Question

我更熟悉php并制作图书馆管理项目。我想在图书馆中打印全书，因为我使用了以下代码：

<?php
$query ="select sum(copies) as totalbooks from booksdetail";
$result=mysqli_query($con, $query);
while($row=mysqli_fetch_array($result)){
echo $row['totalbooks'];     
}
?>

我想打印已发行的书籍总数，因为我使用了以下代码：

<?php
$query ="select count(id) as issuedbooks from issue";
$result=mysqli_query($con, $query);
while($row=mysqli_fetch_array($result)){
echo $row['issuedbooks'];     
}
?>

现在，我想要打印库中可用的书籍数量，因为我必须从issued books中减去total books。

请建议我应该使用哪个代码进行减法？

Answer 1

<?php
$totalBooks=0;
$query ="select sum(copies) as totalbooks from booksdetail";
$result=mysqli_query($con, $query);
while($row=mysqli_fetch_array($result)){
    $totalBooks=$row['totalbooks'];     
}?>

<?php
$issuedBooks=0;
$query ="select count(id) as issuedbooks from issue";
$result=mysqli_query($con, $query);
while($row=mysqli_fetch_array($result)){
    $issuedBooks=$row['issuedbooks'];     
}?>

<?
echo $availableBooks = $totalBooks - $issuedBooks;
?>

Answer 2

就是这样：

<?php
    $query ="select sum(copies) as totalbooks from booksdetail";
    $result=mysqli_query($con, $query);

    $totalbooks = 0;
    while($row=mysqli_fetch_array($result)){
    $totalbooks = $row['totalbooks'];     
    }

    $query ="select count(id) as issuedbooks from issue";
    $result=mysqli_query($con, $query);

$issuedbooks=0;
while($row=mysqli_fetch_array($result)){
    $issuedbooks= $row['issuedbooks'];     
    }

$sub_result = $totalbooks - $issuedbooks;
echo $sub_result;
    ?>

Answer 3

你尝试过这样的事吗？在数据库上查询全部内容比在客户端上进行2次查询和计算要快得多

<?php
$query ="select (totalbooks.val - issuedbooks.val) as result from (select sum(copies) as val from booksdetail) totalbooks, (select count(id) as val from issue) issuedbooks";
$result=mysqli_query($con, $query);
while($row=mysqli_fetch_array($result)){
echo $row['result'];     
}
?>

如何在PHP中执行减法？

3 个答案: