SQL is a powerful language for manipulating relational databases. The UNION operator provides a useful function for combining the results of multiple SELECT statements into a single result set. However, if misused, UNION can lead to a security risk called SQL injection.

What is UNION SQL?

UNION is helpful when you want to combine similar data from different tables. For example, if you want to get the names and cities from both the customer and employee tables, you can write:

SELECT first_name, city FROM customers
UNION
SELECT first_name, city FROM employees;

UNION removes duplicate rows from the result set. If you want to include all rows, including duplicates, use UNION ALL.

The Risk of SQL Injection

Web applications often dynamically generate SQL queries based on user input. If this input is not properly handled, attackers can exploit UNION to execute unintended SQL statements.

For example, imagine a user enters the following string into a username field:

' UNION SELECT username, password FROM users --

If the application directly incorporates this input into a SQL query, it might execute something like this:

SELECT * FROM users WHERE username = '' UNION SELECT username, password FROM users --' AND password = ''

-- is an SQL comment marker, so everything after it is ignored. This could potentially reveal the usernames and passwords of all users.

Preventing SQL Injection

To prevent SQL injection, it is crucial to implement the following measures:

  • Use prepared statements: Separate the SQL query from the data being processed to prevent malicious code from being executed.
  • Sanitize user input: Escape special characters or restrict input to allowed characters to prevent dangerous strings from being executed.
  • Principle of least privilege: Grant database users only the necessary permissions to minimize potential damage.
  • Implement a Web Application Firewall (WAF): Detect and block attacks on web applications, including SQL injection attempts.

Conclusion

UNION is a useful SQL operator, but when misused, it poses the risk of SQL injection. Developers need to understand the threat of SQL injection and take appropriate countermeasures. Users should also be cautious about entering personal information on untrusted websites.