Here are interview questions on technical skills related to SQL Developer Jobs in the USA;

1. Question: What is SQL, and how is it different from other programming languages?

   Answer: SQL (Structured Query Language) is a domain-specific language used for managing and manipulating relational databases. It differs from general-purpose languages in that it specifically deals with database operations, such as querying, updating, and managing data.

2. Question: Explain the differences between DELETE and TRUNCATE statements.

   Answer: DELETE is a DML (Data Manipulation Language) statement that removes specific rows based on a condition, while TRUNCATE is a DDL (Data Definition Language) statement that removes all rows from a table, effectively resetting the table.

3. Question: What is a primary key, and why is it important in a database?

   Answer: A primary key is a unique identifier for a record in a table. It ensures data integrity by preventing duplicate and null values in the key column, and it’s used to establish relationships between tables.

4. Question: How do you optimize a SQL query?

   Answer: Optimization involves using indexes, ensuring appropriate joins, avoiding SELECT * for large tables, and optimizing WHERE clauses. Analyzing execution plans and using tools like SQL Profiler can help identify performance bottlenecks.

5. Question: What is the purpose of an index in a database, and when would you create one?

   Answer: An index improves query performance by allowing the database engine to locate and retrieve data more efficiently. Indexes are created on columns frequently used in WHERE clauses or JOIN conditions.

6. Question: Explain the concept of normalization in the context of database design.

   Answer: Normalization is the process of organizing data to eliminate redundancy and dependency, ensuring data integrity and reducing anomalies. It involves breaking down large tables into smaller, related tables.

7. Question: What is a stored procedure, and how is it different from a function?

   Answer: A stored procedure is a precompiled collection of one or more SQL statements stored in the database. It can return multiple values and doesn’t have to return a value. A function, on the other hand, must return a single value and is called within a SELECT statement.

8. Question: Describe the differences between UNION and UNION ALL.

   Answer: UNION combines the result sets of two SELECT statements, removing duplicate rows, while UNION ALL combines result sets, including duplicate rows.

9. Question: How do you handle NULL values in SQL?

   Answer: NULL represents the absence of data. Functions like IS NULL and IS NOT NULL are used to check for NULL values, and COALESCE or CASE statements can be used to handle them in queries.

10. Question: Explain the concept of a foreign key and its role in maintaining data integrity.

    Answer: A foreign key is a field in a table that refers to the primary key in another table, establishing a relationship between the two. It enforces referential integrity, ensuring that relationships between tables are maintained.

11. Question: What are the differences between INNER JOIN and OUTER JOIN?

    Answer: INNER JOIN returns only matching rows, while OUTER JOIN (LEFT, RIGHT, or FULL) returns matching rows and non-matching rows from one or both tables, filling in the gaps with NULLs.

12. Question: How can you prevent SQL injection in your queries?

    Answer: Use parameterized queries or prepared statements, validate user input, and implement proper authentication and authorization mechanisms to prevent unauthorized access.

13. Question: Explain the purpose of the GROUP BY clause in SQL.

    Answer: GROUP BY is used to arrange identical data into summary rows, typically with an aggregate function (e.g., COUNT, SUM) to perform operations on each group.

14. Question: What is the difference between a clustered and non-clustered index?

    Answer: A clustered index determines the physical order of data in a table, while a non-clustered index does not. A table can have only one clustered index, but multiple non-clustered indexes.

15. Question: How do you approach database version control and schema migrations?

    Answer: Use version control systems like Git for database scripts and tools like Flyway or Liquibase to manage and apply schema changes in a controlled manner.

16. Question: Explain the concept of ACID properties in the context of database transactions.

    Answer: ACID stands for Atomicity, Consistency, Isolation, and Durability. It ensures that database transactions are reliable, complete, maintain data integrity, execute independently of other transactions, and persist changes even after a system failure.

17. Question: What is the purpose of the HAVING clause in SQL?

    Answer: The HAVING clause filters the results of a GROUP BY clause based on specified conditions, allowing you to filter aggregated data.

18. Question: How would you handle a scenario where a database backup fails?

    Answer: Check error logs, ensure sufficient disk space, and verify the backup command syntax. If necessary, consider alternative backup methods or tools.

19. Question: What is the purpose of the ORDER BY clause in SQL?

    Answer: ORDER BY is used to sort the result set of a query in ascending or descending order based on specified columns.

20. Question: How do you optimize a database for better performance?

    Answer: Optimize queries, use appropriate indexing, consider partitioning large tables, monitor and manage server resources, and regularly update statistics.

21. Question: Explain the use of the CASE statement in SQL.

    Answer: The CASE statement is used for conditional logic in SQL queries, similar to a switch statement in other languages. It allows for conditional control within a SELECT, WHERE, or HAVING clause.

22. Question: What is a trigger, and in what situations would you use it?

    Answer: A trigger is a set of instructions that automatically execute in response to a specific event on a particular table or view. Triggers are used to maintain data integrity, enforce business rules, or log changes.

23. Question: How would you handle a situation where a database is slow due to high concurrent connections?

    Answer: Investigate the root cause using performance monitoring tools, optimize queries, and consider connection pooling to efficiently manage and reuse database connections.

24. Question: Describe the process of database normalization and its benefits.

    Answer: Database normalization is the process of organizing data to eliminate redundancy and dependency. It reduces data anomalies, improves data integrity, and makes the database more flexible and scalable.

25. Question: In a scenario where a database table becomes too large, affecting performance, what strategies would you employ to address this issue?

    Answer: Consider partitioning the table, archiving old data, and optimizing queries to focus on necessary subsets. Evaluate hardware resources and scale up or use sharding techniques to distribute data across multiple servers.

Remember to adapt your responses based on your experiences and the specific technologies you have worked with. Good luck in your interviews!