【原创】POSTGRESQL交叉表的实现

这里我来演示下在POSTGRESQL里面如何实现交叉表的展示，至于什么是交叉表，我就不多说了，度娘去哦。
原始表数据如下：

t_girl=# select * from score;
 name | subject | score
-------+---------+-------
 Lucy | English | 100
 Lucy | Physics | 90
 Lucy | Math | 85
 Lily | English | 95
 Lily | Physics | 81
 Lily | Math | 84
 David | English | 100
 David | Physics | 86
 David | Math | 89
 Simon | English | 90
 Simon | Physics | 76
 Simon | Math | 79
(12 rows)


Time: 2.066 ms

想要实现以下的结果：
 

name | English | Physics | Math
-------+---------+---------+------
 Simon | 90 | 76 | 79
 Lucy | 100 | 90 | 85
 Lily | 95 | 81 | 84
 David | 100 | 86 | 89

大致有以下几种方法：


1、用标准SQL展现出来

t_girl=# select name,
t_girl-# sum(case when subject = 'English' then score else 0 end) as "English",
t_girl-# sum(case when subject = 'Physics' then score else 0 end) as "Physics",
t_girl-# sum(case when subject = 'Math' then score else 0 end) as "Math"
t_girl-# from score
t_girl-# group by name order by name desc;
 name | English | Physics | Math
-------+---------+---------+------
 Simon | 90 | 76 | 79
 Lucy | 100 | 90 | 85
 Lily | 95 | 81 | 84
 David | 100 | 86 | 89
(4 rows)


Time: 1.123 ms

2、用PostgreSQL 提供的第三方扩展 tablefunc 带来的函数实现
以下函数crosstab 里面的SQL必须有三个字段，name, 分类以及分类值来作为起始参数，必须以name,分类值作为输出参数。

t_girl=# SELECT *
FROM crosstab('select name,subject,score from score order by name desc',$$values ('English'::text),('Physics'::text),('Math'::text)$$)
AS score(name text, English int, Physics int, Math int);
 name | english | physics | math
-------+---------+---------+------
 Simon | 90 | 76 | 79
 Lucy | 100 | 90 | 85
 Lily | 95 | 81 | 84
 David | 100 | 86 | 89
(4 rows)


Time: 2.059 ms

3、用PostgreSQL 自身的聚合函数实现

t_girl=# select name,split_part(split_part(tmp,',',1),':',2) as "English",
t_girl-# split_part(split_part(tmp,',',2),':',2) as "Physics",
t_girl-# split_part(split_part(tmp,',',3),':',2) as "Math"
t_girl-# from
t_girl-# (
t_girl(# select name,string_agg(subject||':'||score,',') as tmp from score group by name order by name desc
t_girl(# ) as T;
 name | English | Physics | Math
-------+---------+---------+------
 Simon | 90 | 76 | 79
 Lucy | 100 | 90 | 85
 Lily | 95 | 81 | 84
 David | 100 | 86 | 89
(4 rows)


Time: 2.396 ms

4、 存储函数实现

create or replace function func_ytt_crosstab_py ()
returns setof ytt_crosstab
as
$ytt$
  for row in plpy.cursor("select name,string_agg(subject||':'||score,',') as tmp from score group by name order by name desc"):
      a = row['tmp'].split(',')
      yield (row['name'],a[0].split(':')[1],a[1].split(':')[1],a[2].split(':')[1])
$ytt$ language plpythonu;


t_girl=# select name,english,physics,math from func_ytt_crosstab_py();
 name | english | physics | math
-------+---------+---------+------
 Simon | 90 | 76 | 79
 Lucy | 100 | 90 | 85
 Lily | 95 | 81 | 84
 David | 100 | 86 | 89
(4 rows)


Time: 2.687 ms

5、 用PLPGSQL来实现

t_girl=# create type ytt_crosstab as (name text, English text, Physics text, Math text);
CREATE TYPE
Time: 22.518 ms


create or replace function func_ytt_crosstab ()
returns setof ytt_crosstab
as
$ytt$
  declare v_name text := '';
                v_english text := '';
v_physics text := '';
v_math text := '';
v_tmp_result text := '';
  declare cs1 cursor for select name,string_agg(subject||':'||score,',') from score group by name order by name desc;
begin
  open cs1;
  loop
    fetch cs1 into v_name,v_tmp_result;
    exit when not found;
    v_english = split_part(split_part(v_tmp_result,',',1),':',2);
    v_physics = split_part(split_part(v_tmp_result,',',2),':',2);
    v_math = split_part(split_part(v_tmp_result,',',3),':',2);
    return query select v_name,v_english,v_physics,v_math;
  end loop;
end;
$ytt$ language plpgsql;


t_girl=# select name,English,Physics,Math from func_ytt_crosstab();
 name | english | physics | math
-------+---------+---------+------
 Simon | 90 | 76 | 79
 Lucy | 100 | 90 | 85
 Lily | 95 | 81 | 84
 David | 100 | 86 | 89
(4 rows)


Time: 2.127 ms