Get Hire Technologies Inc. is a top staffing firm that offers professional online courses for data analysts. We know how important it is to know SQL in order to be successful in the field. We’ve put together a list of 10 important SQL interview questions and tips from experts on how to answer them so you can be ready for your next interview.
1. What’s the difference between LEFT JOIN and INNER JOIN?
Answer: LEFT JOIN gives you all the rows from the left table plus the matched rows from the right table. INNER JOIN only gives you the rows that match from both tables. If there isn’t a match, NULL values are sent back for the correct table fields.
2. How would you find duplicate records in a table?
Answer: To find duplicate records, you can use the GROUP BY clause along with the HAVING clause.
Mention that this query groups rows by the specified column and returns only those groups with more than one occurrence.
3. What’s the purpose of the COALESCE function in SQL?
COALESCE returns the first non-null value in a list. It’s often used to substitute a default value Provide a practical scenario where COALESCE would be useful, such as handling missing data in reports.
4. Explain the difference between WHERE and HAVING clauses.
Answer: WHERE filters individual rows before grouping, while HAVING filters groups after the GROUP BY clause. WHERE is used with SELECT, UPDATE, and DELETE statements, whereas HAVING is only used with the SELECT statement in conjunction with GROUP BY.
5. How would you calculate a running total in SQL?
Answer: A running total can be calculated using window functions.
Explain that window functions allow for calculations across a set of rows that are related to the current row.
6. What’s the purpose of indexing in databases?
Answer: Indexing improves query performance by allowing faster data retrieval. It creates a data structure that enables the database engine to find and access specific rows more quickly, without having to scan the entire table.
Mention that while indexing speeds up data retrieval, it can slow down data insertion and updates. It’s important to strike a balance based on the specific use case.
7. Explain the concept of subqueries and provide an example.
Answer: A subquery is a query nested inside another query. It can be used in various parts of an SQL statement, such as the SELECT, FROM, or WHERE clauses.
Highlight that subqueries can make complex queries more readable and maintainable.
8. How would you pivot data in SQL?
Answer: Pivoting data involves transforming rows into columns. This can be achieved using conditional aggregation or the PIVOT function (in databases that support it). Mention that while this approach works across most database systems, some have specific PIVOT functions that can simplify the syntax.
9. What’s the difference between UNION and UNION ALL?
Answer: Both UNION and UNION ALL combine result sets from multiple queries. However, UNION removes duplicate rows from the final result set, while UNION ALL includes all rows, including duplicates. UNION ALL is generally faster as it doesn’t need to perform a distinct operation.
10. How would you optimize a slow-running query?
Answer: Query optimization can involve several strategies:
- Ensure proper indexing on frequently queried columns.
- Use JOINs instead of subqueries where possible.
- Limit the use of OR conditions and consider UNION ALL as an alternative.
- Use EXPLAIN to analyze the query execution plan and identify bottlenecks.
Conclusion
Mastering these SQL concepts and being able to articulate your understanding clearly can significantly boost your chances of success in data analyst interviews. At Get Hire Technologies Inc, we offer a comprehensive data analyst course that covers these SQL topics and more, making sure you’re well-prepared for your career in data analysis.
