sql的AVG如何实现保留一位小数（数据库里为int型）？

   
   select b.unitCode,b.unit,AVG(b.age*1.0)    avgAge from  (select ((2015-SUBSTRING(birthday,0,5))*12+(3-SUBSTRING(birthday,5,2)))/12 as age
,a.* from TRT_CADRES a) b where unit is not null group by b.unitCode,b.unit


AVG(b.age*1.0) 

select case when avg(colname*1.0)=cast(avg(colname*1.0) as int) then avg(colname*1.0) else cast(avg(colname*1.0) as decimal(9,1)) end