SQL Top Interview Questions Answer [2024]
SQL interview questions along with their answers:
SQL interview questions along with their answers:
1. What is SQL?
SQL stands for Structured Query Language. It is a programming language designed for managing and querying relational databases.
2. Differentiate between SQL and NoSQL.
SQL databases are relational databases, whereas NoSQL databases are non-relational. SQL databases use structured query language, while NoSQL databases have different query languages.
3. What are the different types of SQL commands?
SQL commands can be categorized into Data Definition Language (DDL), Data Manipulation Language (DML), Data Control Language (DCL), and Transaction Control Language (TCL).
4. Explain the difference between WHERE and HAVING clause.
WHERE clause filters rows before grouping (aggregation) while HAVING clause filters rows after grouping.
Example of WHERE:
SELECT Department, Salary FROM Employees
WHERE Department = 'Marketing';
Example of HAVING:
SELECT Department, AVG(Salary) AS AverageSalary FROM Employees
GROUP BY Department
HAVING AVG(Salary) > 50000;
5. What is the purpose of the GROUP BY clause in SQL?
The GROUP BY clause is used to group rows that have the same values in specified columns and allows performing aggregate functions on them.
6. Explain SQL JOINs and their types.
JOINs are used to retrieve data from multiple tables. Types include INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN.
7. What is a subquery?
A subquery is a query nested within another query. It can be used to return data that will be used in the main query as a condition or to further filter results.
8. Describe the ACID properties in database transactions.
ACID stands for Atomicity, Consistency, Isolation, and Durability—fundamental properties that ensure database transactions are processed reliably.
9. What is normalization in databases?
Normalization is the process of organizing data in a database to minimize redundancy and dependency. It involves dividing large tables into smaller tables and defining relationships between them.
10. What is a primary key?
A primary key is a unique identifier for a record in a database table. It ensures each record in a table is uniquely identifiable.
11. How do you handle NULL values in SQL?
NULL represents missing or unknown data. Handling NULL values can be done using functions like IS NULL, IS NOT NULL, COALESCE, etc.
12. Explain the difference between UNION and UNION ALL.
UNION combines and removes duplicate rows from the result set, while UNION ALL combines all rows, including duplicates.
13. What is a stored procedure?
A stored procedure is a precompiled collection of SQL statements that can be executed by calling the procedure name. It is stored in the database and can be reused.
14. How do you optimize SQL queries?
Query optimization involves various techniques like using indexes, minimizing the use of wildcard characters in WHERE clauses, avoiding unnecessary joins, and optimizing subqueries.
15. What is the purpose of the COMMIT and ROLLBACK statements?
COMMIT is used to save transactions to the database, while ROLLBACK is used to undo transactions that have not been committed.
16. What is the difference between CHAR and VARCHAR data types?
CHAR is a fixed-length character data type, while VARCHAR is a variable-length character data type. CHAR will always use the specified length, padding with spaces if needed, while VARCHAR will only use as much space as needed.
17. Explain the use of the BETWEEN operator in SQL.
BETWEEN is used to filter results within a specific range of values. For example: SELECT * FROM table WHERE column BETWEEN value1 AND value2.
18. What are aggregate functions in SQL?
Aggregate functions perform a calculation on a set of values and return a single value. Common aggregate functions include SUM, AVG, COUNT, MIN, and MAX.
19. How do you add a new record to a table in SQL?
The SQL command used to add a new record is INSERT INTO. For instance: INSERT INTO table_name (column1, column2) VALUES (value1, value2).
20. What is the purpose of the ORDER BY clause?
ORDER BY is used to sort the result set based on specified columns either in ascending (ASC) or descending (DESC) order.
21. Explain the difference between a candidate key, a primary key, and a foreign key.
A candidate key is a column or a set of columns that can uniquely identify a record in a table. A primary key is selected from the candidate keys to uniquely identify records in a table. A foreign key is a column that establishes a relationship between tables by referencing the primary key of another table.
22. What is a self-join?
A self-join is when a table is joined with itself. It's used when a query requires comparing rows within the same table.
23. How do you delete records from a table in SQL?
The DELETE command is used to remove records from a table based on specified conditions, such as: DELETE FROM table_name WHERE condition.
24. Explain the concept of indexing in SQL.
Indexing is a database feature used to improve the speed of data retrieval operations on a table. It creates a separate data structure that allows for faster searching of specific columns.
25. What is a view in SQL?
A view is a virtual table based on the result set of a SELECT query. It doesn't store data itself but presents data from one or more tables, making it easier to query frequently used complex queries.
26. What is the purpose of the DISTINCT keyword in SQL?
DISTINCT is used in a SELECT statement to eliminate duplicate rows from the result set, returning only distinct (unique) values.
27. Explain the concept of a transaction in SQL.
A transaction is a sequence of one or more SQL statements that are executed as a single unit. It follows the ACID properties (Atomicity, Consistency, Isolation, Durability) to ensure the reliability of database operations.
28. What is a trigger in SQL?
A trigger is a set of instructions that are automatically executed (or "triggered") in response to certain events on a particular table or view. Common events include INSERT, UPDATE, and DELETE operations.
29. How do you find the second highest (or nth highest) salary from an employee table?
One way is to use the LIMIT clause with a subquery. For example:
SELECTDISTINCT salary FROM employees ORDERBY salary DESC LIMIT 1OFFSET1;
30. What is the difference between UNION and JOIN?
UNION is used to combine the results of two or more SELECT statements, removing duplicates. JOIN is used to retrieve data from multiple tables based on a related column.
31. Explain the concept of a database index.
An index is a data structure that improves the speed of data retrieval operations on a database table. It is created on one or more columns of a table to provide quick access to rows.
32. How do you update data in a table in SQL?
The UPDATE command is used to modify existing records in a table. For example:
UPDATE table_name SET column1 = value1, column2 = value2 WHEREcondition;
33. What is the purpose of the GROUP_CONCAT function?
GROUP_CONCAT is used to concatenate values from multiple rows into a single string. It is often used with the GROUP BY clause.
34. Explain the concept of normalization and denormalization.
Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. Denormalization involves combining tables and introducing redundancy to improve query performance.
35. What is the role of the HAVING clause in SQL?
The HAVING clause is used in conjunction with the GROUP BY clause to filter the results of aggregate functions based on specified conditions.
36. What is a trigger in SQL?
A trigger is a set of SQL statements that automatically "fires" (executes) in response to certain events on a table, such as INSERT, UPDATE, or DELETE operations.
37. How do you retrieve unique values from a table?
The DISTINCT keyword is used to retrieve unique values from a specific column or set of columns in a table. For example: SELECT DISTINCT column_name FROM table_name.
38. What is the purpose of the GROUPING SETS clause?
GROUPING SETS is used with the GROUP BY clause to specify multiple groupings of columns to generate multiple grouping sets in a single query.
39. Explain the difference between a view and a materialized view.
A view is a virtual table representing the result set of a SELECT query, while a materialized view is a physical copy of the result set stored as a table, updated periodically based on defined conditions.
40. How does the EXISTS operator work in SQL?
The EXISTS operator is used to check the existence of rows in a subquery. It returns true if the subquery returns at least one row; otherwise, it returns false.
41. What is the purpose of the LIKE operator?
The LIKE operator is used in a WHERE clause to search for a specified pattern in a column. It can use wildcard characters such as % (matches any sequence of characters) or _ (matches any single character).
42. Explain the difference between a clustered and non-clustered index.
A clustered index determines the physical order of the rows in a table based on the indexed column(s). A non-clustered index does not alter the physical order of the table and creates a separate structure for faster data retrieval.
43. How do you alter a table in SQL?
The ALTER TABLE statement is used to modify an existing table in various ways, such as adding columns, modifying column data types, or dropping columns.
44. What is a CTE (Common Table Expression) in SQL?
A CTE is a temporary result set that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. It helps simplify complex queries and makes them more readable.
45. Explain the difference between a UNION and a UNION ALL.
UNION merges the results of two or more SELECT statements and removes duplicates, while UNION ALL includes all rows, including duplicates.
46. What is the difference between a database and a schema?
A database is a collection of tables, views, stored procedures, and more. A schema is a collection of database objects (tables, views, etc.) and belongs to a specific database. Multiple schemas can exist within a single database.
47. How do you perform a full-text search in SQL?
Full-text search is performed using the CONTAINS or FREETEXT functions in SQL Server or the MATCH AGAINST clause in MySQL. It allows searching for words or phrases within text columns.
48. Explain the purpose of the RANK() function.
The RANK() function assigns a rank to each row within a result set based on a specified column's values. It allows handling ties by giving the same rank to equal values and leaving gaps in ranking if tied.
49. What is a correlated subquery?
A correlated subquery is a subquery that depends on the outer query for its values. It executes once for every row processed by the outer query.
50. How do you perform transactions in SQL?
Transactions in SQL are handled using the BEGIN TRANSACTION, COMMIT, and ROLLBACK statements. BEGIN TRANSACTION starts a new transaction, COMMIT saves changes to the database, and ROLLBACK undoes changes made during the transaction.
51. Explain the use of the COALESCE function.
The COALESCE function returns the first non-null expression among its arguments. It's useful for handling null values by providing a default value.
SELECTProductName, COALESCE(ProductDescription, 'No description available') AS Description FROM Products;
52. What is the difference between a left join and a right join?
A left join includes all the rows from the left table and matching rows from the right table, while a right join includes all the rows from the right table and matching rows from the left table.
53. How do you handle duplicates in a result set?
Duplicates can be eliminated using the DISTINCT keyword or removed using the GROUP BY clause, depending on the desired outcome.
54. Explain the purpose of the CASE statement.
The CASE statement is used for conditional logic in SQL. It evaluates a set of conditions and returns a result based on the first condition that is true.
SELECT id, name, department, salary,
CASEWHEN salary >=50000THEN'High'
WHEN salary >= 45000THEN'Medium'
ELSE'Low'
ENDASsalary_level
FROM employees;
55. What is the purpose of the TRUNCATE TABLE statement?
TRUNCATE TABLE removes all rows from a table but keeps the table structure intact. It's faster than DELETE as it doesn't log individual row deletions.
56. What is the difference between a view and a materialized view?
A view is a virtual table generated by a query, showing the result set of that query. A materialized view, however, is a physical copy of the result set that is stored on disk, which can be refreshed periodically but requires storage space.
57. Explain the concept of a self-join with an example.
A self-join is when a table is joined with itself. For instance, consider an Employee table with columns like EmployeeID and ManagerID. You can use a self-join to match employees with their managers:
SELECT e1.EmployeeID, e1.EmployeeName, e2.EmployeeName AS ManagerName FROM Employee e1 INNERJOIN Employee e2 ON e1.ManagerID = e2.EmployeeID;
58. How do you retrieve the nth highest (or nth lowest) value from a table?
To get the nth highest value, you can use a query with the LIMIT clause in MySQL or the ROW_NUMBER() function in SQL Server. For example, to get the 5th highest salary:
SELECT Salary FROM Employee ORDERBY Salary DESC LIMIT 1OFFSET4; -- For MySQL-- For SQL Server
SELECT Salary FROM ( SELECT Salary, ROW_NUMBER() OVER (ORDERBY Salary DESC) AS rn FROM Employee ) AS t WHERE rn = 5;
59. Explain the purpose of the HAVING clause in SQL.
The HAVING clause is used with the GROUP BY clause to filter grouped rows based on specified conditions. It's used to filter aggregated data.
60. What is the difference between a constraint and an index in SQL?
Constraints define rules or restrictions for data in a table (e.g., PRIMARY KEY, FOREIGN KEY), ensuring data integrity. Indexes, on the other hand, are used to improve query performance by providing quick access to rows in a table.
61. How do you find duplicate records in a table?
To find duplicate records, you can use a query with GROUP BY and HAVING clauses to identify records where the count of occurrences is more than one for a specific column or set of columns.
62. Explain the concept of a subquery with an example.
A subquery is a query nested within another query. For example, to find employees whose salary is greater than the average salary:
SELECT EmployeeName, Salary FROM Employee WHERE Salary > (SELECTAVG(Salary) FROM Employee);
63. How can you update values in a table using a JOIN?
You can update values in a table using a JOIN in the UPDATE statement, specifying the table and columns to be updated along with the JOIN condition.
UPDATE salaries AS s
JOIN employees AS eONs.employee_id = e.id
SET s.salary = s.salary * 1.1
WHERE e.department = 'Sales';;
64. What is the purpose of the EXISTS operator in SQL?
The EXISTS operator checks for the existence of rows returned by a subquery and returns true if the subquery returns any rows, otherwise false.
SELECT * FROM employees e WHEREEXISTS ( SELECT1FROM departments d WHEREd.id = e.department_id ANDd.name = 'Sales'); ;);
65. How do you retrieve the current date and time in SQL?
The function to get the current date and time varies by database system, such as GETDATE() in SQL Server, CURRENT_TIMESTAMP() in MySQL, and SYSDATE in Oracle.
Scenario Based SQL Questions with Answers
1. Retrieve all unique cities from the customers table.
SELECT DISTINCT city FROM customers;
2. Find the total number of orders placed by each customer.
SELECT customer_id, COUNT(order_id) AS total_orders FROM orders GROUP BY customer_id;
3. Display the top 5 products with the highest sales.
SELECT product_id, SUM(quantity) AS total_sales FROM order_details GROUP BY product_id ORDER BY total_sales DESC LIMIT 5;
4. Calculate the average salary of employees in each department.
SELECT department_id, AVG(salary) AS avg_salary FROM employees GROUP BY department_id;
5. Retrieve the order details along with customer information for a specific order (e.g., Order ID = 123).
SELECT o.order_id, c.customer_name, c.email, od.product_id, od.quantity FROM orders o JOIN customers c ON o.customer_id = c.customer_id JOIN order_details od ON o.order_id = od.order_id WHERE o.order_id = 123;
6. Identify employees whose salary is above the average salary in their department.
SELECT e.employee_id, e.employee_name, e.salary, e.department_id FROM employees e JOIN ( SELECT department_id, AVG(salary) AS avg_salary FROM employees GROUP BY department_id ) dept_avg ON e.department_id = dept_avg.department_id WHERE e.salary > dept_avg.avg_salary;
7. Retrieve the total number of customers and the count of their orders, including customers with zero orders.
SELECT c.customer_id, COUNT(o.order_id) AS order_count FROM customers c LEFT JOIN orders o ON c.customer_id = o.customer_id GROUP BY c.customer_id;
8. Calculate the percentage of total sales for each product.
SELECT product_id, SUM(quantity) AS total_sales, SUM(quantity) * 100.0 / (SELECT SUM(quantity) FROM order_details) AS sales_percentage FROM order_details GROUP BY product_id;
9. Find customers who have made more than 3 orders in the last month.
SELECT customer_id, COUNT(order_id) AS total_orders_last_month FROM orders WHERE order_date >= DATE_SUB(CURDATE(), INTERVAL 1 MONTH) GROUP BY customer_id HAVING total_orders_last_month > 3;
10. Retrieve the employees who have the same job title and salary.
SELECT e1.employee_id, e1.employee_name, e1.job_title, e1.salary, e1.department_id FROM employees e1 JOIN employees e2 ON e1.job_title = e2.job_title AND e1.salary = e2.salary AND e1.employee_id <> e2.employee_id;
11. Retrieve the 3 most recent orders for each customer.
WITH RankedOrders AS ( SELECT customer_id, order_id, order_date, ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY order_date DESC) AS rnk FROM orders ) SELECT customer_id, order_id, order_date FROM RankedOrders WHERE rnk <= 3;
12. Calculate the total revenue for each month.
SELECT EXTRACT(MONTH FROM order_date) AS month, EXTRACT(YEAR FROM order_date) AS year, SUM(unit_price * quantity) AS total_revenue FROM orders JOIN order_details ON orders.order_id = order_details.order_id GROUP BY year, month ORDER BY year, month;
13. Identify customers who have not placed any orders.
SELECT customer_id, customer_name FROM customers WHERE customer_id NOT IN (SELECT DISTINCT customer_id FROM orders);
14. Retrieve the products with a quantity sold greater than the average quantity sold for all products.
WITH ProductAverage AS ( SELECT AVG(quantity) AS avg_quantity FROM order_details ) SELECT product_id, product_name, quantity FROM order_details WHERE quantity > (SELECT avg_quantity FROM ProductAverage);
15. Find the employees who manage more than one department.
SELECT manager_id, COUNT(DISTINCT department_id) AS num_departments_managed FROM employees WHERE manager_id IS NOT NULL GROUP BY manager_id HAVING num_departments_managed > 1;
16. Retrieve the order details for orders that include products with a price greater than $100.
SELECT od.order_id, od.product_id, od.quantity, od.unit_price FROM order_details od JOIN products p ON od.product_id = p.product_id WHERE p.unit_price > 100;
17. Calculate the running total of sales for each product.
SELECT product_id, order_id, quantity, unit_price, SUM(quantity * unit_price) OVER (PARTITION BY product_id ORDER BY order_id) AS running_total FROM order_details;
18. Find the customers who placed orders on consecutive days.
SELECT DISTINCT o1.customer_id, c.customer_name FROM orders o1 JOIN orders o2 ON o1.customer_id = o2.customer_id AND ABS(DATEDIFF(o1.order_date, o2.order_date)) = 1;
19. Identify the top 3 departments with the highest average employee salary.
SELECT department_id, AVG(salary) AS avg_salary FROM employees GROUP BY department_id ORDER BY avg_salary DESC LIMIT 3;
20. Retrieve the products that were never sold.
SELECT product_id, product_name FROM products WHERE product_id NOT IN (SELECT DISTINCT product_id FROM order_details);
21. Retrieve the top 3 departments with the highest average employee salary.
SELECT department_id, AVG(salary) AS avg_salary FROM employees GROUP BY department_id ORDER BY avg_salary DESC LIMIT 3;
22. List all orders along with the latest order date for each customer.
SELECT o.customer_id, MAX(o.order_date) AS latest_order_date FROM orders o GROUP BY o.customer_id;
23. Calculate the total revenue generated by each product category.
SELECT p.category, SUM(od.quantity * p.unit_price) AS total_revenue FROM products p JOIN order_details od ON p.product_id = od.product_id GROUP BY p.category;
24. Retrieve the names and contact information of customers who haven't placed any orders.
SELECT c.customer_name, c.email, c.phone FROM customers c LEFT JOIN orders o ON c.customer_id = o.customer_id WHERE o.order_id IS NULL;
25. Identify the employees who joined before 2000 and are currently in a higher position than their manager.
SELECT e1.employee_id, e1.employee_name, e1.start_date, e1.job_title, e1.manager_id FROM employees e1 JOIN employees e2 ON e1.manager_id = e2.employee_id WHERE e1.start_date < '2000-01-01' AND e1.job_title > e2.job_title;
26. Find customers who have placed orders for more than three different products.
SELECT o.customer_id, COUNT(DISTINCT od.product_id) AS distinct_products_ordered FROM orders o JOIN order_details od ON o.order_id = od.order_id GROUP BY o.customer_id HAVING distinct_products_ordered > 3;
27. Calculate the total number of orders and the total revenue for each month in the current year.
SELECT MONTH(order_date) AS order_month, COUNT(order_id) AS total_orders, SUM(quantity * unit_price) AS total_revenue FROM orders o JOIN order_details od ON o.order_id = od.order_id WHERE YEAR(order_date) = YEAR(CURDATE()) GROUP BY order_month;
28. Retrieve the top 5 customers who have spent the most in total.
SELECT o.customer_id, c.customer_name, SUM(od.quantity * p.unit_price) AS total_spent FROM orders o JOIN order_details od ON o.order_id = od.order_id JOIN products p ON od.product_id = p.product_id JOIN customers c ON o.customer_id = c.customer_id GROUP BY o.customer_id ORDER BY total_spent DESC LIMIT 5;
SQL Server administration interview questions along with their answers
1. What is a clustered index in SQL Server?
Answer: A clustered index is a type of index in SQL Server where the physical order of rows in a table is the same as the order of the rows in the index. It determines the order in which data is stored in the table.
2. Differentiate between CHAR and VARCHAR data types in SQL Server.
Answer:
CHAR: Fixed-length character data type where storage is allocated for the defined length, padding extra spaces if the actual data is shorter.
VARCHAR: Variable-length character data type that only uses the storage necessary for the actual data without padding.
3. Explain the purpose of the "sp_configure" system stored procedure in SQL Server.
Answer: sp_configure is used to view or change server-level configuration settings in SQL Server. It allows modifying various server configuration options such as memory allocation, max degree of parallelism, etc.
4. What is a deadlock in SQL Server? How can you handle it?
Answer: A deadlock occurs when two or more processes permanently block each other by holding resources that the other processes are trying to acquire. To handle deadlocks, techniques like deadlock detection and resolution, using timeouts, or restructuring queries to reduce contention are employed.
5. Explain the concept of SQL Server log shipping.
Answer: SQL Server log shipping involves automatically sending transaction log backups from one database (the primary database) to another (the secondary database) on a different server. This process helps maintain a standby copy of the database for disaster recovery purposes.
6. What are the different recovery models in SQL Server, and how do they differ?
Answer:
Simple: Allows only full backups and doesn’t support transaction log backups. In case of failure, the restore is done to the last full backup.
Full: Allows full and differential backups along with transaction log backups. Provides the ability to restore to a specific point in time.
Bulk-logged: Similar to the full recovery model but minimally logs certain bulk operations to reduce log space usage.
7. What is the purpose of the SQL Server Agent?
Answer: SQL Server Agent is a component that automates administrative tasks and jobs in SQL Server. It schedules and executes tasks like backups, database maintenance, and other administrative functions.
8. Describe the steps involved in performing a database backup in SQL Server.
Answer:
Choose the backup type (full, differential, transaction log).
Use SQL Server Management Studio or T-SQL commands (BACKUP DATABASE or BACKUP LOG) to initiate the backup.
Specify the destination for the backup file.
Set any necessary options such as compression, verification, etc.
Execute the backup.
9. Explain the purpose of SQL Server Profiler.
Answer: SQL Server Profiler is a tool used to monitor and capture SQL Server events and activity in real-time. It helps in troubleshooting, performance tuning, and auditing by capturing events like queries, errors, and other database-related activities.
10. How can you monitor SQL Server performance?
Answer: Performance monitoring can be done using various methods:
Reviewing system views and dynamic management views (DMVs).
Using tools like SQL Server Profiler, SQL Server Management Studio (SSMS), and SQL Server Performance Monitor.
Analyzing query execution plans and tuning poorly performing queries.
Monitoring server health, including CPU, memory, and disk usage.
11. Explain the concept of SQL Server AlwaysOn Availability Groups.
Answer: AlwaysOn Availability Groups is a high-availability and disaster recovery solution that enables you to create a group of user databases that can fail over together. It provides both high availability and disaster recovery capabilities by maintaining multiple copies of the databases.
12. What are the key components of SQL Server AlwaysOn Availability Groups?
Answer:
Availability Group: Contains a set of user databases and associated replicas.
Replicas: Individual database instances that host copies of the databases.
Availability Replica: Represents an instance participating in the availability group, either as a primary or secondary replica.
13. How can you troubleshoot performance issues in SQL Server?
Answer:
Reviewing execution plans and optimizing queries.
Monitoring server resources (CPU, memory, disk usage) using tools like Performance Monitor.
Identifying and resolving blocking and deadlocking issues.
Regular index maintenance and statistics updates.
Using SQL Server Profiler to capture and analyze query behavior.
14. Explain the concept of database mirroring in SQL Server.
Answer: Database mirroring is a solution that maintains a hot standby database (the mirror database) that is synchronized with the principal database. It ensures high availability and automatic failover in case the principal database becomes unavailable.
15. What are the differences between a primary key and a unique key in SQL Server?
Answer:
Primary Key: Ensures uniqueness of each row in a table and doesn’t allow NULL values. Each table can have only one primary key.
Unique Key: Also ensures uniqueness but allows NULL values except in the column(s) defined as unique. Multiple unique keys can exist in a table.
16. How can you manage SQL Server security?
Answer:
Using logins and user accounts to control access.
Assigning appropriate permissions at various levels (database, table, stored procedure).
Auditing and monitoring access using SQL Server Audit and other tools.
Implementing encryption and SSL/TLS for secure connections.
17. Explain the concept of SQL Server backup and restore strategies.
Answer:
Strategies involve full, differential, and transaction log backups to ensure data recoverability.
Full backups capture the entire database.
Differential backups capture changes since the last full backup.
Transaction log backups capture transaction log changes since the last log backup.
18. How do you monitor and manage SQL Server transaction logs?
Answer:
Monitoring log space usage to prevent log file growth issues.
Regularly backing up transaction logs to maintain log file size and enable point-in-time recovery.
Managing log file growth and ensuring it doesn’t fill up the disk.
19. Explain the concept of SQL Server collations and their significance.
Answer:
Collations determine how data is sorted and compared in SQL Server.
They define rules for character set sorting and comparison sensitivity (case sensitivity, accent sensitivity).
Collation settings affect how string data is stored, retrieved, and compared.
20. How can you optimize SQL Server performance for a specific query?
Answer:
Analyzing query execution plans to identify bottlenecks.
Index optimization by adding, removing, or modifying indexes.
Query rewriting or restructuring to improve efficiency.
Parameterizing queries to leverage query plan caching.
Using appropriate hints to guide the query optimizer.
21. Explain the purpose and use of SQL Server Agent Jobs.
Answer: SQL Server Agent Jobs are automated tasks that can be scheduled to perform specific actions such as running queries, executing stored procedures, sending notifications, or performing maintenance activities at specified intervals or times.
22. What is the purpose of SQL Server Profiler and Extended Events?
Answer:
SQL Server Profiler: It's a graphical user interface to monitor an instance of SQL Server, capturing data about each SQL Server process as it occurs. It's used for tracing and troubleshooting.
Extended Events: It's a lightweight performance monitoring system that provides a highly scalable and configurable architecture for collecting different types of data, useful for troubleshooting and monitoring.
23. Explain the role of SQL Server Configuration Manager.
Answer: SQL Server Configuration Manager is a tool used to manage the SQL Server services and network connectivity configuration. It allows configuring and managing SQL Server services, protocols, client configuration, and server network configuration.
24. What are the different types of joins in SQL Server?
Answer:
Inner Join: Retrieves rows when there is a match in both tables.
Left Join (or Left Outer Join): Retrieves all rows from the left table and matching rows from the right table.
Right Join (or Right Outer Join): Retrieves all rows from the right table and matching rows from the left table.
Full Join (or Full Outer Join): Retrieves all rows when there is a match in either table.
25. Explain the process of database restoration in SQL Server.
Answer:
To restore a database, you need a valid backup and appropriate permissions.
Use the RESTORE DATABASE command specifying the backup file and options (like WITH REPLACE to overwrite an existing database).
Choose the appropriate recovery model and backup types (full, differential, or transaction log) to restore to a specific point in time if necessary.
26. What is the purpose of SQL Server's 'tempdb' database?
Answer: tempdb is a system database in SQL Server used to store temporary tables, temporary stored procedures, and other temporary user objects. It's used for sorting, grouping, and other internal operations and is recreated every time SQL Server restarts.
27. Explain the concept of database mirroring in SQL Server.
Answer: Database mirroring is a high-availability technique that involves two copies of a single database that reside on different server instances. It provides automatic failover capabilities in case of a principal database failure.
28. How can you monitor and manage SQL Server transaction logs?
Answer:
Monitoring log space usage regularly to avoid issues with log file growth.
Regularly backing up transaction logs to maintain log file size and enable point-in-time recovery.
Managing log file growth settings and ensuring there is sufficient space on disk.
29. Explain the purpose of the SQL Server Error Log.
Answer: The SQL Server Error Log records information about events, errors, warnings, and messages generated by SQL Server. It's crucial for troubleshooting issues, auditing, and identifying problems within the SQL Server instance.
30. What is the significance of the 'sp_who' and 'sp_who2' stored procedures in SQL Server?
Answer: These stored procedures provide information about current users, sessions, and processes connected to the SQL Server instance. They offer details such as session ID, login information, status, and more, aiding in monitoring and troubleshooting active connections.
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