在Postgres中使用DELETE的返回值进行UPDATE

Question

我需要使用从另一个表中删除的值来更新表.情况是评论投票记分员类似于SO上的评分投票记分员.我正在使用python来处理postgres,但这不应该有所作为.

query="""
UPDATE comment SET score=score-(DELETE FROM history
                                WHERE commentId=%(commentId)s AND
                                      userIdentity=%(userIdentity)s RETURNING vote)
WHERE commentId=%(commentId)s;
"""
cursor.execute(query, data)

出现错误(DELETE FROM; 出现语法错误.我可以用DELETE声明替换SELECT声明,它会起作用,这里有什么我想念的吗？我想在更新中使用返回值.这可能吗？一切都有帮助.

相关模式:

CREATE TABLE history (
    commentId bigint,
    vote int,
    userIdentity varchar(256),
);
CREATE TABLE comment (
    id bigint,
    score bigint,
);

history.vote通常是1或-1.

Answer 1

PostgreSQL不允许将UPDATE和DELETE语句混合为子查询.

您可以使用一点点不同的策略 - 可更新的CTE

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

所以

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)

注意:它需要9.1或更高版本