从两个表中选择 PHP

Question

这是我的数据库:

table 1: dog
   column 1: email
   column 2: password

table 2: cat
   column 1: email
   column 2: password

我想检查一个字符串或数据是否已经存在于两个数据库的列中:

//check if email exists in either dog or cat.
$stmt = $dbh->prepare("SELECT email FROM dog AND cat WHERE email = :email");
$stmt->bindParam(':email', $email);
$stmt->execute();
$row = $stmt->fetch(PDO::FETCH_ASSOC);

//if the email exists in either dog or cat, echo the "taken" message below
if($row['email'] == $email) {
   echo "sorry, the email is taken already!";
}

但它对我不起作用。我怎样才能让它发挥作用？

Answer 1

SQL 语法无效。

我们可以测试 prepare 的返回值，如果它是 FALSE，那么我们就知道发生了错误。

这是一个例子:

  //check if email exists in either dog or cat.
  $sql = "SELECT 'dog' AS src, d.email FROM dog d WHERE d.email = :demail
           UNION ALL
          SELECT 'cat' AS src, c.email FROM cat c WHERE c.email = :cemail";

  if ($stmt = $dbh->prepare($sql)) {
      $stmt->bindParam(':demail', $email);
      $stmt->bindParam(':cemail', $email);
      $stmt->execute();
     ...

  } else {
      // handle error condition
  }