MySQL我可以从一个表还是另一个表中选择数据？

Question

I'm working on a login script (In HTML, PHP and MYSQL) for a website that has two account types: Account "A" and Account "B". Right now I have a radio select that lets the user select which type of account they have. But I want to clear up the login screen a bit and also enable the login form to fit in the header by removing the radio buttons completely.

How do I get the script to check first one table for results (username and password) and if no results are found check the next table (username and password) and then once it finds a match sends it to an account specific page?

In a nutshell, I want to remove the need for the radio buttons.

Edit: I don't get any errors, but the problem is it only selects from the first table: Here's my code:

$q = "SELECT varone, vartwo 
    FROM tablea 
    WHERE (user_email='$e' AND pass=SHA1('$p'))
    UNION
    SELECT varthree, vartwo 
    FROM tableb
    WHERE (contact_email='$e' AND pass=SHA1('$p'))
    ";      
$r = mysqli_query ($dbc, $q) or trigger_error("Query: $q\n<br />MySQL Error: " . mysqli_error($dbc));

Answer 1

SELECT 'first' AS source, ...
FROM firstable
WHERE (username = $user) and (password = $password)

UNION

SELECT 'second' as source, ...
FROM secondtable
WHERE (....)

Then simply check where the results came from in your code.

if ($row['source'] == 'first') { ... came from first table ... }

Should work, assuming the exact same username/password pair wouldn't appear in both tables.

Answer 2

select * from table_A where id = 6 union select * from table_b where other_id = 6;