问题描述

我有两个 while 循环一个接一个地运行(不是在彼此内部) - 我已经稍微简化了代码，因此下面只列出了它的重要部分.当我比较两个回显查询时出现问题，因为第二个 while 循环显然根本没有运行.

I have two while loops running one after the other (not inside of each other) - I've simplified the code a bit so that only the important parts of it are listed below. The problem arises when I compare the two echoed queries because the 2nd while loop apparently isn't running at all.

我在某处读到有人通过对第二个循环使用 for 循环解决了这个问题，但我想深入了解为什么我的代码中没有运行第二个 while 循环.

I read somewhere that someone got around the problem by using a for loop for the second one but I want to get down to why exactly the second while loop is not running in my code.

$query_work_title = "SELECT title FROM works WHERE ";
while ($row = mysql_fetch_assoc($result_work_id)) {
    $query_work_title .= "OR '$work_id' ";
}
echo $query_work_title;
echo '<br />';
$result_work_title = mysql_query($query_work_title) or
    die(mysql_error($cn));

// retrieve the authors for each work in the following query
$query_author_id = "SELECT author_id FROM works_and_authors WHERE ";
while ($row = mysql_fetch_assoc($result_work_id)) {
    $query_author_id .= "work_id = 'hello' ";
}
echo $query_author_id;

推荐答案

MySQL 扩展跟踪每个结果的内部行指针.它在每次调用 mysql_fetch_assoc() 后递增该指针，并且允许您使用 while 循环而不指定何时停止.如果您打算多次遍历结果集，则需要将此内部行指针重置回 0.

The MySQL extension keeps track of an internal row pointer for each result. It increments this pointer after each call to mysql_fetch_assoc(), and is what allows you to use a while loop without specifying when to stop. If you intend on looping through a result set more than once, you need to reset this internal row pointer back to 0.

为此，您将mysql_data_seek() 在第一个循环之后:

To do this, you would mysql_data_seek() after the first loop:

while ($row = mysql_fetch_assoc($result_work_id)) {
    $query_work_title .= "OR '$work_id' ";
}
mysql_data_seek($result_work_id, 0);

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