Question

考虑包含以下字段的表G H I J \ / \ / D E F \ | / \ \ | / | \|/ | B C \ / \ / A A = = A^0 B = A^ = A^1 = A~1 C = A^2 = A^2 D = A^^ = A^1^1 = A~2 E = B^2 = A^^2 F = B^3 = A^^3 G = A^^^ = A^1^1^1 = A~3 H = D^2 = B^^2 = A^^^2 = A~2^2 I = F^ = B^3^ = A^^3^ J = F^2 = B^3^2 = A^^3^2：mdl_files，id，contenthash，timecreated。

此表存储附件文件。

我们认为所有具有相同内容哈希的行都是重复行，我只想保留最旧的行（如果日期相等，则保留第一行）。我该怎么办？

以下查询：

filesize

返回：

SELECT
  id,
  contenthash,
  filesize,
  to_timestamp(timecreated) :: DATE
FROM mdl_files
ORDER BY contenthash;

我想得到这个结果集：

2480229 00002e87605311feb82b70473b61e81f0223c774    18178   2016-10-05
2997411 0000bfd20ef84948eee6811ce5bbac03de42ccb0    1293    2017-03-31
1304839 000280169fc78d704a2d4569bfb6f42ea4a1d5ae    8203    2015-11-10
1364656 000280169fc78d704a2d4569bfb6f42ea4a1d5ae    8203    2015-11-17
71568   0003c6aec5835964870902d697c06d21abf76bf7    139439  2013-04-19
2959945 000419c19d77df7285e669614075b47414e3ab2c    398 2017-03-20
3483049 00061dc0bc2452304107ddc75e7ee2908c729905    28618   2017-08-17
3483047 00061dc0bc2452304107ddc75e7ee2908c729905    28618   2017-08-17

我希望从结果集中删除以下重复的行：

2480229 00002e87605311feb82b70473b61e81f0223c774    18178   2016-10-05
2997411 0000bfd20ef84948eee6811ce5bbac03de42ccb0    1293    2017-03-31
1304839 000280169fc78d704a2d4569bfb6f42ea4a1d5ae    8203    2015-11-10

71568   0003c6aec5835964870902d697c06d21abf76bf7    139439  2013-04-19
2959945 000419c19d77df7285e669614075b47414e3ab2c    398 2017-03-20
3483049 00061dc0bc2452304107ddc75e7ee2908c729905    28618   2017-08-17

Answer 1

使用DISTINCT ON：

SELECT DISTINCT ON (contenthash)
  id,
  contenthash,
  filesize,
  to_timestamp(timecreated) :: DATE
FROM mdl_files
ORDER BY contenthash, timecreated, id;

DISTINCT ON是Postgres扩展名，可确保为括号中每个键的唯一组合返回一行。特定行是根据order by子句找到的第一行。

Answer 2

您可以尝试将ROW_NUMBER()与 windows函数结合使用来制作行号，然后将其删除。

SELECT t.* 
FROM (
SELECT
      id,
      contenthash,
      filesize,
      ROW_NUMBER() OVER (PARTITION BY contenthash,filesize order by timecreated) rn
FROM mdl_files
) t
where t.rn = 1

sqlfiddle

如果您想DELETE复制数据，则可以在where子句中使用EXISTS。

DELETE 
FROM mdl_files f WHERE EXISTS(
  SELECT 1
  FROM (
  SELECT
        id,
        contenthash,
        filesize,
        ROW_NUMBER() OVER (PARTITION BY contenthash,filesize order by timecreated) rn
  FROM mdl_files
  ) t
  where t.rn > 1 and t.id = f.id
)

sqlfiddle

在PostgreSQL中基于选择查询中的字段删除重复的行？

2 个答案: