SQL Interview Questions overview

Q1. Top SQL interview questions for a Data Scientist?

Ans:

1. What is SQL, and why is it important for data science?

2. Explain the difference between SQL and NoSQL databases.

3. What are the primary components of a SQL statement?

4. What is a database schema, and why is it important?

5. What are the basic SQL commands for data retrieval and manipulation?

6. How do you retrieve all records from a table?

7. What is the SELECT statement, and how is it used to retrieve specific data from a table?

8. How do you filter data in SQL using the WHERE clause?

9. Explain the difference between INNER JOIN and OUTER JOIN in SQL.

10. What is an SQL subquery, and when would you use it?

11. How can you aggregate data in SQL? Explain the usage of functions like COUNT, SUM, AVG, MAX, and MIN.

12. What is GROUP BY in SQL, and how is it used to group data?

13. How do you sort data in SQL using the ORDER BY clause?

14. What is the purpose of the HAVING clause in SQL?

15. How do you insert data into a SQL table using the INSERT statement?

16. Explain the UPDATE statement in SQL and how it's used to modify existing records.

17. What is the DELETE statement, and how is it used to remove records from a table?

18. What is normalization in the context of database design, and why is it important?

19. How do you create an index in a database, and why might you use it?

20. What are SQL views, and why would you create one?

21. Explain the concept of ACID properties in the context of database transactions.

22. How can you handle missing or NULL values in SQL queries?

23. What is the difference between a primary key and a foreign key?

24. How do you optimize SQL queries for better performance?

25. Can you explain the use of window functions in SQL, and provide an example?

26. What is the purpose of the LIMIT (or equivalent) clause in SQL, and when would you use it?

27. Describe the difference between UNION and UNION ALL in SQL.

28. How do you back up and restore a database using SQL?

29. Explain the concept of SQL injection and how to prevent it.

30. Can you write a SQL query to find the second-highest salary in an employee table?

Q2. Top 50 SQL queries for a Data Scientist interview questions?

Ans:

Basic SQL Queries:

1. Retrieve all records from a table.
2. Select distinct values from a column.
3. Retrieve records based on a specific condition using the WHERE clause.
4. Count the number of records in a table.
5. Calculate the sum of values in a column.
6. Find the average value in a column.
7. Determine the minimum and maximum values in a column.
8. Retrieve records sorted in ascending order by a specific column.
9. Retrieve records sorted in descending order by a specific column.
10. Limit the number of records returned in a query.

JOIN Operations:

11. Perform an INNER JOIN between two tables.
12. Perform a LEFT JOIN (or LEFT OUTER JOIN) between two tables.
13. Perform a RIGHT JOIN (or RIGHT OUTER JOIN) between two tables.
14. Perform a FULL JOIN (or FULL OUTER JOIN) between two tables.
15. Use a self-join to retrieve related records from the same table.

Subqueries and Aggregation:

16. Write a subquery to find records that meet a specific condition.
17. Use the EXISTS clause to check for the existence of related records.
18. Calculate the total sum of a column using GROUP BY.
19. Find the maximum value within each group using GROUP BY.
20. Retrieve records meeting specific conditions after grouping using the HAVING clause.

Advanced SQL Queries:

21. Use the CASE statement to create custom columns in the result set.
22. Find the Nth highest (or lowest) value in a column.
23. Write a query to calculate the running total of a column.
24. Rank records based on a specific column.
25. Calculate the percentage of each record's value relative to the total.

Window Functions:

26. Use the ROW_NUMBER() function to number records in a result set.
27. Calculate the cumulative sum using the SUM() window function.
28. Retrieve the first and last record within each group using window functions.
29. Calculate the difference between a record's value and the previous record using window functions.

Working with Dates:

30. Extract the year, month, or day from a date column.
31. Calculate the difference between two dates.
32. Find records within a specific date range.
33. Group records by month and calculate aggregate values.

String Operations:

34. Concatenate values from multiple columns into a single column.
35. Extract a substring from a text column.
36. Convert text to uppercase or lowercase.

Data Modification:

37. Insert records into a table.
38. Update records in a table based on a condition.
39. Delete records from a table based on a condition.

More Advanced Topics:

40. Create a new table from an existing table.
41. Use a Common Table Expression (CTE) to simplify complex queries.
42. Perform bulk data loading using the COPY or BULK INSERT command.

Database Administration:

43. Create a new user and grant them specific privileges.
44. Backup a database and restore it from a backup.
45. Find information about table indexes and analyze their usage.

SQL Optimization:

46. Identify and explain the concept of indexing in SQL.
47. Optimize a slow-performing SQL query.
48. Explain the difference between INNER JOIN and LEFT JOIN in terms of performance.

Security and Best Practices:

49. Describe SQL injection and how to prevent it.
50. Explain the importance of parameterized queries for security.

These SQL queries cover a wide range of topics and complexities that a Data Scientist might encounter in their work. Be prepared to not only answer questions about these queries but also write and execute them during the interview to demonstrate your SQL skills.