postgresql多行合并string_agg函数

1 有时候我们会需要将多条数据根据一些特别的字段做一些合并。比如下面这个查询，正常会查询出3条数据，但是我们会希望根据create_by 分成两列显示

2 这时候需要用到string_agg函数，先通过group by分组，在进行合并，当然查询结果需要满足group  by的限制；sql语句：

select create_by,string_agg(videoname,',') as videonames from w008_video_addr_info where id in (4248,538,546)
group by create_by

查询结果：

3 复杂一些的应用场景（子查询）：

下面的语句是我用来查询一个学生在什么时间看了哪些视频：

select 
	sa.id,
	info.nickname,  
	(select string_agg(v.videoname,',')  
		from w008_school_assign_video sv   
		join w008_video_addr_info v on sv.videoaddrinfo =v.id 
		where sv.schoolassignment=sa.id and v.is_removed=0 and sv.is_removed=0  
		group by v.is_removed) as videos,
    (select string_agg(to_char(sv.create_date, 'MM-DD HH24:MI'),',')  
		from w008_school_assign_video sv  
		join w008_video_addr_info v on sv.videoaddrinfo =v.id  where         
        sv.schoolassignment=sa.id and v.is_removed=0  
		and sv.is_removed=0 group by v.is_removed) as viewtime 
from w008_school_assignment sa   
join w008_user_business_info info on sa.userlongid=info.id  where sa.shchoolworkid=2514505674916356

结果：

当然，string_agg(field,'分隔符');分隔符可以填写其他任意的字符，方便后期处理即可；