那不是INSERT.这是 UPDATE . INSERT语句插入 new 行. UPDATE语句更新现有行.

That not an INSERT. That's an UPDATE. INSERT statements insert a new row. UPDATE statements update an existing row.

UPDATE issued_books
SET Book_4 = '$IssuedBookNumber'
WHERE Student_ID = '$Student_ID'

(我假设您已经正确逃脱了$IssuedBookNumber和$Student_ID)

(I'm assuming you've properly escaped $IssuedBookNumber and $Student_ID)