为什么此函数不能返回正确的结果？

Question

我正在通过创建两个函数“ daySum（）”和“ yesterdaySum（）”从数据库中获取数据。 调用这些函数时，它将分别返回正确的值54000和51000。

然后，将它们放入变量$ day和$ yesterday，但是在执行变量操作“ $ day + $ yesterday”时，它将显示0。这将不允许我创建项目所需的条件。

你能帮我吗？

提前致谢！

//DEFINING 2 FUNCTIONS: daySum() and yesterdaySum()

function daySum() {
    //connect to database
    require 'connect.php';

    // query
    $query = "SELECT sum(till_amount) FROM Cash ";
    $query .= "WHERE date ='2017-09-05'";
    $result = mysqli_query($connect, $query);

    // fetch
    while($row = mysqli_fetch_assoc($result)) {
        print_r($row["sum(till_amount)"]);
    }
}

function yesterdaySum() {
    //connect to database
    require 'connect.php';

    // query
    $query = "SELECT sum(till_amount) FROM Cash ";
    $query .= "WHERE date ='2017-09-04'";
    $result = mysqli_query($connect, $query);

    // fetch
    while($row = mysqli_fetch_assoc($result)) {
        print_r($row["sum(till_amount)"]);
    }
}

// PUTTING THEM INTO VARIABLE 

$day = daySum(); // sum is 54000
$yesterday = yesterdaySum(); // sum is 51000

    // adding $day plus $yesterday should print 105000
    // and here is the problem...

print($day + $yesterday); // it prints 0

//WHY????

Answer 1

您的功能只是打印东西 。这就是为什么添加不起作用的原因。

您必须像下面那样更改您的代码（ 也有一些改进，也有评论）：

<?php
require 'connect.php'; //don't add multiple time
function daySum() {
    $query = "SELECT sum(till_amount) FROM Cash WHERE date ='2017-09-05'";
    $result = mysqli_query($connect, $query);
    while($row = mysqli_fetch_assoc($result)) {
        return $row["sum(till_amount)"]); //return value
    }
}

function yesterdaySum() {
    $query = "SELECT sum(till_amount) FROM Cash WHERE date ='2017-09-04'";
    $result = mysqli_query($connect, $query);
    while($row = mysqli_fetch_assoc($result)) {
       return $row["sum(till_amount)"]); //return value
    }
}

$day = daySum(); // now $day have value
$yesterday = yesterdaySum(); //now $yesterday have value


print((int)$day + (int)$yesterday); // it will work fine now

现在最简单的是 使用BETWEEN查询 ：

<?php
require 'connect.php'; //don't add multiple time
function daySum() {
    $query = "SELECT sum(till_amount) FROM Cash WHERE date BETWEEN '2017-09-04' AND '2017-09-05'";
    $result = mysqli_query($connect, $query);
    while($row = mysqli_fetch_assoc($result)) {
        return $row["sum(till_amount)"]);
    }
}

$day = daySum();

print($day);

注意：-也许这是您的家庭作业，但是从今天起开始学习prepared statements ，以防止您从SQL INJECTION查询。这是重要且必要的。