Question

I need some help here i have this 2 tables :

table "clients"
+------+-------------+-----------+
   id  |  email      |  otherinfo|
+------+-------------+-----------+
   1   |[e-mail]   |   .....   |
+------+-------------+-----------+
   2   |test2@.ts.ts |   ....    |
+------+-------------+-----------+

table "comptes"
+------+-------------+---------------+
   id  |  login      |   id_clients  | 
+------+-------------+---------------+
 1     |     test    | 1             |
+------+-------------+---------------+
 1     |     test2   | 2             |
+------+-------------+---------------+
 etc.  |    etc.     |       etc..   |
+------+-------------+---------------+

In my website when a user crate an account he give the information of both tables. So i want to test if LOGIN or EMAIL exist in the database before adding them, something like this

'select clients.email,comptes.login    
from clients,comptes     
where clients.email='test2@.ts.ts'
 or comptes.login ='test';

But this query return empty result, i tired other combination but none give the right results .So what i am messing here

Answer 1

You don’t need a join at all to see if they exist. The following query returns the id of any matching record:

select c.id, 'email' as matchtype
from clients c
where c.email = <email>
union all
select c.id, 'login' as matchtype
from comptes c
where c.login = <login>

This gives you the matching ids and tells you where the duplicates are appearing (if that is of interest). If you just want a 0 or 1 flag to specify the existence of duplicates, do this:

select count(*) as numdups
from ((select c.id, 'email' as matchtype
       from clients c
       where c.email = <email>
      )
      union all
      (select c.id, 'login' as matchtype
       from comptes c
       where c.login = <login>
     )
    ) t