SQL update set table if value in table A is equals to value in table B

Question

this query is working fine.

UPDATE data
   SET unit_id='a3110a89'
 WHERE unit_id='7d18289f';

Now, I need to run this query over 30 times so I made csv file import it to the DB with this command:

COPY mytable FROM 'D:/test.csv' WITH CSV HEADER DELIMITER AS ','

Now I have table called my table with 2 columns OLD and NEW i want to search the table "data" in column unit_id anywhere there if the value equals to the value in table "mytable.old" replace it with the value "mytable.new" on the same row.

I tried to run this query but I get an error:

UPDATE data
   SET unit_id=(SELECT mytable."old" FROM public.mytable)
   WHERE unit_id=(SELECT mytable."new" FROM public.mytable)

error:

more than one row returned by a subquery used as an expression

I think i'm just trying to do it in the wrong way...

thx for the help!

by the way Im using PostgreSQL

Answer 1

Your subqueries need to be correlated to the outer update:

UPDATE data
   SET unit_id = (SELECT mytable."new" FROM public.mytable where data.old = mytable.old)
   WHERE unit_id in (SELECT mytable."old" FROM public.mytable);

That is, set the unit_id to the "new" value, when you find the "old" value in the table.

Answer 2

Can you try like this,

UPDATE data A
   SET A.unit_id=B.old
   FROM (SELECT mytable."old",mytable."new" FROM public.mytable) B
   WHERE A.unit_id=B.new

Answer 3

UPDATE data A
  SET unit_id = B."old"
  FROM public.mytable B
  WHERE A.unit_id = B."new"
  ;

BTW: it looks like you also have old and new swapped in your question. Do you really want A's value to be set to B's old field?