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Joins SQL Interview Questions By ScholarHat

Joins SQL Interview Questions By ScholarHat

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Joins SQL Interview Questions By ScholarHat

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  1. Top SQL Joins Interview Questions You Need to Know SQL Joins Questions You Must Know If you're getting ready for an SQL interview, it's important to understand SQL joins. You might be asked to explain how different types of SQL joins work or write queries during the interview. Knowing how to use INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN is very helpful. In this interview tutorial, we will go through common SQL joins interview questions with easy examples and clear explanations. By the end, you'll feel more confident and ready to answer any join question. These skills will also help you with real-world database tasks. Get ready to succeed in your SQL interview! What can you expect in SQL joins interview questions? Join interviews, which are designed to test your understanding of databases and how well you solve problems. It is common for you to be asked about different types of joins like INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN and how to use them in real-world situations.

  2. Whether you are a fresher or have experience, it is likely that you will be asked to write SQL queries, explain join concepts, and improve query performance. It is important for you to be well- prepared for these questions, including tricky cases, to increase your chances of success. Read More: SQL Queries Interview Questions and Answers. Top 20 SQL joins interview questions for freshers Example Tables: Student: ROLL_NO NAME AGE CITY 101 102 Ravi Kumar 21 Delhi 103 Priya Patel 22 Mumbai Arjun Singh 20 Bangalore Course: COURSE_ID COURSE_NAME ROLL_NO C101 Data Science 101 C102 Machine Learning 102 C103 Cloud Computing 103 C104 Web Development 101 1. What is a JOIN in SQL? A JOIN in SQL is used to combine rows from two or more tables based on a related column. For example, in the Student and Course tables, we can use ROLL_NO to match rows in each table and view which student is taking which course. 2. Can you explain an INNER JOIN?

  3. An INNER JOIN selects records that have matching values in both tables. So, if we want to find students who are enrolled in a course, we can use an INNER JOIN on ROLL_NO: SELECT Student.NAME, Course.COURSE_NAME FROM Student INNERJOIN Course ON Student.ROLL_NO = Course.ROLL_NO; 3. What is a LEFT JOIN? A LEFT JOIN returns all records from the left table (Student) and the matched records from the right table (Course). If there is no match, NULL values are returned for columns from the right table. For example: SELECT Student.NAME, Course.COURSE_NAME FROM Student LEFTJOIN Course ON Student.ROLL_NO = Course.ROLL_NO; 4. What is a RIGHT JOIN?

  4. A RIGHT JOIN returns all records from the right table (Course) and the matched records from the left table (Student). If no match is found, NULL values appear in columns from the left table. For example: SELECT Student.NAME, Course.COURSE_NAME FROM Student RIGHTJOIN Course ON Student.ROLL_NO = Course.ROLL_NO; 5. What is a FULL JOIN? A FULL JOIN returns all records when there is a match in either the left or right table. If there is no match, NULLs are returned for the unmatched rows. For example: SELECT Student.NAME, Course.COURSE_NAME FROM Student FULLJOIN Course ON Student.ROLL_NO = Course.ROLL_NO; 6. Can we use multiple JOINs in a single query? Yes, multiple JOINs can be used to combine data from more than two tables. Each JOIN needs to specify the matching column for the tables being joined. 7. What is a CROSS JOIN?

  5. A CROSS JOIN returns the Cartesian product of both tables. This means every row in the Student table is combined with every row in the table. Course SELECT Student.NAME, Course.COURSE_NAME FROM Student CROSSJOIN Course; 8. How do you perform a SELF JOIN? A SELF JOIN in sql is a join of a table with itself. It's helpful for comparing rows within the same table. You use table aliases to differentiate the instances. 9. What’s the difference between Inner join and Outer join? An INNER JOIN returns only the matching rows from both tables, while an (LEFT, OUTER JOIN RIGHT, or FULL) returns all rows from one or both tables, even if there’s no match. 10. How does a NATURAL JOIN work?

  6. A NATURAL JOIN automatically joins tables based on columns with the same name and datatype. It’s a shortcut but can cause issues if there are unexpected columns with the same name in both tables. 11. What is the purpose of using table aliases in JOINs? Table aliases make queries easier to read, especially in JOINs with multiple tables or a SELF JOIN. For example: SELECT S.NAME, C.COURSE_NAME FROM Student AS S INNER JOIN Course AS C ON S.ROLL_NO = C.ROLL_NO; 12. How can you join more than two tables? You can join multiple tables by adding additional JOIN statements. Each JOIN specifies which tables and columns should match.

  7. 13. What is the difference between JOIN and UNION? A JOIN combines columns from different tables, while a combines rows from different UNION queries or tables, making it a vertical combination. 14. How do you filter results in a JOIN using WHERE clause? Use the WHERE clause to apply conditions after performing the JOIN. For instance, to find students from Delhi in a course: SELECT Student.NAME, Course.COURSE_NAME FROM Student INNERJOIN Course ON Student.ROLL_NO = Course.ROLL_NO WHERE Student.CITY = 'Delhi'; 15. Can JOINs be used with aggregate functions? , etc., allowing you to calculate SUM, AVG Yes, JOINs can be used with aggregate functions like grouped results across joined tables. 16. What’s the impact of indexing on JOIN performance? Indexes on the joined columns (e.g., ) improve JOIN performance by speeding up data ROLL_NO retrieval, especially for large tables. 17. Can we join a table with a subquery? Yes, you can join a table with a subquery by treating the subquery as a temporary table. It’s helpful in combining filtered data with other tables. 18. What is an equi join?

  8. An equi join is a type of join where we match rows based on equality conditions, usually with = . Most joins are equi joins. 19. How can you exclude unmatched rows in a JOIN? To exclude unmatched rows, use an . It only returns rows with matching values in INNER JOIN both tables. 20. How do you handle NULL values in JOINs? NULL values can cause mismatches in JOINs. Using or can help handle IS NULL COALESCE NULLs effectively, especially in OUTER JOINs. NAME COURSE_NAME Ravi Kumar Data Science Priya Patel Machine Learning Arjun Singh Cloud Computing

  9. NAME COURSE_NAME Ravi Kumar Web Development SQL joins interview questions for experienced 21. What is a NATURAL JOIN? A NATURAL JOIN automatically joins tables based on columns with the same name and data type in both tables. Unlike an inner join, you don’t explicitly specify the condition for the join. SELECT Student.NAME, Course.COURSE_NAME FROM Student NATURALJOIN Course; NAME COURSE_NAME Ravi Kumar Data Science Priya Patel Machine Learning Arjun Singh Cloud Computing 22. How would you use multiple JOINs in a query? You can join multiple tables in a query by chaining statements. This allows you to fetch JOIN related data from different tables. Each should have an appropriate condition. JOIN SELECT Student.NAME, Course.COURSE_NAME, Instructor.INSTRUCTOR_NAME FROM Student INNERJOIN Course ON Student.ROLL_NO = Course.ROLL_NO INNERJOIN Instructor ON Course.COURSE_ID = Instructor.COURSE_ID; NAME COURSE_NAME INSTRUCTOR_NAME Ravi Kumar Data Science Dr. Sharma Priya Patel Machine Learning Prof. Verma Arjun Singh Cloud Computing Dr. Kumar

  10. 23. Can you explain the concept of using aliases in joins? Aliases are used to give temporary names to tables and columns, especially when working with self-joins or when the same table is involved multiple times in the query. This makes the query more readable. SELECT S1.NAME AS Student_Name, S2.NAME AS Friend_Name FROM Student S1 JOIN Student S2 ON S1.ROLL_NO = S2.ROLL_NO; Student_Name Friend_Name Ravi Kumar Priya Patel Arjun Singh Arjun Singh 24. What is the difference between JOIN and UNION? While both JOIN and UNION combine data from multiple tables. The key difference is that JOIN combines rows based on a condition while UNION appends the result sets of two queries, ensuring no duplicates (use to include duplicates). UNION ALL SELECT Student.NAME FROM Student UNION SELECT Teacher.NAME FROM Teacher; NAME Ravi Kumar Priya Patel Prof. Sharma 25. What is a CROSS JOIN? A CROSS JOIN returns the Cartesian product of both tables. Every row in the first table is joined with all rows in the second table. This can result in a very large result set. SELECT Student.NAME, Course.COURSE_NAME FROM Student

  11. CROSSJOIN Course; NAME COURSE_NAME Ravi Kumar Data Science Ravi Kumar Machine Learning Ravi Kumar Cloud Computing Priya Patel Data Science Priya Patel Machine Learning Priya Patel Cloud Computing 26. How do you prevent duplicate rows in a JOIN query? To prevent duplicate rows in a JOIN query, use the keyword. This eliminates duplicate DISTINCT records from the result set. SELECTDISTINCT Student.NAME, Course.COURSE_NAME FROM Student INNERJOIN Course ON Student.ROLL_NO = Course.ROLL_NO; NAME COURSE_NAME Ravi Kumar Data Science Priya Patel Machine Learning 27. How would you find rows with matching values in only one table? Use an OUTER JOIN (left or right) and check for NULL in the joined table to identify rows from the main table that do not have matching rows in the second table. SELECT Student.NAME, Course.COURSE_NAME FROM Student LEFTJOIN Course ON Student.ROLL_NO = Course.ROLL_NO WHERE Course.COURSE_NAME ISNULL;

  12. NAME COURSE_NAME Rajesh Kumar NULL 28. How do you handle NULL values in JOIN operations? In SQL joins, NULL values can affect the results. You can handle them by using the COALESCE() function to replace NULL with a default value or handle them in the clause. WHERE SELECT Student.NAME, COALESCE(Course.COURSE_NAME, 'No Course') AS COURSE_NAME FROM Student LEFTJOIN Course ON Student.ROLL_NO = Course.ROLL_NO; NAME COURSE_NAME Ravi Kumar Data Science Rajesh Kumar No Course 29. What is a JOIN with multiple conditions? You can use multiple conditions in the ON clause of a JOIN to specify more complex matching logic, such as combining multiple columns for the join condition. SELECT Student.NAME, Course.COURSE_NAME FROM Student JOIN Course ON Student.ROLL_NO = Course.ROLL_NO AND Course.YEAR = 2024; NAME COURSE_NAME Ravi Kumar Data Science Priya Patel Machine Learning 30. What is an example of using JOIN with a subquery? JOINs can be combined with subqueries to retrieve data that meets specific criteria from another query.

  13. SELECT Student.NAME, Course.COURSE_NAME FROM Student INNERJOIN Course ON Student.ROLL_NO = Course.ROLL_NO WHERE Course.COURSE_NAME IN (SELECT COURSE_NAME FROM Course WHEREYEAR = 2024); NAME COURSE_NAME Ravi Kumar Data Science Priya Patel Machine Learning 31. How would you JOIN three or more tables? To join three or more tables, chain multiple JOINs, each with an appropriate condition. The process remains the same as joining two tables, but you repeat the JOIN operation for each additional table. SELECT Student.NAME, Course.COURSE_NAME, Instructor.NAME FROM Student JOIN Course ON Student.ROLL_NO = Course.ROLL_NO JOIN Instructor ON Course.COURSE_ID = Instructor.COURSE_ID; NAME COURSE_NAME INSTRUCTOR_NAME Ravi Kumar Data Science Dr. Sharma Priya Patel Machine Learning Prof. Verma 32. What is a FULL OUTER JOIN? A returns all records when there is a match in either the left or righttable. It FULL OUTER JOIN returns for non-matching rows from both tables. NULL SELECT Student.NAME, Course.COURSE_NAME FROM Student FULLOUTERJOIN Course ON Student.ROLL_NO = Course.ROLL_NO; NAME COURSE_NAME Ravi Kumar Data Science

  14. NAME COURSE_NAME Priya Patel Machine Learning Arjun Singh Cloud Computing Rajesh Kumar NULL 33. What is the difference between LEFT JOIN and RIGHT JOIN? are types of OUTER JOINs, but: RIGHT JOIN Both and LEFT JOIN returns all records from the left table and the matched records from the LEFT JOINin SQL right table. returns all records from the right table and the matched records from RIGHT JOIN in SQL the left table. SELECT Student.NAME, Course.COURSE_NAME FROM Student LEFTJOIN Course ON Student.ROLL_NO = Course.ROLL_NO; NAME COURSE_NAME Ravi Kumar Data Science Priya Patel Machine Learning Arjun Singh Cloud Computing Rajesh Kumar NULL 34. How do you write a JOIN query to get data from two tables with conditions on both? To join two tables with conditions on both, you specify multiple conditions in the clause or use ON the clause to filter rows after joining. WHERE SELECT Student.NAME, Course.COURSE_NAME FROM Student

  15. JOIN Course ON Student.ROLL_NO = Course.ROLL_NO WHERE Course.YEAR = 2024 AND Student.AGE > 20; NAME COURSE_NAME Ravi Kumar Data Science Priya Patel Machine Learning 35. What is the difference between a JOIN and a UNION in SQL? JOIN combines data from two or more tables based on a common column while UNION combines results from two queries, stacking one result set below the other. UNION Also removes duplicaterows by default. SELECT Student.NAME FROM Student UNION SELECT Teacher.NAME FROM Teacher; NAME Ravi Kumar Priya Patel Prof. Sharma 36. How would you perform a JOIN between more than two tables? To perform a JOIN with multiple tables, chain the conditions one after the other, specifying JOIN how each table is related to the others. SELECT Student.NAME, Course.COURSE_NAME, Instructor.NAME FROM Student JOIN Course ON Student.ROLL_NO = Course.ROLL_NO JOIN Instructor ON Course.COURSE_ID = Instructor.COURSE_ID; NAME COURSE_NAME INSTRUCTOR_NAME Ravi Kumar Data Science Dr. Sharma

  16. NAME COURSE_NAME INSTRUCTOR_NAME Priya Patel Machine Learning Prof. Verma 37. What is the role of the clause in a JOIN? ON The ON clause in a JOIN specifies the condition that defines how two tables are related. It is used to filter rows based on matching columns from both tables. SELECT Student.NAME, Course.COURSE_NAME FROM Student INNERJOIN Course ON Student.ROLL_NO = Course.ROLL_NO; NAME COURSE_NAME Ravi Kumar Data Science Priya Patel Machine Learning 38. What is the impact of using with JOIN? DISTINCT Using DISTINCT in a JOIN query ensures that the result set does not contain duplicate rows. It’s useful when you expect multiple matching rows from one table and only want to return unique combinations. SELECTDISTINCT Student.NAME, Course.COURSE_NAME FROM Student INNERJOIN Course ON Student.ROLL_NO = Course.ROLL_NO; NAME COURSE_NAME Ravi Kumar Data Science Priya Patel Machine Learning 39. How would you handle multiple NULL values in JOIN operations? In JOIN operations, NULL values in one or both tables can cause rows to be excluded. Use COALESCE() or IFNULL()functions to replace NULLs with a default value.

  17. SELECT Student.NAME, COALESCE(Course.COURSE_NAME, 'No Course') AS COURSE_NAME FROM Student LEFTJOIN Course ON Student.ROLL_NO = Course.ROLL_NO; NAME COURSE_NAME Ravi Kumar Data Science Rajesh Kumar No Course 40. How can you join a table with a subquery in SQL? You can join a table with a subquery in the FROM clause. The subquery acts as a derived table that is treated like a regular table in the JOIN operation. SELECT Student.NAME, SubQuery.COURSE_NAME FROM Student JOIN (SELECT COURSE_NAME, ROLL_NO FROM Course WHERE YEAR = 2024) AS SubQuery ON Student.ROLL_NO = SubQuery.ROLL_NO; NAME COURSE_NAME Ravi Kumar Data Science Priya Patel Machine Learning Summary SQL join interview questions often focus on understanding different types of joins, such as INNER, LEFT, RIGHT, and FULL OUTER JOIN, as well as their use cases. Candidates should be prepared to explain how joins combine data from multiple tables based on related columns and demonstrate writing queries to illustrate these concepts. Practical knowledge of handling NULL values and complex join conditions can also be essential. Get Interview-Ready: Boost your SQL expertise and ace your next technical interview with the comprehensive ScholarHat SQL Server Course – your key to mastering database concepts and practical applications!

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