为什么fetch_assoc会继续工作？

#1 code

$row = $result->fetch_assoc());
while ($row) {
// code here
}

#2 code

while ($row = $result->fetch_assoc());) {
// code here
}

Why #1 code and #2 code give different result?

Because in #1 you execute fetch_assoc() only once, while in #2 you execute it on every loop iteration.

In other words, in #2 fetch_assoc() will keep on returning rows as long as there are rows available in the result set. #1 will enter an infinite loop if there is at least one row available.