Using return value from DELETE for UPDATE in Postgres

Question

I need to update a table using a value deleted from another table. The situation is a comment vote scorekeeper similar to the one on SO. I\'m using python to work the postgres,

Answer 1

PostgreSQL doesn't allow mix UPDATE and DELETE statements as subquery.

You can use a little bit different strategy - updateable CTE

postgres=# WITH t1 AS (DELETE FROM foo RETURNING *), 
                t2 AS (INSERT INTO deleted 
                          SELECT * FROM t1 RETURNING *) 
             SELECT max(a) FROM t2;

so

postgres=# CREATE TABLE comment(id int, score int);
CREATE TABLE
postgres=# CREATE TABLE history(id int, comment_id int, vote int);
CREATE TABLE
postgres=# INSERT INTO comment values(1,10);
INSERT 0 1
postgres=# INSERT INTO comment values(2,20);
INSERT 0 1
postgres=# INSERT INTO history values(1,1,5);
INSERT 0 1
postgres=# WITH t1 AS (DELETE FROM history 
                       WHERE id=1 
                       RETURNING comment_id, vote) 
           UPDATE comment SET score=score-t1.vote 
           FROM t1 
           WHERE t1.comment_id=comment.id;
UPDATE 1
postgres=# select * from comment;
 id | score 
----+-------
  2 |    20
  1 |     5
(2 rows)

Attention: It require 9.1 or newer