1

在 PostgreSQL 8.3 中,假设我有一个名为widgets以下内​​容的表:

 id  | type | count
--------------------
  1  | A    |   21
  2  | A    |   29
  3  | C    |    4
  4  | B    |    1
  5  | C    |    4
  6  | C    |    3
  7  | B    |   14

我想根据列删除重复项,只留下表中列值type最高的那些。count最终数据如下所示:

 id  | type | count
--------------------
  2  | A    |   29
  3  | C    |    4  /* `id` for this record might be '5' depending on your query */
  7  | B    |   14

我觉得我已经很接近了,但我似乎无法围绕一个可以消除重复列的查询。

4

3 回答 3

2

count是一个 sql 保留字,因此必须以某种方式对其进行转义。我不记得在我头顶的 Postgres 中执行此操作的语法,所以我只是用方括号将其包围(如果不正确,请更改它)。无论如何,以下理论上应该有效(但我实际上并没有测试它):

delete from widgets where id not in (
select max(w2.id) from widgets as w2 inner join 
(select max(w1.[count]) as [count], type from widgets as w1 group by w1.type) as sq
on sq.[count]=w2.[count] and sq.type=w2.type group by w2.[count]
);
于 2009-11-02T05:54:07.217 回答
1

有一个比 Asaph 更简单的答案,使用 EXISTS SQL 运算符:

DELETE FROM widgets AS a 
WHERE EXISTS 
  (SELECT * FROM widgets AS b 
   WHERE (a.type = b.type AND b.count > a.count) 
          OR (b.id > a.id AND a.type = b.type AND b.count = a.count))

如果以下 SQL 语句返回至少一条记录,则 EXISTS 运算符返回 TRUE。

于 2009-11-02T16:48:43.920 回答
0

根据您的要求,在我看来这应该可行:

DELETE 
FROM widgets
WHERE type NOT IN 
(
    SELECT type, MAX(count) 
    FROM widgets 
    GROUP BY type
)
于 2009-11-02T05:57:52.330 回答