MySQL and PostgreSQL – Creating Unique Constraints with Different Names

I do agree with Aaron, that not creating a PK (or unique constraint) is a very strange requirement

But you could do something like this:

insert into b (id_b, id_a)
select 1, 2 
where exists (select 1 from a where id_a = 2);

or alternatively:

insert into b (id_b, id_a)
select 1, id_a 
from a where id_a = 2

If you want to insert multiple rows, you can use a values clause:

insert into b (id_b, id_a)
select * 
from (
   values 
      (1,2), (2,3), (3,4)
) as t(id_b, id_a) 
where exists (select 1  
              from a 
              where a.id_a = t.id_a);

This will only make sure that this specific insert will insert correct data. It will not prevent others that don't follow this pattern to insert invalid data. And it is not safe in a multi-user environment where different transactions run the inserts concurrently.

@dezso had the completely right idea:

All this means that some data has been updated, right? Try to update them back, using a temp table where you copy the original data

The only thing left now was to make it happen.

So here's what I did. I took a leaf out of his book and manually edited the dump-file to use a table named table_backup. Then I created said table using the definition provided in my pgAdmin (but it can be done manually, too).

I left out triggers and constraints, as well as Foreign Keys, and then proceeded to "updating" the original table with the data from the backup table like the following:

BEGIN TRANSACTION;
ALTER TABLE table DISABLE TRIGGER ALL;

UPDATE table SET 
    (column1, column2, ...) = 
    (table_backup.column1, table_backup.colum2, ...)
FROM table_backup WHERE table.pk_column = table_backup.pk_column;

ALTER TABLE table ENABLE TRIGGER ALL;
-- I didn't but you can drop table_backup here
COMMIT;

So I am finally back with my original data, ready for the next testrun ;)