: In MySQL, this character marks the rest of the query as a comment, effectively "breaking" any remaining original code that might cause the attack to fail. Why are you seeing this?
: This is the "injection" part. It tells the database to append a second result set (containing the number 34 ) to the first (empty) result set. If the page then displays "34", the attacker knows the site is vulnerable and that they can use this spot to extract sensitive data like usernames or passwords.
The string you've provided, "-1697 UNION ALL SELECT 34#" , is a classic example of a payload. This specific syntax is often used by security researchers or attackers to test if a database-driven website is vulnerable to unauthorized data access. What this string does: