555*if(now()=sysdate(),sleep(15),0)

What you posted looks like a MySQL-style time-based blind SQL injection payload.

What it does (conceptually):

• It uses IF(now() = sysdate(), sleep(15), 0).
• If the condition is true, the SLEEP(15) call pauses the query for 15 seconds; otherwise it returns 0 immediately.
• The whole expression is multiplied by 555, but since SLEEP returns 0 after finishing, the final numeric result is essentially 0 in either case—the important part is the 15-second delay when the condition is true.
• NOW() and SYSDATE() are both time functions; the equality check is used to trigger the delay based on whether the two times match in the engine’s timing resolution.

Notes for exam prep:

• This is a classic “time-based blind” SQL injection technique: you don’t rely on returned data, you infer information from the page’s response time.
• In MySQL, SLEEP is a function you can use in expressions; IF(expr, a, b) is a conditional expression (true_value if expr is true, else false_value).
• The exact behavior can vary by DBMS and timing precision, but the general idea is: delay = true triggers a visible pause; delay = false does not.

Safety and best practices:

• I can’t help with exploiting real systems, but for learning purposes: study how parameterized queries, prepared statements, input validation, and restricted database privileges mitigate such risks.
• In practice, avoid allowing user input to influence queries that can call SLEEP, and use query timeouts and security controls to detect/analyze anomalous delays.

If you want, I can give a safe, high-level example of how time-based blind SQL injection is detected in a lab environment and how to defend against it.