sql的max之类的聚合函数只能针对同一列的n行运算，如果对n列运算，一般都用case 语句来判断，如果列少还比较容易写，列多了就麻烦了。
--------------------------------------------------------------------------------
/*
测试名称：利用 XML 求任意列之间的聚合
测试功能：对一张表的列数据做 min 、 max 、 sum 和 avg 运算
运行原理：字段合并为 xml 后做 xquery 查询转为行集后聚合
*/ -- 建立测试环境 declare @t table (
id smallint ,
a smallint , b smallint ,
c smallint , d smallint ,
e smallint , f smallint )
insert into @t
select 1, 1, 2, 3, 4, 6, 7 union all
select 2, 34, 45, 56, 54, 9, 6 -- 测试语句 selecta.*, c.*
from @t a outer apply(
select doc=(
select * from @t as doc where id= a. idfor xml path ( '' ), type)
) b
outer apply(
select
min ( r) as minValue,
max ( r) as maxValue,
sum ( r) as sumValue,
avg ( r) as avgValue
from (
select cast ( cast ( d. n. query( 'text()' ) as varchar ( max )) as int ) as r
from doc. nodes( '/a,b,c,d,e,f' ) D( n)) tt
) c /* 测试结果 idabcdefminValuemaxValuesumValueavgValue
------ ------ ------ ------ ------ ------ ------ ----------- ----------- ----------- -----------
112346717233
2344556549665620434
*/
