此 PDO::FETCH_ASSOC` 查询跳过返回的第一个结果

2021-12-26 00:00:00 fetch php pdo

我正在过渡到 PDO 准备好的语句，但我在使用 WHILE 语句的基本 SELECT 查询的语法方面遇到了问题.

I'm transitioning over to PDO prepared statements, and I'm having trouble with the syntax for a basic SELECT query with a WHILE statement.

下面的 foreach 语句回显了正确的结果，但是 PDO::FETCH_ASSOC 查询跳过了返回的第一个结果(因此它总是回显小于它的一个结果应该).

The foreach statement below echos the correct results, but the PDO::FETCH_ASSOC query is skipping the 1rst result that's returned (so it always echo's one result less than it should).

PDO::FETCH_ASSOC

$stmt = $conn->prepare("SELECT * FROM products"); 
$stmt->execute();
$row = $stmt->fetch();
while($row = $stmt->fetch(PDO::FETCH_ASSOC)) { 
    echo $row['product_name'].' '.$row['price'].' '.$row['color'], "<br />"; 
}

foreach

foreach($conn->query('SELECT * FROM products') as $row) {
    echo $row['product_name'].' '.$row['price'].' '.$row['color'], "<br />";  
}

推荐答案

您已经在 while 循环之前获取了第一行 $row = $stmt->fetch();.如果您删除此行，它将按预期工作.

You already fetched the first row before the while loop $row = $stmt->fetch();. If you remove this line, it will work as expected.

由于 while 循环会在每次迭代时覆盖 $row，看起来您是从第二行开始的，但实际发生的是 $row 的值在首先覆盖 while 循环迭代.

Since the while loop will overwrite $row on each iteration, it looks like you start with the second row, but what happens is the value of $row at the first while loop iteration is overwritten.

要让循环按照您编写的方式工作，您需要使用 do-while 构造:

To have the loop work the way you have written, you would need to use a do-while construct:

$row = $stmt->fetch();
do {
     echo $row['product_name'].' '.$row['price'].' '.$row['color'], "<br />";
} while($row = $stmt->fetch(PDO::FETCH_ASSOC));

这里首先打印$row的值，然后被while条件覆盖.

Here the value of $row will be printed first, before it is overwritten by the while condition.

在这种特殊情况下，当没有任何结果时，我不想回显任何内容

In this particular case I don't want to echo anything when there aren't any results

如果是这种情况，请先检查您的查询是否返回了任何结果.在这里，我在检查中是明确的，因为如果您删除外部 if，您的 while 循环仍会遵循您的意图 - 也就是说，它不会回显任何内容如果没有任何结果.

If that's the case, then check to see if your query returned any results first. Here I'm being explicit in the check, because if you removed the outer if, your while loop would still follow your intentions - that is, it won't echo anything if there aren't any results.

但是，在您的代码中有明确的意图总是好的:

However, it is always good to have clear intent in your code:

if ($stmt->columnCount()) {
   while($row = $stmt->fetch(PDO::FETCH_ASSOC)) { 
       echo $row['product_name'].' '.$row['price'].' '.$row['color'], "<br />"; 
   }
}

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