Overview
Ensuring data integrity in a SQL database is fundamental to maintaining accurate, reliable, and consistent data across the database system. Data integrity involves various mechanisms, constraints, and protocols to prevent unauthorized access or corruption of data, ensuring that the data remains intact and unaltered during transactions, retrievals, or updates.
Key Concepts
- Constraint-based Integrity: Utilizing primary keys, foreign keys, unique constraints, and check constraints to enforce data rules and relationships.
- Transactional Integrity: Ensuring the ACID properties (Atomicity, Consistency, Isolation, Durability) are maintained during database transactions.
- Data Validation and Sanitization: Implementing input validation and sanitization techniques to prevent SQL injection and other forms of data corruption.
Common Interview Questions
Basic Level
- What are the different types of constraints in SQL that help maintain data integrity?
- How does a PRIMARY KEY constraint ensure data integrity in a SQL database?
Intermediate Level
- Explain the role of ACID properties in maintaining data integrity in SQL databases.
Advanced Level
- How can you prevent SQL injection attacks, thereby ensuring data integrity?
Detailed Answers
1. What are the different types of constraints in SQL that help maintain data integrity?
Answer: SQL supports several types of constraints to ensure data integrity, including:
- PRIMARY KEY: Ensures each row in a table has a unique identifier.
- FOREIGN KEY: Maintains referential integrity by ensuring a valid relationship between the data in two tables.
- UNIQUE: Guarantees that all values in a column are different.
- CHECK: Ensures the data in a column meets a specific condition.
- NOT NULL: Prevents null values from being entered into a specified column of a table.
Key Points:
- Constraints are rules applied to the data fields of a table.
- They are used to limit the type of data that can go into a table, ensuring the accuracy and reliability of the data.
- Applying constraints is a critical step in database schema design to prevent invalid data entry.
Example:
// Example of SQL statements applying constraints
CREATE TABLE Employees (
EmployeeID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
CONSTRAINT PK_Employee PRIMARY KEY (EmployeeID),
CONSTRAINT CHK_Employee_Age CHECK (Age>=18)
);
2. How does a PRIMARY KEY constraint ensure data integrity in a SQL database?
Answer: A PRIMARY KEY constraint uniquely identifies each record in a database table. It ensures that no duplicate values exist in the primary key column, and it cannot accept NULL values. This enforces uniqueness and ensures that each row in a table can be uniquely identified, maintaining data integrity by preventing duplicate and undefined values.
Key Points:
- Prevents duplicate entries in the key column.
- Ensures every row can be uniquely identified, facilitating faster data retrieval.
- A table can only have one PRIMARY KEY, which can consist of single or multiple columns (composite key).
Example:
// Creating a table with a PRIMARY KEY constraint
CREATE TABLE Products (
ProductID int NOT NULL,
ProductName varchar(255) NOT NULL,
Price money NOT NULL,
CONSTRAINT PK_Product PRIMARY KEY (ProductID)
);
3. Explain the role of ACID properties in maintaining data integrity in SQL databases.
Answer: The ACID properties (Atomicity, Consistency, Isolation, Durability) play a crucial role in transaction management, ensuring data integrity in SQL databases:
- Atomicity: Ensures a transaction is all-or-nothing. If one part fails, the entire transaction fails, leaving the database unchanged.
- Consistency: Guarantees that a transaction never leaves the database in a half-finished state. Upon completion, all data must be consistent according to all rules, constraints, and triggers.
- Isolation: Ensures transactions are executed in isolation from each other, preventing concurrent transactions from affecting each other's execution and outcome.
- Durability: Once a transaction has been committed, it remains so, even in the event of a system failure, ensuring data is never lost.
Key Points:
- ACID properties are essential for transaction management systems.
- They help maintain integrity by ensuring that transactions are processed reliably.
- Enforcing these properties can prevent data anomalies and ensure the database remains in a consistent state.
Example:
// No direct C# code example for ACID properties as they are concepts applied within the SQL database management system during transaction processing.
4. How can you prevent SQL injection attacks, thereby ensuring data integrity?
Answer: SQL injection attacks can be prevented by:
- Using Prepared Statements with Parameterized Queries: These do not allow SQL code execution within the parameter values, limiting the attacker's ability to manipulate the query.
- Employing Stored Procedures: They encapsulate the SQL statements and treat all input as data, not executable code.
- Validating and Sanitizing User Inputs: Ensure inputs are checked for type, length, format, and range.
- Limiting Database Permissions: Use the principle of least privilege for database access.
Key Points:
- Prevention of SQL injection is crucial for protecting databases from unauthorized access and modification.
- Validating user input is a first line of defense but should not be the only method employed to prevent SQL injection.
- Using ORM (Object-Relational Mapping) frameworks can also abstract SQL code and reduce the risk of injection attacks.
Example:
// Using a parameterized query in C#
string query = "SELECT * FROM Users WHERE Username = @username AND Password = @password";
SqlCommand command = new SqlCommand(query, connection);
command.Parameters.AddWithValue("@username", username);
command.Parameters.AddWithValue("@password", password);
SqlDataReader reader = command.ExecuteReader();
This approach ensures that the values of @username and @password are treated as parameters, not part of the SQL to be executed, thereby preventing SQL injection.
