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

That looks like a MySQL-based time‑based injection payload. It’s not intended for normal application logic; it’s used to test or exploit a vulnerability by causing the query to sleep for a while.

What it does, component by component:

• 555* is just a multiplier (often used to try to bypass simple filters or to make the payload look like part of a numeric expression).
• IF(now()=sysdate(), sleep(15), 0) is the core expression:
  • now() and sysdate() both return the current date/time, but they can differ slightly depending on how MySQL evaluates them.
  • If they are equal at evaluation time, the IF condition is true, and sleep(15) is executed, delaying the query by 15 seconds.
  • If they are not equal, the expression yields 0 and there is no delay.
• The overall effect in a vulnerable query is to cause a 15-second delay when the condition is true, which can be detected by measuring response time. Such timing differences can be used for blind SQL injection attacks to infer information.

Notes for exam context and safety:

• This is an educational example of a timing-based (blind) SQL injection technique. Do not run it on systems you do not own or have permission to test.
• In a defensive context, preventing this kind of payload is important. Common mitigations:
  • Use parameterized queries/prepared statements instead of string-concatenated SQL.
  • Do not rely on input validation alone to block injections.
  • Least-privilege database accounts (no SLEEP权限 or '../../../../' style exploits).
  • Input sanitization and proper escaping where appropriate, plus Web Application Firewalls (WAF) with anomaly detection.
  • Disable or restrict the use of dangerous MySQL functions if feasible, and monitor for unusual sleep() calls or long-running queries.

If this is for an exam, you might be asked to explain:

• Why NOW() and SYSDATE() can be used in such a test and how their timing semantics affect the condition.
• How a timing-based attack works and what it leaks (or proves) about a vulnerable application.
• How to defend against this class of attack with concrete coding and infrastructure practices.