关系数据库的表comment查询

--Oracle
SELECT rownum,COMMENTS as table_comment FROM all_tab_comments 
WHERE OWNER  = '%s' 
AND TABLE_NAME ='%s' AND rownum=1

--SQLserver
select top 1 t.name as TableName,
f.value as [table_comment]
from %1$s.sys.tables as t
left join %1$s.sys.sysindexes as i
on t.object_id = i.id
left join
%1$s.sys.extended_properties f
on  t.object_id=f.major_id
where t.name='%2$s' and    i.indid <=1

--postgresql
select relname as tabname,
cast(obj_description(relfilenode,'pg_class') as varchar) as table_comment 
from pg_class c 
where   relname ='%s'

--mysql
select TABLE_COMMENT as table_comment from information_schema.TABLES t 
where table_schema='%s' and  table_name='%s'