SQL：如何获取具有唯一列值的记录并对另一列中的值求和

Question

I have this table called file

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

I want to get all the records with a unique hash value (that is a single instance of duplicated files) and then sum the values in the size column to the total bytes of disk space I'll need to back up a single copy of each file.

Answer 1

This returns only unique hashes, ie no duplicates exist:

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

Edit: This return only one row per hash:

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

Both queries are Standard SQL, but PostgreSQL also supports proprietary syntax:

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