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

Short answer: it’s a MySQL-like expression that will evaluate to 0.

What it does (step by step):

• NOW() and SYSDATE() are two ways to get the current timestamp. NOW() is the time at the start of the statement; SYSDATE() is the time when the function is evaluated.
• The expression checks if now() = sysdate().
• If true, it evaluates sleep(0); otherwise it evaluates 0. Sleep(0) returns 0 (and sleeps for 0 seconds).
• The result of the IF is then multiplied by 555.
• Since both possible branches return 0, the final result is 0.

Notes you might need for exams:

• In MySQL, SLEEP(n) sleeps for n seconds and returns 0 after sleeping. SLEEP(0) returns 0 immediately, so it has no noticeable delay.
• Therefore 555 * IF(now() = sysdate(), SLEEP(0), 0) yields 0 in all cases.
• Difference between NOW() and SYSDATE():
  • NOW() returns the timestamp at the start of the statement (constant for that statement).
  • SYSDATE() returns the time at the moment it is evaluated and can differ within a single statement. The equality now() = sysdate() is rarely guaranteed; it would only be true if both timestamps are exactly the same.

If your goal was to test a time-based condition for injection demonstrations, remember that using SLEEP(n) with n > 0 would create a delay; here the delay is effectively none because sleep(0) = 0.