select a.*,b.b_name
from a_001.dbo.hist a inner join b_001.dbo.bhis b on a.mid=b.mid

------解决方案--------------------
SQL code

select a.*,b.Bname from A_001.dbo.HIST a, B_001.DBO.BHIS b
where a.MID=b.MID

------解决方案--------------------
SELECT  a.* , b.b_name
FROM    a_001..hist a
INNER JOIN b_001..bhis b
ON      a.mid = b.mid

垮库的引用的时候带上库名就可以了

库名.架构名.表名  
架构名可以省略掉，就时当前用户默认架构
库名..表