How to spread a table into another one - sql

Question

We have a legacy table
create table table1 (col1 int);
insert into table1 values(1);
insert into table1 values(2);
insert into table1 values(3);
SELECT * FROM table1;
1
2
3
now it gets a new column
alter table table1 add column col2 int;
alter table table1 ADD CONSTRAINT unique1 UNIQUE (col2);
SELECT * FROM table1;
1;null
2;null
3;null
then we have another table
create table table2 (col1 int);
insert into table2 values(7);
insert into table2 values(8);
insert into table2 values(9);
SELECT * FROM table2;
7
8
9
now we want to spread the values of table 2 into table1.col2
UPDATE table1 up
SET col2 = (SELECT col1
FROM table2 t2
WHERE NOT EXISTS (SELECT 1 FROM table1 t1 WHERE t1.col2=t2.col1)
LIMIT 1);
but the update statement does not see the already updated rows
ERROR: duplicate key value violates unique constraint "unique1"
Any ideas how to do that? It would be ok, if table1 remains with some rows col2=null if table2 has less rows than table1

Answer 1

This seems much easier with a join:
with t2 as (
select t2.*, row_number() over (order by col1) as seqnum
from table2 t2
)
update table1 t1
set col2 = t2.col1
from t2
where t1.col1 = t2.seqnum;
If col1 in table1 is not strictly sequential, you can still do this:
with t2 as (
select t2.*, row_number() over (order by col1) as seqnum
from table2 t2
),
t1 as (
select t1.*, row_number() over (order by col1) as seqnum
from table1 t1
)
update table1 toupdate
set col2 = t2.col1
from t1 join
t2
on t1.seqnum = t2.seqnum
where toupdate.col1 = t1.col1;

Answer 2

i think i found a solution
WITH rownumbers AS (
SELECT col1, row_number() over (partition by 1 ORDER BY col1) FROM table1
)
UPDATE table1 up SET col2 =
(SELECT col1 FROM table2 t2 WHERE NOT EXISTS (SELECT 1 FROM table1 t1 WHERE t1.col2=t2.col1)
LIMIT 1 OFFSET (
SELECT row_number-1 FROM rownumbers WHERE col1=up.col1
)
)
any cons about it?