Question

I have table1 with columns ( id, id_user ) and table2 with columns (id, username).
The users are registered in table2. So what I want is to insert the table2.id of the new registered users in table1.id_user.

I have this query:

INSERT INTO table1 (id_user) SELECT id FROM table2;

but the result is:

for example I have in table1.id:

 id
 1
 2
 3

and when the new users register in table2 the table1.id looks like this:

id
1
2
3
1
2
3
4

and it repeats all the data every time there is a new registered user.
How can I fix this to only add the newly registered user?

Answer 1

INSERT INTO table1  (id_user) 
    SELECT id FROM table2 where id not in (select id_user from table1)

If you use a DB trigger then you could insert the new user id from table2 in table1 automatically.