PostgreSQL中的group_concat使用
group_concat是mysql中的一个聚集函数，挺好用的，mysql的group_concat使用可参考：http://my.oschina.net/Kenyon/blog/70480。在postgresql中实现这个功能倒也容易,可以用array的转换或者函数string_agg()来做。DB环境：postgresql 9.1.2
一.测试数据准备
postgres=# create table t_kenyon(id int,name text);
CREATE TABLE
postgres=# insert into t_kenyon values(1,'kenyon'),(1,'chinese'),(1,'china'),('2','american'),('3','japan'),('3','russian');
INSERT 0 6
postgres=# select * from t_kenyon order by 1;
id | name
----+----------
1 | kenyon
1 | chinese
1 | china
2 | american
3 | japan
3 | russian
(6 rows)
二.实现过程
1.以逗号为分隔符聚集
postgres=# select array_to_string(ARRAY(SELECT unnest(array_agg(name))),',') from t_kenyon ;
array_to_string
---------------------------------------------
kenyon,chinese,china,american,japan,russian
(1 row)
2.结合order by排序
postgres=# select array_to_string(ARRAY(SELECT unnest(array_agg(name)) order by 1),',') from t_kenyon;
array_to_string
---------------------------------------------
american,china,chinese,japan,kenyon,russian
(1 row)
3.结合group聚集
postgres=# SELECT id, array_to_string(ARRAY(SELECT unnest(array_agg(name))
),',') FROM t_kenyon GROUP BY id;
id | array_to_string
----+----------------------
1 | china,chinese,kenyon
2 | american
3 | japan,russian
(3 rows) 4.以其他分隔符聚集,如*_*
postgres=# SELECT id, array_to_string(ARRAY(SELECT unnest(array_agg(name))
order by 1),'*_*') FROM t_kenyon GROUP BY id ORDER BY id;
id | array_to_string
----+--------------------------
1 | china*_*chinese*_*kenyon
2 | american
3 | japan*_*russian
(3 rows)
还有一个函数更简:string_agg
CREATE TABLE
postgres=# insert into t_test values(1,'kk'),(2,'ddd'),(1,'yy'),(3,'ty');
INSERT 0 4
postgres=# select * from t_test;
id | vv
----+-----
1 | kk
2 | ddd
1 | yy
3 | ty
(4 rows)
postgres=# select id,string_agg(vv,'*^*') from t_test group by id;
id | string_agg
----+------------
1 | kk*^*yy
2 | ddd
3 | ty
(3 rows)
三.总结
postgresql也可以很简单的实现mysql中的group_concat功能，而且更加丰富，可以基于此写一个同名的函数。