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

That looks like a MySQL timing-based blind SQL injection payload.

What it does (conceptually):

• The expression is IF(now() = sysdate(), sleep(15), 0).
• If the condition is true, the database sleeps for 15 seconds; if false, it returns 0 immediately.
• In many MySQL setups, NOW() and SYSDATE() can be equal within the same evaluation, so the condition may be true and cause a 15-second delay. Attackers use this kind of delay to infer information by measuring response times.

Important cautions:

• Do not run this against systems you don’t own or don’t have permission to test. It can disrupt services and is considered unauthorized access.
• It’s a classic technique used in blind SQL injection to test for vulnerabilities.

For study/defense:

• In legitimate testing, use a controlled lab environment (e.g., DVWA, WebGoat, or a local MySQL instance) to observe how timing-based injections behave.
• Defenses: use parameterized queries/prepared statements, avoid exposing dangerous functions to user inputs, implement least-privilege database accounts, validate/sanitize inputs, and consider a firewall or web app firewall that can detect anomalous sleep-based payloads.

If you’d like, I can provide a safe, step-by-step lab example in a local MySQL setup or summarize the MySQL behavior differences between NOW() and SYSdate for exam prep.