10 Dynamic SQL Interview Questions and Answers

Dynamic SQL is a powerful feature in SQL that allows for the construction and execution of SQL statements at runtime. This capability is essential for creating flexible and efficient database applications, enabling developers to build queries that can adapt to varying conditions and inputs. Dynamic SQL is particularly useful in scenarios where static SQL cannot accommodate the complexity or variability of the required operations.

This article provides a curated selection of dynamic SQL interview questions designed to help you demonstrate your proficiency in this advanced SQL technique. By working through these questions, you will gain a deeper understanding of dynamic SQL’s intricacies and be better prepared to showcase your expertise in a technical interview setting.

Dynamic SQL Interview Questions and Answers

1. Write a Dynamic SQL query to select data from a table where the table name is passed as a parameter.

Dynamic SQL is a technique that allows you to construct and execute SQL queries at runtime. This is useful when the table name or other parameters are not known until execution. However, it is important to handle dynamic SQL carefully to avoid SQL injection attacks.

Here is an example of writing a dynamic SQL query to select data from a table where the table name is passed as a parameter:

import sqlite3

def select_from_table(table_name):
    query = f"SELECT * FROM {table_name}"
    return query

# Example usage
table_name = "employees"
query = select_from_table(table_name)
print(query)

In this example, the select_from_table function constructs a SQL query string using the table name passed as a parameter. This query can then be executed using a database connection.

2. What are the security risks associated with Dynamic SQL and how can they be mitigated?

Dynamic SQL poses security risks, notably SQL injection attacks. SQL injection occurs when an attacker manipulates a query by injecting malicious SQL code, potentially gaining unauthorized access to the database or executing harmful operations.

To mitigate these risks, you can use the following strategies:

• Parameterized Queries: Using parameterized queries ensures that user input is treated as data, not executable code. This is the most effective way to prevent SQL injection.
• Stored Procedures: Encapsulating SQL code within stored procedures can limit the exposure of SQL statements to user input.
• Input Validation: Validating and sanitizing user inputs can help ensure that only expected data types and formats are accepted.
• Least Privilege Principle: Granting the minimum necessary permissions to database users can limit the potential damage of a successful SQL injection attack.

Example of using parameterized queries to mitigate SQL injection:

import sqlite3

def safe_query(user_input):
    conn = sqlite3.connect('example.db')
    cursor = conn.cursor()
    
    # Using parameterized query to prevent SQL injection
    cursor.execute("SELECT * FROM users WHERE username = ?", (user_input,))
    
    results = cursor.fetchall()
    conn.close()
    return results

user_input = "admin' OR '1'='1"
print(safe_query(user_input))

3. Demonstrate how to use Dynamic SQL to update a column value based on a condition provided at runtime.

To use Dynamic SQL for updating a column value based on a runtime condition, you can use the EXEC or sp_executesql stored procedure in SQL Server. Here is an example using sp_executesql:

DECLARE @sql NVARCHAR(MAX)
DECLARE @columnName NVARCHAR(50) = 'ColumnName'
DECLARE @newValue NVARCHAR(50) = 'NewValue'
DECLARE @condition NVARCHAR(50) = 'ConditionValue'

SET @sql = N'UPDATE TableName SET ' + @columnName + ' = @newValue WHERE SomeColumn = @condition'

EXEC sp_executesql @sql, N'@newValue NVARCHAR(50), @condition NVARCHAR(50)', @newValue, @condition

In this example:

• @columnName is the name of the column to be updated.
• @newValue is the new value to be set.
• @condition is the condition that determines which rows to update.

The sp_executesql procedure is used to execute the dynamically constructed SQL statement, with parameters passed to ensure proper handling of the values.

4. Write a Dynamic SQL query that joins two tables where the table names and join conditions are provided at runtime.

Dynamic SQL allows you to construct and execute SQL queries at runtime. This is useful when the structure of the query, such as table names and join conditions, is not known until the application is running. In Python, you can use libraries like sqlite3 or pyodbc to execute dynamic SQL queries.

Example:

def dynamic_sql_join(table1, table2, join_condition):
    query = f"SELECT * FROM {table1} INNER JOIN {table2} ON {join_condition}"
    return query

# Example usage
table1 = "employees"
table2 = "departments"
join_condition = "employees.department_id = departments.id"

query = dynamic_sql_join(table1, table2, join_condition)
print(query)
# Output: SELECT * FROM employees INNER JOIN departments ON employees.department_id = departments.id

5. Explain the role of the QUOTENAME function in Dynamic SQL. Provide an example.

The QUOTENAME function in SQL Server is used to safely delimit identifiers, such as table names or column names, to prevent SQL injection attacks and syntax errors. This function is particularly useful in dynamic SQL, where SQL statements are constructed and executed at runtime. By using QUOTENAME, you can ensure that the identifiers are properly quoted, thus avoiding potential issues with special characters or reserved keywords.

Example:

DECLARE @TableName NVARCHAR(128) = 'MyTable'
DECLARE @SQL NVARCHAR(MAX)

SET @SQL = 'SELECT * FROM ' + QUOTENAME(@TableName)
EXEC sp_executesql @SQL

In this example, the QUOTENAME function is used to safely quote the table name ‘MyTable’. This ensures that the table name is properly delimited, preventing any potential SQL injection attacks or syntax errors.

6. Write a Dynamic SQL query that executes a stored procedure with parameters provided at runtime.

Dynamic SQL is a technique that allows you to construct and execute SQL statements at runtime. This is useful when you need to execute a stored procedure with parameters that are not known until runtime.

Here is an example of how to write a dynamic SQL query to execute a stored procedure with parameters provided at runtime:

DECLARE @sql NVARCHAR(MAX)
DECLARE @param1 INT = 10
DECLARE @param2 NVARCHAR(50) = 'example'

SET @sql = N'EXEC dbo.MyStoredProcedure @Param1 = @p1, @Param2 = @p2'

EXEC sp_executesql @sql, N'@p1 INT, @p2 NVARCHAR(50)', @p1 = @param1, @p2 = @param2

In this example, we declare a variable @sql to hold the dynamic SQL statement. We then set the value of @sql to the EXEC statement that calls the stored procedure MyStoredProcedure with parameters @Param1 and @Param2. The sp_executesql command is used to execute the dynamic SQL statement, with the parameters passed in as arguments.

7. How do you handle errors in Dynamic SQL? Provide an example.

Error handling in Dynamic SQL is important to ensure that your SQL code runs smoothly and to handle any unexpected issues that may arise during execution. Proper error handling can help you identify and resolve issues quickly, maintain data integrity, and improve the overall reliability of your SQL scripts.

In SQL Server, you can use the TRY…CATCH construct to handle errors in Dynamic SQL. This allows you to catch errors and take appropriate actions, such as logging the error, rolling back transactions, or providing user-friendly error messages.

Example:

BEGIN TRY
    DECLARE @sql NVARCHAR(MAX);
    SET @sql = N'SELECT * FROM NonExistentTable';
    EXEC sp_executesql @sql;
END TRY
BEGIN CATCH
    DECLARE @ErrorMessage NVARCHAR(4000);
    DECLARE @ErrorSeverity INT;
    DECLARE @ErrorState INT;

    SELECT 
        @ErrorMessage = ERROR_MESSAGE(),
        @ErrorSeverity = ERROR_SEVERITY(),
        @ErrorState = ERROR_STATE();

    RAISERROR (@ErrorMessage, @ErrorSeverity, @ErrorState);
END CATCH;

8. Explain how to use parameters in sp_executesql. Provide an example.

sp_executesql is a system stored procedure in SQL Server that allows for the execution of a dynamically built SQL statement. One of the key advantages of using sp_executesql is the ability to pass parameters, which helps in preventing SQL injection attacks and improves performance by allowing SQL Server to reuse execution plans.

To use parameters in sp_executesql, you need to define the parameter names and types, and then pass the actual values when executing the dynamic SQL statement.

Example:

DECLARE @sql NVARCHAR(MAX);
DECLARE @paramDefinition NVARCHAR(MAX);
DECLARE @FirstName NVARCHAR(50);
DECLARE @LastName NVARCHAR(50);

SET @FirstName = 'John';
SET @LastName = 'Doe';

SET @sql = N'SELECT * FROM Employees WHERE FirstName = @FirstName AND LastName = @LastName';
SET @paramDefinition = N'@FirstName NVARCHAR(50), @LastName NVARCHAR(50)';

EXEC sp_executesql @sql, @paramDefinition, @FirstName = @FirstName, @LastName = @LastName;

In this example, the dynamic SQL statement is built with placeholders for the parameters @FirstName and @LastName. The parameter definitions are specified in the @paramDefinition variable, and the actual values are passed when calling sp_executesql.

9. How do you prevent SQL injection in Dynamic SQL? Provide an example.

SQL injection is a code injection technique that exploits vulnerabilities in an application’s software by inserting malicious SQL statements into an entry field for execution. This can lead to unauthorized access to the database, data leakage, or even data manipulation.

To prevent SQL injection in Dynamic SQL, the best practice is to use parameterized queries or prepared statements. These methods ensure that user input is treated as data and not executable code, thereby mitigating the risk of SQL injection.

Example:

import sqlite3

def get_user_data(user_id):
    conn = sqlite3.connect('example.db')
    cursor = conn.cursor()
    
    # Using parameterized query to prevent SQL injection
    cursor.execute("SELECT * FROM users WHERE id = ?", (user_id,))
    
    result = cursor.fetchall()
    conn.close()
    return result

# Example usage
user_data = get_user_data(1)
print(user_data)

In this example, the ? placeholder is used in the SQL query, and the actual value of user_id is passed as a parameter. This ensures that the input is safely handled and not executed as part of the SQL command.

10. Write a Dynamic SQL query that executes within a loop to perform repetitive tasks.

Dynamic SQL allows you to construct and execute SQL statements at runtime. This is useful when you need to perform repetitive tasks that involve varying SQL statements. By using dynamic SQL within a loop, you can automate these tasks efficiently.

Here is an example of how to use dynamic SQL within a loop in SQL Server:

DECLARE @i INT = 1;
DECLARE @sql NVARCHAR(MAX);

WHILE @i <= 10
BEGIN
    SET @sql = N'INSERT INTO MyTable (Column1) VALUES (' + CAST(@i AS NVARCHAR(10)) + N')';
    EXEC sp_executesql @sql;
    SET @i = @i + 1;
END

In this example, the loop runs from 1 to 10, and for each iteration, it constructs a dynamic SQL statement to insert a value into a table. The sp_executesql stored procedure is used to execute the dynamically constructed SQL statement.