如果第一个查询没有返回结果，则使用第二个查询

Question

我正在制作一个系统，需要在其中检查第一张表或第二张表中是否存在电子邮件。

    $results = mysqli_query($db,"SELECT * FROM table_1 WHERE email='$email'");
    if(count($results) == 0)
    {
        $results = mysqli_query($db,"SELECT * FROM table_2 WHERE email='$email'");
    }

我想制作一个MySQL，这样就不需要两个了。 由于两个表结构不同，因此UNION不能提供适当的结果。 有没有UNION or JOINS的方法吗

我一直在尝试与UNION

SELECT * FROM ( SELECT *, 1 as preference FROM table_1 WHERE email = 'doc@demo.com' UNION SELECT *, 2 as preference FROM table_2 WHERE email = 'doc@demo.com' ) T ORDER BY preference LIMIT 1

但是问题是如果表2的大小写满足，则考虑表1的字段并且所有值都不匹配

Answer 1

我找到了自己问题的解决方案，所以只想分享一下

SELECT * FROM (
  SELECT email,password,user_role, id as customer_id,id as user_id, 1 as preference 
    FROM table1 WHERE email = 'raj@demo.com'
   UNION 
  SELECT email,password,user_role, customer_id,id, 2 as preference 
    FROM table2 WHERE email = 'doc@demo.com' 
) T ORDER BY preference
LIMIT 1