types of Joins present in SQL?
1.5.
5. Explain the difference between inner join and left join.
1.6.
6. Explain the difference between left join and right join.
1.7.
7. What is the difference between inner and outer join?
2.
Medium SQL joins interview questions
2.1.
1. What information is required to perform a Join query?
2.2.
2. What is Merge Join in SQL?
2.3.
3. How can I write an SQL query to join three tables?
2.4.
4. What are the types of join based on comparing operators?
2.5.
5. What are Non-Equi Joins?
2.6.
6. Is there any way to join a table to itself?
2.7.
7. Explain nested joins in SQL.
3.
Hard SQL joins interview questions
3.1.
1. What is the difference between Join and Subquery?
3.2.
2. What pre-calculation is done by RDBMS before executing JOINs and subqueries?
3.3.
3. Is it required that the Join condition should always be equal?
3.4.
4. Explain the term CTE (Common Table Expression) SQL.
3.5.
5. What do you understand by hash join in SQL?
3.6.
6. Why is a Join clause different from a Union clause?
3.7.
7. Distinguish between types of subquery and join operations.
3.8.
8. How can I write SQL syntax to join three tables?
3.9.
9. How can I formulate a query to retrieve the Customer Name and Product for individuals who have been assigned to more than one project?
3.10.
10. How can I write a query to display the name of customers who have not been assigned any products?
3.11.
11. Difference between Cross join and Natural join.
4.
Conclusion
Last Updated: Mar 27, 2024
Medium

Top SQL Joins Interview Questions and Answers (Updated)

Author Amit Singh
0 upvote

SQL stands for Structured Query Language. It is a special type of language that is used to maintain databases. It is meant for storing, searching, manipulating, updating, and retrieving data present in a table in a relational database.

SQL Joins Interview Questions

This article will be focused on the most commonly asked sql joins interview questions that will help you prepare for interviews with big MNCs.Let's continue with some of the top SQL joins interview questions.


Click on the following link to read further: Java OOPs Interview Questions and Operating System Interview Questions

Easy SQL Joins Interview Questions

1. What are joins in SQL?

A join clause of an Sql command combines records from multiple tables. It also retrieves data from these tables based on a common field (column) between them. SELECT statement and a join condition can be used to join the tables. Using the JOIN clause, records can be fetched from two or more tables and can be combined in a Database. In general, they are used when users retrieve data from tables containing many-to-many or one-to-many relationships between them.

2. Why are SQL joins essential in database management?

SQL joins are essential in database management because of the following reasons:

  • It is a way to join databases to make them easy to use and read.
     
  • It is used to maintain a normalized database. 
     
  • Joins are faster and more efficient.
     
  • It is quicker to retrieve data using a join than a subquery.
     
  • It also helps in reducing the workload on the databases by using joins.

3. Does SQL join help in maintaining a normalized database?

Data normalization keeps data redundancy low. Thus when we update or delete a record, we have fewer data anomalies in our application. With the help of joins, we connect these normalized databases. Hence, it helps in maintaining a normalized database.

4. What are the different types of Joins present in SQL?

There are many types of joins available in SQL. Their use depends on the use case you have. There are mainly four types of joins as follows:

  1. Inner Join: This method returns datasets with the same values in both tables.
  2. Full Join: This is also known as a full outer join. It combines all rows from the left and right tables to create the result set. This Join Query will return records from both tables, even if they have NULL values. The joined table (result set) will display NULL values if there are no matching rows.
  3. Right Join: This is also known as the Right outer join. It helps the user by returning all the records from the right table along with any records that match from the left table.
  4. Left Join: This is also known as the Left outer join. It helps the user by returning all the records from the left table along with any records that match from the right table.
diagram

5. Explain the difference between inner join and left join.

How an inner join and a left join handle rows without a corresponding row in the other table is the main difference between them.

Only the rows from both tables with a matching row in the other table are returned by an inner join. It is used to aggregate data from two connected tables and is the most used type of join.

Even if there is no matching row in the right table, a left join returns every entry from the left table. The values NULL will be returned for any columns from the right table that don't have a corresponding entry in the left table.

6. Explain the difference between left join and right join.

Even if there is no matching row in the right table, a left join returns every entry from the left table. The values NULL will be returned for any columns from the right table that don't have a corresponding entry in the left table.

Even if there is no matching row in the left table, a right join retrieves every row from the right table. The values NULL will be returned for any columns from the left table that don't have a corresponding entry in the right table.

7. What is the difference between inner and outer join?

Inner and outer join are two types of SQL Equi join.

  1. SQL Inner Join returns all rows from tables where one table's key record is equal to another table's key records.
  2. SQL Outer Join returns all rows from one table and only those from the secondary table that meet the joined requirement. It is where the columns in both tables are equal.

Learn more, Html interview questions

Medium SQL joins interview questions

1. What information is required to perform a Join query?

To perform a JOIN query, we require the following information:

  • Name of the tables between whose Join query is to be performed.
     
  • Name of the columns based on which a condition for joining will perform. It can be of two or more tables.
     
  • Syntax: SELECT * FROM table1 join_type table2 ON (join_condition)

2. What is Merge Join in SQL?

The Merge join is also known as sort-merge join. It is a join process that is used in the application of an RDBMS. This merge join has a basic trick for the joining process. It is to find each unique value of the join attribute and the set of tuples in every relation that output that value.

3. How can I write an SQL query to join three tables?

Use the steps below to write SQL syntax to join three tables:

  • Step 1: Determine the columns that join the three tables.
  • Step 2: All the columns from the three tables you want to return should be included in a SELECT statement.
  • Step 3: Utilising the columns that you determined in Step 1, combine the three tables together using the JOIN Keyword.

4. What are the types of join based on comparing operators?

The are two types of SQL JOINS depending on comparing operators:-

  1. SQL Equi Join is a simple SQL join that uses the comparison operator equal sign(=) for the condition. It is of two types SQL Inner join and SQL Outer join.
  2. SQL Non-Equi Join is a join that uses a comparison operator other than the equal sign with the condition. It can be >, <, >=, or <=.

5. What are Non-Equi Joins?

The non-equi joins are all those JOIN types that use some other comparison operators. It can be greater than (>), less than (<), less than or equal to (<=), or greater than or equal to (>=).

There are three main uses for this type of join:

  1. They are used to find duplicates in tables.
  2. They are used to join tables on a range of values.
  3. They are used to calculate running totals.

6. Is there any way to join a table to itself?

You can join a table to itself in SQL using a type of join known as a SELF JOIN. It joins a table to itself. A minimum of one column (let's say X) that acts as the primary key and one column (let's say Y) that has values that can be matched with those in X are required in order to accomplish a self-join. Column X does not always need to have the same value as Column Y for every row, and Column Y's value may be null in certain rows.

7. Explain nested joins in SQL.

A Nested Join uses two tables. One joining table is an outer input table, while the other is an inner input table. A Nested Loop Join involves:

  1. Retrieving one row from the outside table.
  2. Searching for it in the inner table.
  3. Repeating this procedure until all the output rows from the outer table are searched for in the inner table.

Naive Nested, Indexed Nested, and Temporary Index Nested Loop Join are subcategories of Nested Loop Join.

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Hard SQL joins interview questions

1. What is the difference between Join and Subquery?

  1. JOINs are mostly faster than a subquery, and the opposite is rare. 
  2. RDBMS has some pre-calculations in JOINs and hence helps save time, unlike the subquery. 
  3. A JOIN first checks conditions, put them into a table, and displays them. In contrast, a subquery takes a separate temp table internally and checks the condition.
  4. When joins are used, there should be a connection between two or more tables. Also, each table should have a relation with the other. In comparison, subquery means a query inside another query. It does not need to be related. It works on conditions and columns.

2. What pre-calculation is done by RDBMS before executing JOINs and subqueries?

RDBMS calculates an execution plan in JOINs. This helps them to predict what data will be loaded and how much processing time it will require. This helps to save time while joining databases and performing queries on them. While executing subqueries, no pre-calculation is done. RDBMS runs all the queries and loads data for processing. Thus we can say JOINs are better than subqueries.

3. Is it required that the Join condition should always be equal?

No, there are non-equi conditions also present in joins. Join clauses have common comparison operators, like <, <=, >, >=, !=, BETWEEN. For example, listing unique pairs, listing records, and identifying duplicate records are a few situations where non-equi joins are useful.

4. Explain the term CTE (Common Table Expression) SQL.

CTE stands for Common Table Expressions. They are temporarily named result sets that you referred to within a SELECT, INSERT, DELETE, or UPDATE statement. These CTEs are defined by adding a WITH before a SELECT, INSERT, DELETE, UPDATE, OR MERGE statement. Many CTEs are used within the WITH clause and are separated by commas.

5. What do you understand by hash join in SQL?

Hash join is also a type of join in SQL. It requires two inputs: the inner table (probe input) and the outer table (build input). This join involves using a hash table to identify rows matching between two tables. The hash join is preferred when no other join is preferred. It is due to the absence of indexing or sorting etc. Hash joins are best for joining large data sets that are non-indexed and unsorted.

6. Why is a Join clause different from a Union clause?

You can use either JOINS or UNIONS in your queries to combine data from two or more tables.

  1. Join: Joins let you horizontally combine the data into new columns. A SQL join clause is used when combining records from various tables or retrieving data from them based on the presence of a shared field (column). The columns of the linking tables may be different when using JOIN. 
  2. Union: When more than one SELECT statement's results need to be combined, you can use the term UNION in SQL. You can merge data into new vertical rows using union. All queries must comply with UNION's requirements about the number of columns and their order.

7. Distinguish between types of subquery and join operations.

  1. Join: A join operation is a binary operation. It combines the rows or data based on a common field from two or more tables. Different types of joins include INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN, etc.
  2. Subquery: It is a query enclosed within another query. In this, the inner query is known as a subquery. While the outer query is known as the main query. There are two types of subqueries:-
    1. Nested Query: It first executes the inner query, and only once. Then an outer query is executed depending on the results of the inner query. Therefore the inner query is used to execute the outer query.
    2. Correlated Query: It first executes the outer query. For every outer query row, the inner query is executed. Therefore, values from the outer query are used in the inner query.

8. How can I write SQL syntax to join three tables?

Here is an example of an SQL query to join three tables:

SELECT students.std_id, students.first_name, students.last_name, books.book_id, books.pub_date, book.total_amount
FROM students
JOIN books ON students.std_id = books.std_id
JOIN books_items ON books.book_id = books_items.books_id;

 

This query will join the students, books, and books_items tables together, using the std_id and book_id columns.

9. How can I formulate a query to retrieve the Customer Name and Product for individuals who have been assigned to more than one project?

The following is an example of a complete SQL query to retrieve the Customer Name and Product for individuals who have been assigned to more than one project:

SELECT Customers.CustomerName, Products.Product, Projects.ProjectName
FROM Customers
JOIN ProjectAssignments ON Customers.CustomerID = ProjectAssignments.CustomerID
JOIN Projects ON ProjectAssignments.CustomerID = Projects.CustomerID
WHERE ProjectAssignments.CustomerID IN (SELECT CustomerID FROM ProjectAssignments GROUP BY CustomerID HAVING COUNT(*) > 1)
ORDER BY Customers.CustomerName;

 

This will provide you a list of all the customers who have more than one project assigned to them, along with the products they are working on and the projects they are assigned to.

10. How can I write a query to display the name of customers who have not been assigned any products?

The following is an example of a complete SQL query to display the name of customers who have not been assigned any products:

SELECT Customers.CustomerName
FROM Customers
LEFT JOIN ProductAssignments ON Customers.CustomerID = ProductAssignments.CustomerID
WHERE ProductAssignments.CustomerID IS NULL;

 

This query will return a list of all customers without any assigned products.

11. Difference between Cross join and Natural join.

Natural Join combines two tables based on similar attribute names and data types, and the resulting table consists of the attributes from both tables by keeping only one copy of each common column. Therefore, two tables are combined on the basis of their similar column names, and the table in output consists of only one instance of the common column.

When the user specifies no condition, then the rows are returned on the basis of common columns.

Syntax

SELECT * FROM Table_Name1 NATURAL JOIN Table_Name2;

 

Cross Join provides a cartesian product of two tables and is used for combining each row from one table to the row of another table. The resulting table consists of the cartesian product, i.e., attributes of both tables (duplicate columns are included).

When the user specifies no condition, then all the possible pairs of rows from the provided tables are returned, even if they are unmatched or matched.

Syntax

SELECT * FROM Table_Name1 CROSS JOIN Table_Name2;

Conclusion

In this article, we have discussed the top 30 sql joins interview questions. All these questions were the most commonly asked interview questions. The questions ranged from basic to intermediate to hard advanced questions. 

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