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 (9.5.0) Type "help" for help. mydb=> select * from trade; tradeno | accountid | fee | game_id ---------+------------+-----+--------- 1000006 | yyb_100001 | 10 | 2555 1000011 | yyb_100002 | 100 | 2555 1001859 | yyb_100001 | 10 | 2555 1001861 | yyb_100003 | 20 | 2555 1001854 | yyb_100004 | 6 | 2555 1001881 | yyb_100002 | 328 | 2555 (6 rows) mydb=> select distinct accountid from trade; accountid ------------ yyb_100001 yyb_100004 yyb_100002 yyb_100003 (4 rows) mydb=> select distinct accountid,game_id from trade; accountid | game_id ------------+--------- yyb_100001 | 2555 yyb_100003 | 2555 yyb_100004 | 2555 yyb_100002 | 2555 (4 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 | 10 yyb_100002 | 100 yyb_100003 | 20 yyb_100004 | 6 (4 rows) mydb=> select distinct on (game_id) accountid,fee from trade; accountid | fee ------------+----- yyb_100001 | 10 (1 row) mydb=> select distinct on (game_id) accountid,fee from trade order by game_id, fee desc; accountid | fee ------------+----- yyb_100002 | 328 (1 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 | 10 yyb_100002 | 328 yyb_100003 | 20 yyb_100004 | 6 (4 rows)