第二个while循环未运行.为什么? [英] Second while loop not running. Why?

问题描述

我有两个while循环一个接一个地运行(彼此之间不在内部)-我对代码进行了一些简化，因此下面仅列出了其中的重要部分.比较两个回显查询时会出现问题，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.

我读到某个地方有人通过使用for循环来解决第二个问题，但是我想深入了解为什么第二个while循环在我的代码中没有运行.

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扩展会跟踪每个结果的内部行指针.在每次调用mysql_fetch_assoc()之后，它都会递增该指针，这是允许您使用while循环而无需指定停止时间的原因.如果打算多次遍历结果集，则需要将此内部行指针重置为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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