Top SQL Joins Interview Questions You Need to Know

SELECT Student.NAME, Course.COURSE_NAME
FROM Student
RIGHT JOIN Course
ON Student.ROLL_NO = Course.ROLL_NO;

Q 5. What is a FULL JOIN?

A FULL JOIN returns all rows from both the left and right tables. If there’s no match between the rows, NULL values are returned for the missing side. This JOIN is useful when you want to retrieve all data from both tables, regardless of whether they have a matching row.

• For example, when you want to see all students and all courses, even if some students aren’t enrolled in any course or some courses don't have any students enrolled.

SELECT Student.NAME, Course.COURSE_NAME
FROM Student
FULL JOIN Course
ON Student.ROLL_NO = Course.ROLL_NO;

Q 6. Can we use multiple JOINs in a single query?

Yes, you can use multiple JOINs in a single query to combine data from more than two tables. In fact, it’s very common to join multiple tables, especially when you need information from different sources. You just need to ensure that you correctly specify the conditions that relate to the tables.

• Each JOIN must have a matching condition (like a shared column) to connect the tables together.

SELECT Student.NAME, Course.COURSE_NAME, Teacher.TEACHER_NAME
FROM Student
INNER JOIN Course ON Student.ROLL_NO = Course.ROLL_NO
INNER JOIN Teacher ON Course.TEACHER_ID = Teacher.TEACHER_ID;

Q 7. What is a CROSS JOIN?

A CROSS JOIN returns the Cartesian product of both tables, meaning it will combine each row from the first table with each row from the second table.

• This can result in a large number of rows, depending on the size of the tables.
• CROSS JOIN is typically used when you need to create combinations of every row in both tables, but it’s not commonly used for everyday queries due to its high resource demand.

SELECT Student.NAME, Course.COURSE_NAME
FROM Student
CROSS JOIN Course;

Q 8. How do you perform a SELF JOIN?

A SELF JOIN is a special type of join where a table is joined with itself. This is useful when you need to compare rows within the same table. You differentiate between the instances of the table by using aliases. A typical scenario is when you want to find employees who manage other employees, as the employee and manager data exist in the same table.

SELECT A.EMP_NAME AS Employee, B.EMP_NAME AS Manager
FROM Employee A
INNER JOIN Employee B ON A.MANAGER_ID = B.EMP_ID;

Q 9. What’s the difference between Inner join and Outer join?

An INNER JOIN returns only the rows where there’s a match between the tables. If there’s no match, those rows are excluded.

In contrast, an OUTER JOIN (LEFT, RIGHT, or FULL) returns all rows from one or both tables, even if there’s no match. This makes OUTER JOINs useful when you need to include all records, regardless of whether they have matching data in the other table.

-- INNER JOIN Example:
SELECT Student.NAME, Course.COURSE_NAME
FROM Student
INNER JOIN Course ON Student.ROLL_NO = Course.ROLL_NO;

-- LEFT OUTER JOIN Example:
SELECT Student.NAME, Course.COURSE_NAME
FROM Student
LEFT JOIN Course ON Student.ROLL_NO = Course.ROLL_NO;

Q 10. How does a NATURAL JOIN work?

A NATURAL JOIN automatically joins tables based on columns that have the same name and datatype in both tables. It simplifies queries by eliminating the need to specify the join condition explicitly. However, it can lead to issues if the tables contain columns with the same name but different meanings, so it’s important to be cautious when using this type of join.

SELECT Student.NAME, Course.COURSE_NAME
FROM Student
NATURAL JOIN Course;

Q 11. What is the purpose of using table aliases in JOINs?

Table aliases make queries easier to read and write, especially in JOINs involving multiple tables or a SELF JOIN. They help you avoid repeating long table names and improve query clarity. For example:

SELECT S.NAME, C.COURSE_NAME
FROM Student AS S
INNER JOIN Course AS C
ON S.ROLL_NO = C.ROLL_NO;

Q 12. How can you join more than two tables?

You can join multiple tables by adding extra JOIN statements for each table you want to include. Each JOIN specifies how tables and columns should match. This way, you can combine data from several tables into one result set.

Q 13. What is the difference between JOIN and UNION?

A JOIN combines columns from different tables into a single result set by matching rows, whereas a UNION combines rows from different queries or tables into one result, stacking them vertically. Each serves a distinct purpose based on how you want to combine your data.

Q 14. How do you filter results in a JOIN using the WHERE clause?

You use the WHERE clause to apply conditions after performing the JOIN. For instance, if you want to find students from Delhi enrolled in a course, you can filter the results like this:

SELECT Student.NAME, Course.COURSE_NAME
FROM Student
INNER JOIN Course
ON Student.ROLL_NO = Course.ROLL_NO
WHERE Student.CITY = 'Delhi';

Q 15. Can JOINs be used with aggregate functions?

Yes, JOINs work seamlessly with aggregate functions like SUM, AVG, and COUNT. You can calculate grouped results by joining tables and applying these functions, such as finding the total course fees for all students.

Q 16. What’s the impact of indexing on JOIN performance?

Indexing on the joined columns (e.g., ROLL_NO) significantly improves JOIN performance. It speeds up data retrieval by enabling the database to quickly locate matching rows, which is especially beneficial for large datasets.

Q 17. Can we join a table with a subquery?

Yes, you can join a table with a subquery. The subquery acts as a temporary table and helps you combine filtered or pre-processed data with other tables. This technique is useful when you need additional conditions or aggregations.

Q 18. How should data be organized to support Join operations in a one-to-many relationship?

To support JOIN operations in a one-to-many relationship, store the "one" side in a primary table with a primary key (PK) and the "many" side in a related table with a foreign key (FK) referencing the primary table.

Q 19. How should data be structured to support Join Operations in a many-to-many relationship?

To support JOIN operations in a many-to-many relationship, use a junction table (also called a bridge table) that connects the two main tables. This junction table contains foreign keys referencing the primary keys of both tables, allowing efficient linking between multiple records in both directions.

Q 20. What is an equi join?

An equi join is a type of join that matches rows using equality conditions, typically with =. Most INNER JOINs are equi joins since they rely on equal column values to combine rows.

Q 21. How can you exclude unmatched rows in a JOIN?

To exclude unmatched rows, you can use an INNER JOIN. It only returns rows where matching values exist in both tables, ensuring your results are clean and precise.

Q 22. Explain about Hash JOIN in SQL.

A HASH JOIN in SQL joins two tables by creating a temporary hash table from the smaller table and then matching it with rows from the larger table. It is useful for joining large tables without indexes but needs more memory.

Q 24. What is NESTED JOIN in SQL?

A NESTED JOIN (or Nested Loop Join) in SQL is a method where for each row in the first table, SQL checks every row in the second table to find matching data. It works well for small tables or when an index helps speed up the search but can be slow for large datasets.

Q 25. What is CTE (Common Table Expression) SQL?

A CTE (Common Table Expression) in SQL is a temporary result set that you can use within a query to make it easier to read and manage. It is created using the WITH keyword and helps break complex queries into smaller, more understandable parts.

Q 26. Does the Join condition always have to be based on equality?

No, the JOIN condition does not always have to be based on equality. While INNER JOIN and OUTER JOIN typically use equality (=) to match rows, NON-EQUI JOINS can use other operators like <, >, <=, >=, or BETWEEN to join tables based on a range of values instead of exact matches.

Q 27. How do you handle NULL values in JOINs?

NULL values can cause mismatches during JOINs. You can use COALESCE to replace NULLs with default values or IS NULL to filter out or handle rows with NULLs, especially in OUTER JOINs.

SQL Joins Interview Questions for Experienced

Q 28. 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
NATURAL JOIN Course;

Q 29. How would you use multiple JOINs in a query?

You can join multiple tables in a query by chaining JOIN statements. This allows you to fetch related data from different tables. Each JOIN should have an appropriate condition.

SELECT Student.NAME, Course.COURSE_NAME, Instructor.INSTRUCTOR_NAME
FROM Student
INNER JOIN Course ON Student.ROLL_NO = Course.ROLL_NO
INNER JOIN Instructor ON Course.COURSE_ID = Instructor.COURSE_ID;

Q 30. 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;

Q 31. 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 UNION ALL to include duplicates).

SELECT Student.NAME FROM Student
UNION
SELECT Teacher.NAME FROM Teacher;

Q 32. 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
CROSS JOIN Course;

Q 33. How do you prevent duplicate rows in a JOIN query?

To prevent duplicate rows in a JOIN query, use the DISTINCT keyword. This eliminates duplicate records from the result set.

SELECT DISTINCT Student.NAME, Course.COURSE_NAME
FROM Student
INNER JOIN Course ON Student.ROLL_NO = Course.ROLL_NO;

Q 34. What is the use of the ON keyword in a JOIN?

The ON keyword is used to specify the condition for the join between two tables. This condition determines how the rows from each table are matched.

SELECT Student.NAME, Course.COURSE_NAME
FROM Student
INNER JOIN Course ON Student.ROLL_NO = Course.ROLL_NO;

Q 35. What is a SELF JOIN?

A SELF JOIN is when a table is joined with itself. You can use a SELF JOIN to compare rows within the same table. It is often used to find relationships within the same dataset.

SELECT A.NAME, B.NAME FROM Employee A
INNER JOIN Employee B ON A.MANAGER_ID = B.EMPLOYEE_ID;

Q 36. What is the difference between INNER JOIN and LEFT JOIN?

The INNER JOIN returns only the rows that have matching records in both tables. In contrast, a LEFT JOIN returns all rows from the left table, along with matching rows from the right table. If there is no match, the result will contain NULL for the right table's columns.

SELECT Student.NAME, Course.COURSE_NAME
FROM Student
LEFT JOIN Course ON Student.ROLL_NO = Course.ROLL_NO;

Q 37. How does the RIGHT JOIN work?

A RIGHT JOIN is similar to a LEFT JOIN, but it returns all rows from the right table along with matching rows from the left table. If there is no match, the result will contain NULL for the left table's columns.

SELECT Student.NAME, Course.COURSE_NAME
FROM Student
RIGHT JOIN Course ON Student.ROLL_NO = Course.ROLL_NO;

Q 38. 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;

Q 39. What is a FULL OUTER JOIN?

A FULL OUTER JOIN returns all records when there is a match in either the left or right table. It returns NULL for non-matching rows from both tables.

SELECT Student.NAME, Course.COURSE_NAME
FROM Student
FULL OUTER JOIN Course ON Student.ROLL_NO = Course.ROLL_NO;

Q 40. What is the difference between LEFT JOIN and RIGHT JOIN?

Both LEFT JOIN and RIGHT JOIN are types of OUTER JOINs, but:

• LEFT JOINin SQLreturns all records from the left table and the matched records from the right table.
• RIGHT JOIN in SQL returns all records from the right table and the matched records from the left table.

SELECT Student.NAME, Course.COURSE_NAME
FROM Student
LEFT JOIN Course ON Student.ROLL_NO = Course.ROLL_NO;

Q 41. 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 ON clause or use the WHERE clause to filter rows after joining.

SELECT Student.NAME, Course.COURSE_NAME
FROM Student
JOIN Course ON Student.ROLL_NO = Course.ROLL_NO
WHERE Course.YEAR = 2024 AND Student.AGE > 20;

Q 42. 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 duplicate rows by default.

SELECT Student.NAME FROM Student
UNION
SELECT Teacher.NAME FROM Teacher;

Q 43. How would you perform a JOIN between more than two tables?

To perform a JOIN with multiple tables, chain the JOIN conditions one after the other, specifying 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;

Q 44. What is the role of the ON clause in a JOIN?

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
INNER JOIN Course ON Student.ROLL_NO = Course.ROLL_NO;

Q 45. What is the impact of using DISTINCT with JOIN?

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.

SELECT DISTINCT Student.NAME, Course.COURSE_NAME
FROM Student
INNER JOIN Course ON Student.ROLL_NO = Course.ROLL_NO;

Q 46. 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.

SELECT Student.NAME, COALESCE(Course.COURSE_NAME, 'No Course') AS COURSE_NAME
FROM Student
LEFT JOIN Course ON Student.ROLL_NO = Course.ROLL_NO;

Q 47. 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;

Summary

This article covered essential SQL Joins, a critical concept for working with relational databases. The explanation included different types of joins, such as INNER JOIN, LEFT OUTER JOIN, RIGHT OUTER JOIN, and FULL OUTER JOIN, along with their syntax and use cases. Understanding these joins is vital for querying data from multiple tables effectively.

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