postgresql之distinct用法

1. 去重；关键字distinct去重功能  在其他数据库（oracle，mysql）是存在；当然postgresql也有这个功能

[postgres@sdserver40_210 ~]$ psql mydb lottu
psql (9.5.)
Type "help" for help.

mydb=> select * from trade;
 tradeno | accountid  | fee | game_id
---------+------------+-----+---------
  | yyb_100001 |   |
  | yyb_100002 |  |
  | yyb_100001 |   |
  | yyb_100003 |   |
  | yyb_100004 |    |
  | yyb_100002 |  |
( rows)

mydb=> select distinct accountid from trade;
 accountid
------------
 yyb_100001
 yyb_100004
 yyb_100002
 yyb_100003
( rows)

mydb=> select distinct accountid,game_id from trade;
 accountid  | game_id
------------+---------
 yyb_100001 |
 yyb_100003 |
 yyb_100004 |
 yyb_100002 |
( rows)

2. 跟on一起用； 使用DISTINCT ON实现用窗口函数实现的取第一名的功能

    这个功能oracle，mysql是没有的；当然它们有其他的分析函数可以替换；顶替；例如row_number, fisrt_values等等

mydb=> select distinct on (accountid) accountid,fee from trade;
 accountid  | fee
------------+-----
 yyb_100001 |
 yyb_100002 |
 yyb_100003 |
 yyb_100004 |
( rows)

mydb=> select distinct on (game_id) accountid,fee from trade;
 accountid  | fee
------------+-----
 yyb_100001 |
( row)

mydb=> select distinct on (game_id) accountid,fee from trade order by game_id, fee desc;
 accountid  | fee
------------+-----
 yyb_100002 |
( row)

--例如取每个帐号充值最大的一笔
mydb=> select distinct on (accountid) accountid,fee from trade order by accountid, fee desc;
 accountid  | fee
------------+-----
 yyb_100001 |
 yyb_100002 |
 yyb_100003 |
 yyb_100004 |
( rows)