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

The expression you provided:

if(now()=sysdate(), sleep(15), 0)
What this means:


now() and sysdate() are both MySQL functions that return the current date and time.


In most MySQL configurations, now() returns the precise current timestamp at the start of the statement execution.


sysdate() returns the exact time when the function is executed.


So, now() = sysdate() will typically be false because sysdate() can differ slightly from now() within the same query execution.


However, depending on the timing, they may sometimes be equal.
The full behavior:


The if condition checks if now() equals sysdate().


If they are equal, the function will execute sleep(15) – which causes the query to pause for 15 seconds.


If they are not equal, it returns 0 immediately.


Important notes:


It’s unusual for a query to include something like this because now() and sysdate() almost never equal each other in the same instant.


sleep(15) will block the query for 15 seconds.


This can be useful for simulating a delay in testing but is not recommended for production use.

This expression might be used for debugging or throttling.

Example use case:
If you want to artificially delay a query only when a certain time condition is met (e.g., when certain timestamps align), this could be a way, but since now() and sysdate() rarely equal, the delay almost never happens.

Summary:


if(now()=sysdate(), sleep(15), 0) attempts to sleep for 15 seconds only if the current timestamp from now() equals that from sysdate().


Because of how these functions work, the condition usually evaluates to false, so it typically returns 0 immediately.


Use with caution due to the potential blocking behavior and the almost impossible equality condition.