POSTGRESQL数据库如何实现交叉表

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：

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    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：
 

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    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

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    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。

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    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

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    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

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    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

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    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