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# Top 10 SQL Scenario-Based Interview Questions for Experienced Professionals

SQL interviews for experienced professionals often dive into complex scenarios that test not only your knowledge but also your problem-solving skills. This post will outline ten challenging SQL scenario-based questions you might encounter, along with explanations to help you understand the solutions.

## Handling Missing Data

In many real-world databases, missing data is a common issue. Interviewers may ask how to handle scenarios where you need to replace missing values or identify records with incomplete data.

Example Question: How would you update all NULL values in a salary column to the average salary of the department?

Solution: You can use the `COALESCE` function along with a subquery to calculate the average salary.

``````UPDATE employees
SET salary = COALESCE(salary, (SELECT AVG(salary) FROM employees e WHERE e.department_id = employees.department_id))
WHERE salary IS NULL;``````

## Data Deduplication

Deduplicating data is essential to maintain data integrity. This question tests your ability to identify and remove duplicate records efficiently.

Example Question: How would you delete duplicate rows from a table without using temporary tables?

Solution: Use the `ROW_NUMBER()` window function to identify duplicates.

``````WITH CTE AS (
SELECT id, ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY id) AS rn
FROM your_table
)
DELETE FROM your_table WHERE id IN (SELECT id FROM CTE WHERE rn > 1);``````

Advanced join operations, such as self-joins or cross joins, are often explored in interviews to assess your understanding of data relationships.

Example Question: How would you find employees who are also managers?

Solution: A self-join can be used here.

``````SELECT e1.name AS Employee, e2.name AS Manager
FROM employees e1
JOIN employees e2 ON e1.manager_id = e2.id;``````

## Recursive Queries

Recursive queries are useful for hierarchical data. Interviewers may ask how to retrieve all child elements from a parent-child relationship.

Example Question: Write a query to get all subordinates of a given manager.

Solution: Use Common Table Expressions (CTEs) for recursive querying.

``````WITH RECURSIVE Subordinates AS (
SELECT id, name, manager_id FROM employees WHERE manager_id = ?
UNION
SELECT e.id, e.name, e.manager_id FROM employees e
INNER JOIN Subordinates s ON s.id = e.manager_id
)
SELECT * FROM Subordinates;``````

## Data Aggregation

Aggregating data is a fundamental SQL skill, and scenarios often involve grouping and summarizing data.

Example Question: How would you calculate the total sales for each product category?

Solution: Utilize the `GROUP BY` clause.

``````SELECT category, SUM(sales) AS total_sales
FROM sales_data
GROUP BY category;``````

## Window Functions

Window functions allow for complex calculations across rows. They’re frequently used for ranking, running totals, and moving averages.

Example Question: How would you calculate the running total of sales for each product?

Solution: Use the `SUM()` function as a window function.

``````SELECT product_id, sales, SUM(sales) OVER (ORDER BY date) AS running_total
FROM sales_data;``````

## Handling Transactions

Transactions ensure data integrity and consistency. Interview questions may explore your knowledge of committing, rolling back, and isolating transactions.

Example Question: How would you handle a situation where you need to update two related tables and ensure both updates succeed?

Solution: Use transactions with commit and rollback.

``````BEGIN TRANSACTION;
UPDATE table1 SET column1 = 'value' WHERE condition;
UPDATE table2 SET column2 = 'value' WHERE condition;
COMMIT;
-- or
ROLLBACK;``````

## Optimizing Query Performance

Performance tuning is crucial for large databases. Questions often revolve around indexing, query plans, and optimization strategies.

Example Question: What strategies would you use to optimize a slow query?

Solution: Possible strategies include:

• Analyzing query plans
• Reducing the number of joins
• Query restructuring

## Data Migration Strategies

Migrating data between systems can be challenging. This often involves understanding schema differences and data transformation.

Example Question: How would you migrate data from an old schema to a new schema with different column names and types?

Solution: Use a mapping table and data transformation scripts.

``````INSERT INTO new_table (new_col1, new_col2)
SELECT old_col1, CAST(old_col2 AS new_type)
FROM old_table;``````

## Security Considerations

SQL injection and data privacy are critical concerns in database management.

Example Question: How would you protect a web application from SQL injection?

Solution: Use prepared statements and parameterized queries.

``````String sql = "SELECT * FROM users WHERE username = ? AND password = ?";
PreparedStatement pstmt = connection.prepareStatement(sql);
ResultSet rs = pstmt.executeQuery();``````

## Conclusion

SQL interviews for experienced professionals can be challenging, but being prepared with knowledge of advanced SQL scenarios can make a significant difference. Review these questions and understand the underlying concepts to excel in your next interview.

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