【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*fromscore;name|subject&n.

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