我有三个表，我想查看其中任何一个表中是否存在一行。
这是我的三个查询。我怎样才能将它们合二为一？
$dbh->prepare("SELECT id FROM users WHERE email = :email");
$dbh->prepare("SELECT id FROM employees WHERE email = :email");
$dbh->prepare("SELECT id FROM teachers WHERE email = :email");
if(!$row) { // If row not found
// continue with registration
}
您可以使用UNION查询。如果用户id在多个表中有不同的值，这将返回多行，但因为您只需要知道是否存在一行或多行就可以了。
SELECT id FROM users WHERE email = :email
UNION
SELECT id FROM employees WHERE email = :email
UNION
SELECT id FROM teachers WHERE email = :email
或者，您可以使用EXISTS表达式从查询中获取简单的是/否结果：
SELECT EXISTS (SELECT * FROM users WHERE email = :email)
OR EXISTS (SELECT * FROM employees WHERE email = :email)
OR EXISTS (SELECT * FROM teachers WHERE email = :email)
如果您想知道电子邮件地址在哪个表中，您可以修改UNION查询以返回该信息：
SELECT 'users' AS `table`, id FROM users WHERE email = :email
UNION
SELECT 'employees', id FROM employees WHERE email = :email
UNION
SELECT 'teachers', id FROM teachers WHERE email = :email
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