如何在更新前检查其他表格？

I have these two tables:

 // users
+----+-------+-----------------------+--------+
| id | name  |       email           | active |
+----+-------+-----------------------+--------+
| 1  | peter | peter12@hotmail.com   | NULL   |
| 2  | jack  | most_wanted@gmail.com | NULL   |
| 3  | john  | john_20016@yahoo.com  | NULL   |
+----+-------+-----------------------+--------+

// activate
+----+---------+---------------------+
| id | post_id |   random_string     |
+----+---------+---------------------+
| 1  | 2       | fewklw23523kf       |
+----+---------+---------------------+

Also I have an URL like this:

http://example.com/activate.php?str=fewklw23523kf

All I'm trying to do:

• Comparing GET['str'] with random_string column from activate table
• Checking active column for NULL where id = post_id.

And then (if there is matched row) set 1 the active column from users table. How can I do that?

$str = $_GET['str'];

$stm = $db_con->prepare( "UPDATE users SET active = 1
                          WHERE ( SELECT 1 FROM activate WHERE random_string = ? ) t AND
                                ( SELECT 1 FROM users WHERE t.post_id = id AND
                                                            active IS NULL ) ");

$stmt->execute(array($str));

My query doesn't work as expected.

You can use join

UPDATE users 
INNER JOIN activate on activate.post_id = user.id
SET active = 1        
WHERE activate.random_string = ? 
AND user.active IS NULL;

I do believe this one will do the trick

UPDATE users, activate SET active = 1 
WHERE users.id = post_id and active is null and random_string=? 

try to change

( SELECT 1 FROM users WHERE t.post_id = id AND active IS NULL )

to this

( SELECT 1 FROM users, activate as t WHERE t.post_id = id AND active IS NULL)