Hello Dev, if you are an experienced SQL Server professional looking for a job change or preparing for interviews, you need to be well-prepared for the interview process. The interview process may be rigorous and challenging, and your interviewer may test your knowledge and expertise in SQL Server. This article will provide you with a comprehensive guide of SQL Server interview questions for 10 years of experience. It will help you to refresh your knowledge and prepare well for the interview.
1. What is SQL Server?
Microsoft SQL Server is a relational database management system (RDBMS) developed by Microsoft. It is used to store and retrieve data as requested by other software applications. It is widely used in organizations for storing, managing, and retrieving data.
SQL Server Architecture
The SQL Server architecture consists of four main components:
Component |
Description |
|---|---|
Database Engine |
It is the core component that manages the data storage, processing, and security. |
Analysis Services |
It is used for analyzing and reporting data. |
Integration Services |
It is used for transferring and transforming data from one source to another. |
Reporting Services |
It is used for creating, managing, and delivering reports. |
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2. What is a clustered index?
A clustered index is an index structure that determines the physical order of data in a table. It is created on a single column or a combination of columns that are used frequently in the WHERE clause of the SQL queries. When a table has a clustered index, the data is stored physically in the order of the clustered index key. A table can have only one clustered index.
Advantages of Clustered Index
Some of the advantages of clustered index are:
- Improves query performance by reducing the number of I/O operations.
- Provides fast retrieval of data.
- Minimizes fragmentation and improves data storage.
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3. What is a non-clustered index?
A non-clustered index is an index structure that is separate from the data storage. It is created on a single column or a combination of columns that are frequently used in the queries. It stores a copy of the indexed columns along with a pointer to the actual data. A table can have multiple non-clustered indexes.
Advantages of Non-clustered Index
Some of the advantages of non-clustered index are:
- Improves query performance by reducing the number of I/O operations.
- Provides fast retrieval of data.
- Allows fast searching and sorting of data.
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4. What is normalization?
Normalization is the process of organizing data in a database to eliminate redundancy and improve data integrity. It involves splitting a table into two or more tables and defining relationships between them. Normalization is used to prevent data anomalies and inconsistencies by ensuring that each piece of data is stored in only one place.
Types of Normalization
There are several levels of normalization, known as normal forms. The most commonly used normal forms are:
Normal Form |
Description |
|---|---|
First Normal Form (1NF) |
Eliminates repeating groups and ensures that each column contains only atomic values. |
Second Normal Form (2NF) |
Eliminates partial dependencies by separating the table into two or more tables. |
Third Normal Form (3NF) |
Eliminates transitive dependencies by splitting tables into two or more tables. |
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5. What is a transaction?
A transaction is a sequence of one or more database operations that are treated as a single logical unit of work. It is used to ensure data consistency and integrity in a database. A transaction can be either committed or rolled back. When a transaction is committed, all changes made to the database during the transaction are saved permanently. When a transaction is rolled back, all changes made during the transaction are undone.
ACID Properties
A transaction must follow the ACID properties to ensure data consistency and integrity:
- Atomicity: A transaction must be treated as a single logical unit of work, which means that all changes made during the transaction must be either committed or rolled back.
- Consistency: A transaction must leave the database in a consistent state, which means that all constraints and rules must be satisfied.
- Isolation: A transaction must be isolated from other transactions, which means that the changes made during a transaction must be invisible to other transactions until the transaction is committed.
- Durability: A transaction must be durable, which means that once a transaction is committed, its changes must be permanent and survive any subsequent failures.
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6. What is an execution plan?
An execution plan is a detailed description of how SQL Server will execute a specific SQL query. It shows the order in which tables will be accessed, how the data will be retrieved and sorted, and how the results will be returned. The execution plan is generated by the query optimizer and is used to optimize the query performance.
Types of Execution Plan
There are two types of execution plans:
- Estimated Execution Plan: It is generated by the query optimizer without actually executing the query. It provides an estimate of the query execution plan.
- Actual Execution Plan: It is generated by the query optimizer after the query is executed. It provides the actual execution plan, including the actual rows processed, execution time, and resource usage.
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7. What is a deadlock?
A deadlock occurs when two or more transactions are blocked, waiting for each other to release the resources they need to complete their transactions. It is a situation where a transaction holds a lock on a resource that another transaction needs, and that transaction holds a lock on a resource that the first transaction needs. As a result, both transactions are stuck and cannot proceed.
Deadlock Prevention and Resolution
To prevent deadlocks, you can use the following techniques:
- Use the appropriate isolation level.
- Minimize the transaction time.
- Reduce the number of locks held in a transaction.
To resolve deadlocks, you can use the following techniques:
- Kill one of the transactions.
- Roll back one of the transactions.
- Implement retry logic in the application.
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8. What is a trigger?
A trigger is a special type of stored procedure that is automatically executed in response to certain database events. It can be used to enforce business rules, validate data, or perform complex calculations. A trigger can be defined to execute before or after an INSERT, UPDATE, or DELETE operation on a table.
Types of Triggers
There are two types of triggers:
- After Triggers: It is executed after the data modification operation has taken place.
- Instead of Triggers: It is executed instead of the data modification operation. It is used for complex views and security purposes.
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9. What is a view?
A view is a virtual table that is based on the result of a SQL query. It does not store any data, but instead provides a filtered or simplified view of the data in one or more underlying tables. A view can be used to enforce security, simplify data access, or provide a custom view of the data.
Types of Views
There are two types of views:
- Simple Views: It is a view that consists of a single SELECT statement and does not contain any subqueries or joins.
- Complex Views: It is a view that consists of multiple SELECT statements with subqueries, joins, or GROUP BY clauses.
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10. What is backup and restore?
Backup and restore are the processes of creating a copy of a database and restoring it to its original state in case of data loss or corruption. It is essential to have a backup plan to ensure data availability and minimize downtime.
Types of Backup
There are several types of backup:
- Full Backup: It is a backup of the entire database.
- Differential Backup: It is a backup of the data that has changed since the last full backup.
- Transaction Log Backup: It is a backup of the transaction log that contains all the changes made to the database since the last transaction log backup.
To restore a backup, you can use the RESTORE command with the appropriate options.
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11. What is data warehousing?
Data warehousing is the process of storing and managing large volumes of data from different sources in a centralized repository. It is used to support business intelligence and decision-making processes. A data warehouse is designed to provide a unified view of the data and to support complex queries and analysis. It is a separate database optimized for reporting and analysis.
Data Warehousing Concepts
Some of the key concepts in data warehousing are:
- ETL (Extract, Transform, Load): It is the process of extracting data from different sources, transforming it to a common format, and loading it into the data warehouse.
- Data Mart: It is a subset of the data warehouse that is designed for a specific business function or user group.
- Dimensional Model: It is a data model that organizes data into facts and dimensions. Facts are the measures of the data, and dimensions are the attributes that describe the facts.
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12. What is a stored procedure?
A stored procedure is a precompiled set of SQL statements that is stored in a database and can be called by other SQL statements or applications. It is used to improve performance, security, and maintainability. A stored procedure can be used to perform complex queries, data manipulation, or business logic.
Advantages of Stored Procedure
Some of the advantages of stored procedure are:
- Improves performance by reducing network traffic and reducing the number of SQL statements sent to the server.
- Improves security by allowing you to control access to database objects.
- Improves maintainability by centralizing the code and making it easier to modify and update.
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13. What is a database snapshot?
A database snapshot is a read-only copy of a database at a specific point in time. It is used to create a point-in-time view of a database for reporting or analysis purposes. A database snapshot is created using the CREATE DATABASE SNAPSHOT command.
Advantages of Database Snapshot
Some of the advantages of database snapshot are:
- Provides a read-only copy of the database at a specific point in time.
- Allows you to perform reporting and analysis on the snapshot without affecting the original database.
- Can be used to recover data from a specific point in time.
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14. What is database mirroring?
Database mirroring is a high-availability feature of SQL Server that provides database-level redundancy and automatic failover. It involves creating a copy of the database on another server, called the mirror server, and synchronizing the changes between the principal server and the mirror server.
Types of Database Mirroring
There are three types of database mirroring:
- High-Safety Mode: It ensures that the mirror server is always synchronized with the principal server by ensuring that transactions are committed on both servers.
- High-Performance Mode: It optimized performance by allowing the mirror server to lag behind the principal server by a specific amount of time.
- High-Availability Mode: It provides automatic failover in case of a server failure.
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15. What is database replication?
Database replication is the process of copying and distributing data from one database to one or more target databases. It is used to improve performance, scalability, and availability. Database replication involves creating a copy of the data in the source database and sending it to one or more target databases.
Types of Database Replication
There are several types of database replication:
- Snapshot Replication: It involves creating a copy of the entire database and sending it to the target database at a specific interval.
- Transactional Replication: It involves sending only the changes made to the source database to the target database.
- Merge Replication: It involves sending changes made to the source database and the target database and merging them together.
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16. What is SQL injection?
SQL injection is a technique used by attackers to exploit vulnerabilities in SQL Server applications. It involves inserting malicious SQL code into a SQL statement that is executed by the database. SQL injection can be used to bypass authentication, access sensitive data, or execute arbitrary SQL code.
Preventing SQL Injection
To prevent SQL injection, you can use the following techniques:
- Use parameterized queries.
- Validate user input.
- Use stored procedures instead of dynamic SQL.
- Implement strict security policies and access control.
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17. What is indexing?
Indexing is the process of creating an index, which is a data structure that allows faster data retrieval. It is used to improve query performance by reducing the number of I/O operations required to retrieve data. An index is created on one or more columns of a table