PostgreSQL：根据分组来将多行合并成一行

select		
b.bid, b.oid, b.bed_name, b.meta, b.create_time, b.update_time,		
string_agg(g.group_name,',') AS groupNames		
from bed as b				
LEFT JOIN bed_group bg ON bg.bid = b.bid		
LEFT JOIN bgroup g ON g.gid = bg.gid		
GROUP BY b.bid