问题

我正在尝试检查数据库是否有记录，以防止重复。我正在使用以下代码：

$stmt = $db->prepare('SELECT * FROM newsletter WHERE useremail=:useremail');
$stmt->bindParam(':useremail', $_GET['useremail'], PDO::PARAM_INT);
$stmt->execute();
$row = $stmt->fetch(PDO::FETCH_ASSOC);
if ($row) {
echo "";
} else {
$kaydet = $db->prepare("INSERT INTO newsletter SET useremail=:useremail");
$insert = $kaydet->execute(array('useremail' => $_POST['useremail']));

	if ($insert) {
		header("Location:../index.php?status=success");
		exit;
	} else {
		header("Location:../index.php?status=error");
		exit;
	}
}

<details>
<summary>英文:</summary>

I am trying to check if the database has the record to prevent duplicates. I am using the following code:

$stmt = $db->prepare('SELECT * FROM newsletter WHERE useremail=:useremail');
$stmt->bindParam(':useremail', $_GET['useremail'], PDO::PARAM_INT);
$stmt->execute();
$row = $stmt->fetch(PDO::FETCH_ASSOC);
if ($row) {
echo "<script type='text/javascript'>Swal.fire('Info', 'This email is already registered.','info')";
} else {
$kaydet = $db->prepare("INSERT INTO newsletter SET useremail=:useremail");
$insert = $kaydet->execute(array('useremail' => $_POST['useremail']));

	if ($insert) {
		header("Location:../index.php?status=success");
		exit;
	} else {
		header("Location:../index.php?status=error");
		exit;
	}
}

I aim to prevent form submission for duplicate records. But this is not working. Any help would be appreciated.
Thank you.

</details>


# 答案1
**得分**: 0

在PHP中执行`SELECT`、检查结果、执行`INSERT`的问题在于它容易受到竞态条件的影响。一个线程可能正要执行`insert`，而另一个线程正在执行`select`，此时未返回任何行并且即将执行`insert`，这样就会产生重复数据。

面对这种情况，让数据库来处理。

为表上的`useremail`（或主键）添加一个唯一索引可以解决这个问题。

尝试执行插入操作，如果出现重复键异常，那就触发`'This email is already registered.'`，否则就是成功添加。

<details>
<summary>英文:</summary>

The problem with doing `SELECT`, check result, do `INSERT`, in PHP is that it is subject to race conditions. One thread could be about to do the `insert` just as the other `select`s no rows and about to also do an `insert`, and now you have duplicates.

When confronted with this situation, let the database handle it.

A unique index on `useremail` (or primary key), for the table will resolve this.

Attempt the insert, if you get a duplicate key exception, then that&#39;s when you fire `&#39;This email is already registered.&#39;`, otherwise its a successful addition.

</details>



# 答案2
**得分**: -1

我认为问题出在

```php
$stmt->bindParam(':useremail', $colour, PDO::PARAM_STR);

使用上述代码而不是param int
https://www.php.net/manual/en/pdostatement.bindparam.php

$stmt->bindParam(':useremail', $colour, PDO::PARAM_STR);