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|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
(12rows)
Time:2.066ms

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=#selectname,t_girl-#sum(casewhensubject='English'thenscoreelse0end)as"English",t_girl-#sum(casewhensubject='Physics'thenscoreelse0end)as"Physics",t_girl-#sum(casewhensubject='Math'thenscoreelse0end)as"Math"
t_girl-#fromscore
t_girl-#groupbynameorderbynamedesc;
name|English|Physics|Math
-------+---------+---------+------
Simon|90|76|79
Lucy|100|90|85
Lily|95|81|84
David|100|86|89
(4rows)
Time:1.123ms

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*
FROMcrosstab('selectname,subject,scorefromscoreorderbynamedesc',$$values('English'::text),('Physics'::text),('Math'::text)$$)
ASscore(nametext,Englishint,Physicsint,Mathint);
name|english|physics|math
-------+---------+---------+------
Simon|90|76|79
Lucy|100|90|85
Lily|95|81|84
David|100|86|89
(4rows)
Time:2.059ms

3、用Postgresql 自身的聚合函数实现

t_girl=#selectname,split_part(split_part(tmp,',1),':',2)as"English",t_girl-#split_part(split_part(tmp,2),2)as"Physics",3),2)as"Math"
t_girl-#from
t_girl-#(
t_girl(#selectname,string_agg(subject||':'||score,')astmpfromscoregroupbynameorderbynamedesc
t_girl(#)asT;
name|English|Physics|Math
-------+---------+---------+------
Simon|90|76|79
Lucy|100|90|85
Lily|95|81|84
David|100|86|89
(4rows)
Time:2.396ms

4、 存储函数实现

createorreplacefunctionfunc_ytt_crosstab_py()
returnssetofytt_crosstab
as
$ytt$
forrowinplpy.cursor("selectname,')astmpfromscoregroupbynameorderbynamedesc"):
a=row['tmp'].split(',')
yield(row['name'],a[0].split(':')[1],a[1].split(':')[1],a[2].split(':')[1])
$ytt$languageplpythonu;
t_girl=#selectname,english,physics,mathfromfunc_ytt_crosstab_py();
name|english|physics|math
-------+---------+---------+------
Simon|90|76|79
Lucy|100|90|85
Lily|95|81|84
David|100|86|89
(4rows)
Time:2.687ms

5、 用PLPGsql来实现

t_girl=#createtypeytt_crosstabas(nametext,Englishtext,Physicstext,Mathtext);
CREATETYPE
Time:22.518ms
createorreplacefunctionfunc_ytt_crosstab()
returnssetofytt_crosstab
as
$ytt$
declarev_nametext:='';
v_englishtext:='';
v_physicstext:='';
v_mathtext:='';
v_tmp_resulttext:='';
declarecs1cursorforselectname,')fromscoregroupbynameorderbynamedesc;
begin
opencs1;
loop
fetchcs1intov_name,v_tmp_result;
exitwhennotfound;
v_english=split_part(split_part(v_tmp_result,2);
v_physics=split_part(split_part(v_tmp_result,2);
v_math=split_part(split_part(v_tmp_result,2);
returnqueryselectv_name,v_english,v_physics,v_math;
endloop;
end;
$ytt$languageplpgsql;
t_girl=#selectname,English,Physics,Mathfromfunc_ytt_crosstab();
name|english|physics|math
-------+---------+---------+------
Simon|90|76|79
Lucy|100|90|85
Lily|95|81|84
David|100|86|89
(4rows)
Time:2.127ms