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

What you posted is a MySQL-style time-based expression often seen in blind SQL injection tests.

• The core is IF(now()=sysdate(), sleep(15), 0).
• If the condition is true, the statement executes sleep(15), which blocks for 15 seconds. If false, it returns 0.
• The outer 555*(...) multiplies the result by 555, but since sleep(...) returns 0 after finishing, the overall numeric value ends up 0 in either case. The important part is the side effect (the delay) when the condition is true.

Why this matters:

• This is a classic time-based blind SQL injection technique: by measuring the response time, an attacker can infer whether the condition was true.

Notes for exam understanding:

• NOW() vs SYSdate(): NOW() returns the current timestamp (often fixed for the duration of the statement), while SYSDATE() returns the actual current time at the moment it’s evaluated. The equality check is typically true only under specific timing within a statement.
• Sleep introduces a delay; its purpose is to create a measurable difference in response time.
• The 555 multiplier doesn’t affect the delay; it only affects the numeric result (which stays 0 because sleep returns 0).

If you’re studying this for defense:

• Use prepared statements and parameterized queries.
• Apply least-privilege accounts and input validation.
• In lab settings, test in a controlled environment (dummy data, legal authorization).

If you want, I can give a safe, exam-style explanation or a defender-oriented variant (how to detect and prevent such patterns).