Important MS SQL Questions

1. What is SQL? Explain its purpose and common use cases.

??Answer: SQL stands for Structured Query Language. It is a programming language used to manage and manipulate relational databases. SQL is primarily used for tasks such as retrieving data, inserting, updating, and deleting records in a database. Common use cases include data analysis, reporting, and application development.

2. What is the difference between SQL and NoSQL databases?

??Answer: SQL databases are relational databases that use structured data and have predefined schemas, while NoSQL databases are non-relational and use unstructured or semi-structured data. SQL databases provide ACID transactions and have strong consistency, while NoSQL databases offer high scalability, flexibility, and eventual consistency.

3. What are the different types of SQL joins? Provide an example of each.

??Answer: The different types of SQL joins are:

??- INNER JOIN: Returns matching records from both tables based on a specified condition.

???Example: SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id;

??- LEFT JOIN: Returns all records from the left table and the matched records from the right table.

???Example: SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id;

??- RIGHT JOIN: Returns all records from the right table and the matched records from the left table.

???Example: SELECT * FROM table1 RIGHT JOIN table2 ON table1.id = table2.id;

??- FULL JOIN: Returns all records when there is a match in either the left or right table.

???Example: SELECT * FROM table1 FULL JOIN table2 ON table1.id = table2.id;

4. Explain the difference between the INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN operations in SQL.

??Answer:?

??- INNER JOIN returns only the matching records from both tables.

??- LEFT JOIN returns all records from the left table and the matching records from the right table.

??- RIGHT JOIN returns all records from the right table and the matching records from the left table.

??- FULL JOIN returns all records when there is a match in either the left or right table, and includes unmatched records from both.

5. What is a subquery in SQL? How is it different from a regular query?

??Answer: A subquery is a query nested inside another query. It is enclosed within parentheses and used as a part of a larger query. A subquery is different from a regular query in that it is used within another query and provides intermediate results that are used to perform further operations or filtering.

6. What are aggregate functions in SQL? Provide examples of commonly used aggregate functions.

??Answer: Aggregate functions in SQL perform calculations on a set of values and return a single value. Commonly used aggregate functions include:

??- COUNT: Returns the number of rows in a table.

??- SUM: Calculates the sum of a numeric column.

??- AVG: Calculates the average of a numeric column.

??- MAX: Returns the maximum value from a column.

??- MIN: Returns the minimum value from a column.

7. What is the difference between the HAVING clause and the WHERE clause in SQL?

??Answer: The WHERE clause is used to filter rows before they are grouped or aggregated. It is applied to individual rows and operates on column values. The HAVING clause, on the other hand, is used to filter rows after they have been grouped or aggregated. It is applied to groups of rows and operates on the result of an aggregate function.

8. What is normalization in database design? Explain the different normal forms.

??Answer: Normalization is the process of organizing data in a database to eliminate redundancy and improve data integrity. The different normal forms are:

??- First Normal Form (1NF): Ensures that each column contains only atomic values and there are no repeating groups.

??- Second Normal Form (2NF): Builds on 1NF and requires that non-key columns be functionally dependent on the entire primary key.

??- Third Normal Form (3NF): Builds on 2NF and ensures that non-key columns are not transitively dependent on the primary key.

??- Fourth Normal Form (4NF): Addresses multi-valued dependencies and ensures that there are no non-trivial multi-valued dependencies between columns.

??- Fifth Normal Form (5NF): Addresses join dependencies and ensures that there are no non-trivial join dependencies between tables.

9. What are indexes in SQL? How do they improve database performance?

??Answer: Indexes in SQL are data structures that improve the speed of data retrieval operations on database tables. They allow the database engine to quickly locate and retrieve the requested data by creating a separate data structure that contains a sorted copy of the indexed column(s). Indexes improve database performance by reducing the number of data pages that need to be accessed and by enabling faster lookup and search operations.

10. Explain the concept of transactions in SQL. What are ACID properties?

??Answer: Transactions in SQL are a sequence of database operations that are treated as a single logical unit of work. Transactions ensure data consistency and integrity by following the ACID properties:

??- Atomicity: Ensures that a transaction is treated as a single indivisible operation. Either all the changes within a transaction are committed, or none of them are.

??- Consistency: Ensures that a transaction brings the database from one consistent state to another. The database should satisfy all integrity constraints before and after the transaction.

??- Isolation: Ensures that concurrent transactions are isolated from each other, so that the intermediate results of one transaction are not visible to other transactions until the transaction is committed.

??- Durability: Ensures that once a transaction is committed, its changes are permanently saved in the database, even in the event of system failures.

11. What are stored procedures in SQL? How do they differ from functions?

??Answer: Stored procedures in SQL are a set of pre-compiled SQL statements stored in the database catalog. They can be called by applications or other SQL statements. Stored procedures differ from functions in that they can have both input and output parameters, can modify database data, and can execute procedural logic. Functions, on the other hand, are designed to return a single value and are primarily used in SQL expressions.

12. What is the purpose of the GROUP BY clause in SQL? Provide an example.

??Answer: The GROUP BY clause in SQL is used to group rows based on one or more columns. It is often used in combination with aggregate functions to perform calculations on groups of data. Here's an example:

??Suppose we have a table called "Sales" with columns "Product" and "QuantitySold". We want to calculate the total quantity sold for each product. We can use the GROUP BY clause as follows:

??SELECT Product, SUM(QuantitySold) AS TotalQuantity

??FROM Sales

??GROUP BY Product;

??This query will group the rows by the "Product" column and calculate the sum of "QuantitySold" for each product.

13. What is the difference between UNION and UNION ALL in SQL?

??Answer: UNION and UNION ALL are used to combine the result sets of two or more SELECT statements. The difference between them is that UNION removes duplicate rows from the result set, while UNION ALL does not. UNION ALL simply concatenates the rows from all the SELECT statements, including any duplicate rows.

14. Explain the concept of database transactions. What is the significance of rollback and commit?

??Answer: A database transaction is a sequence of database operations that are treated as a single logical unit of work. Transactions ensure data integrity and consistency. The significance of rollback and commit is as follows:

??- ROLLBACK: If a transaction encounters an error or fails to complete successfully, the ROLLBACK statement is used to undo the changes made within the transaction and restore the database to its previous state.

??- COMMIT: When a transaction completes successfully, the COMMIT statement is used to permanently save the changes made within the transaction. Once committed, the changes become permanent and cannot be rolled back.

15. What is the purpose of the FOREIGN KEY constraint in SQL? How does it maintain referential integrity?

??Answer: The FOREIGN KEY constraint in SQL is used to enforce referential integrity between two tables. It ensures that values in a column (the foreign key) of one table match values in another table's primary key. The FOREIGN KEY constraint maintains referential integrity by either allowing or preventing changes or deletions in the referenced table that could result in orphaned or inconsistent data. If a foreign key constraint is violated, the database engine will prevent the operation or cascade the changes to maintain consistency.

These answers should provide a good understanding of the SQL concepts and help you evaluate candidates during an interview. Feel free to ask for further clarification or more questions if needed!