SQL：如何獲取具有唯一列值的記錄並對另一列中的值求和

Question

我有一張叫做file表

id         integer primary key,
created_on timestamp
updated_on timestamp 
file_name  text not null
path       text not null unique
hash       text not null
size       bigint not null
size_mb    bigint not null

我想獲取具有唯一hash值的所有記錄（即重復文件的單個實例），然后將size列中的值與磁盤空間的總字節數相加，我需要備份每個記錄的單個副本文件。

Answer 1

這僅返回唯一的哈希值，即不存在重復項：

select *, 
   -- group sum of all files
   sum(size) over () 
from
 (
   select *, 
      -- rows per hash
      count(*) over (partition by hash) as cnt
   from file
 ) as dt
where cnt = 1

編輯：這每個哈希只返回一行：

select *, 
   -- group sum of all files
   sum(size) over () 
from
 (
   select *, 
      -- unique number per hash
      row_number(*) over (partition by hash order by hash) as rn
   from file
 ) as dt
where rn = 1

這兩個查詢都是標准 SQL，但 PostgreSQL 也支持專有語法：

select *, 
   -- group sum of all files
   sum(size) over () 
from
 (
   select DISTINCT ON (hash) *
   from file
   order by hash
 ) as dt