PostgreSQL的行转列函数使用一例

开发同事说使用postgres的扩展行转列应用时有一个问题，示例如下

该扩展包初步使用参考： http://my.oschina.net/Kenyon/blog/54357

一、环境
OS:CentOS 6.3
DB:Postgresql 9.3.0

二、场景

create table t(day date,equipment varchar(20),output integer);
insert into t values('2010-04-01','DAT501',100);
insert into t values('2010-04-01','DAT502',120);
insert into t values('2010-04-01','DAT503',130);
insert into t values('2010-04-02',110);
insert into t values('2010-04-02',105); 
insert into t values('2010-04-03',125);
insert into t values('2010-04-04',100);
insert into t values('2010-04-04',200);
--想得到如下结果
    day     | dat501 | dat502 | dat503 
------------+--------+--------+--------
 2010-04-01 |    100 |    120 |    130
 2010-04-02 |    110 |    105 |       
 2010-04-03 |        |        |    125
 2010-04-04 |    100 |        |    200     
(4 rows)

--但是直接使用crosstab会导致第3,4行不准确，也就是说中间项为Null就会不准
test=# SELECT * FROM crosstab('select day,equipment,output from t order by 1,2')  AS t(day date,DAT501 integer,DAT502 integer,DAT503 integer);
    day     | dat501 | dat502 | dat503 
------------+--------+--------+--------
 2010-04-01 |    100 |    120 |    130
 2010-04-02 |    110 |    105 |       
 2010-04-03 |    125 |        |       
 2010-04-04 |    100 |    200 |       
(4 rows)

三、解决
crosstab还有一个包含两个输入参数的用法，用这个可以解决上述问题

test=# SELECT * FROM crosstab('select day,2',$$values('DAT501'::text),('DAT502'::text),('DAT503'::text)$$)  AS t(day date,DAT503 integer);
    day     | dat501 | dat502 | dat503 
------------+--------+--------+--------
 2010-04-01 |    100 |    120 |    130
 2010-04-02 |    110 |    105 |       
 2010-04-03 |        |        |    125
 2010-04-04 |    100 |        |    200
(4 rows)

--其他的写法,本质都一样

test=# SELECT * FROM crosstab('select day,output from t order by 1','select distinct equipment  from t order by 1')  AS t(day date,DAT503 integer);
    day     | dat501 | dat502 | dat503 
------------+--------+--------+--------
 2010-04-01 |    100 |    120 |    130
 2010-04-02 |    110 |    105 |       
 2010-04-03 |        |        |    125
 2010-04-04 |    100 |        |    200
(4 rows)

四、说明
使用两个参数的crosstab其实更安全，推荐使用，其基础用法是

crosstab(text source_sql,text category_sql)

参考：
http://www.postgresql.org/docs/9.2/static/tablefunc.html