Php mysql SUM 来自不同表中的多个记录的 1 个字段

我有一个脚本，可以从员工在holiday_entitlement表中的当前记录中获取他们的假期权利。

然后，此权利数字将用于数据库中要从中减去的每个假日请求。

例如：权利天数 = 27

假日请求天数 = 剩余 2>>>> 个权利天数 = 25

假日请求天数 = 剩余 4>>>> 个权利天数 = 21

假日请求天数 = 剩余 1>>>> 个权利天数 = 20等等....

在我的脚本中，剩余的权利天数是$remainder

我正在努力解决的一点是能够在每次批准请求时将此$remainder保存到holiday_entitlement表中，或者在SQL中计算出原始权利数字减去该员工的所有请求，该学年以及是否获得批准。

   <?php
    $is_business_result = mysql_query('SELECT * FROM holiday_entitlement WHERE employee = ''' . $username . ''' AND academic_year = ''' . $acyear . ''' ');
    if($is_business = mysql_fetch_array($is_business_result)) {
    echo'<div style="float:left; width:400px;">';
    echo'<table width="100%">
    <tr>
    <td><strong>Name:</strong></td>
    <td>'.$is_business['employee'].'</td>
    </tr>
    <tr>
    <td><strong>Entitlement:</strong></td>
    <td>'.$is_business['new_entitlement'].' '.$is_business['units'].'</td>
    </tr>
    <tr>
    <td><strong>Department / Division:</strong></td>
    <td>'.$is_business['division'].'</td>
    </tr>
    <tr>
    <td><strong>Line Manager:</strong></td>
    <td>'.$is_business['line_manager'].'</td>
    </tr>
    </table>';
    echo'</div>';


    echo'<table class="dataTable" id="business_days_table" cellpadding="2" cellspacing="2" width="100%">
    <thead>
    <th>Start Date</th>
    <th>End Date</th>
    <th>Days to be taken</th>
    <th>Days remaining</th>
    </thead>';
    echo '<tr>';
    echo '<td>-</td>';
    echo '<td>-</td>';
    echo '<td>-</td>';
    echo '<td>'.$is_business['new_entitlement'].'</td>';
    echo '</tr>';

    $input = $is_business['new_entitlement'];
    }
    else {echo 'You currently dont have a record for this academic year. ';}

    $requests_result = mysql_query('SELECT * FROM requests WHERE employee = ''' . $username . ''' AND approved = 1 AND academic_year = ''' . $acyear . ''' ORDER BY start_date ASC');
    $remainder = 0;
    while($requests = mysql_fetch_array($requests_result)) {
    $start_date = new DateTime($requests['start_date']);
    $end_date = new DateTime($requests['end_date']);
    $timestamp_start_date = $start_date->getTimestamp();
    $timestamp_end_date = $end_date->getTimestamp();
    $formatted_start_date = date("d M Y", $timestamp_start_date);           
    $formatted_end_date = date("d M Y", $timestamp_end_date);   
    $remainder = ($remainder == 0) ? $input : $remainder;
    $out = $remainder - $requests['days'];
    if($out < 0){
          break;
    }
    $remainder = $out;

    echo'<tr>';
    echo'<td>'.$formatted_start_date.'</td>';
    echo'<td>'.$formatted_end_date.'</td>';
    echo'<td>'.$requests['days'].'</td>';
    echo'<td>'.$remainder.'</td>';
    echo'</tr>';
    }
    echo'</table>';
    ?>

select new_entitlement-(select sum(days) from requests where employee="sashn" and academic_year=2012 and approved=1) from holiday_entitlement where employee="sashn";

update holiday_entitlement set new_entitlement=27-(select sum(days) from requests where employee="sashn" and academic_year=2012 and approved=1) where employee="sashn" and academic_year=2012;