PostgreSQL行列转换(兼容oracle listagg)

oracle11g开始支持的listagg函数替代了wmconcat来实现行列转换的功能。

listagg函数的用法：

oracle行列转换例子：
–建表

SQL> create table b (id number,name varchar2(20));

Table created.

–插入数据

insert into b values(1,'zhang');
insert into b values(1,'wang');
insert into b values(2,'li');
insert into b values(2,'zhao');

–行列转换

SQL> select id,LISTAGG(name,',') within group(order by name desc) as bb from b group by id;

        ID
----------
BB
--------------------------------------------------------------------------------
         1
zhang,wang

         2
zhao,li

pg中是没有listagg和wmconcat函数的，那么我们要怎么在pg中如何实现同样的功能呢？pg中的string_agg函数可以用来代替listagg，实现同样的功能。

pg行列转换例子：
–建表

bill=# create table tbl1 (gid int, val text, ts timestamp default clock_timestamp()); 
CREATE TABLE

–插入数据

bill=# insert into tbl1 values (1,'a'),(1,'b'),(1,null),(2,'test'),(2,'a""b"c'),(3,'fw'); 
INSERT 0 6

bill=# select * from tbl1;  
 gid |  val   |             ts             
-----+--------+----------------------------
   1 | a      | 2020-01-05 14:08:42.995435
   1 | b      | 2020-01-05 14:08:42.995636
   1 |        | 2020-01-05 14:08:42.995642
   2 | test   | 2020-01-05 14:08:42.995645
   2 | a""b"c | 2020-01-05 14:08:42.995648
   3 | fw     | 2020-01-05 14:08:42.995717
(6 rows)

–行列转换
逆向聚合，双引号作为quote字符，转义文本内的双引号，空值使用NULL表示。

bill=# select gid, string_agg(coalesce('"'||replace(val,'"','\"')||'"','NULL'),',' order by ts desc) from tbl1 group by gid;  
 gid |     string_agg     
-----+--------------------
   1 | NULL,"b","a"
   2 | "a\"\"b\"c","test"
   3 | "fw"
(3 rows)

正向聚合，双引号作为quote字符，转义文本内的双引号，空值使用NULL表示。

bill=# select gid, string_agg(coalesce('"'||replace(val,'"','\"')||'"','NULL'),',' order by ts) from tbl1 group by gid;  
 gid |     string_agg     
-----+--------------------
   1 | "a","b",NULL
   2 | "test","a\"\"b\"c"
   3 | "fw"
(3 rows)

正向聚合，不使用QUOTE，直接去除NULL值

bill=# select gid, string_agg(val,',' order by ts) from tbl1 group by gid;  
 gid | string_agg  
-----+-------------
   1 | a,b
   2 | test,a""b"c
   3 | fw
(3 rows)