简单的SQL问题！

SQL code:

declare @t1 table(数值 decimal(10,2))
insert into @t1 values(200)
insert into @t1 values(150)
insert into @t1 values(199)
insert into @t1 values(200)

select sum(数值)as a ,a*2 as b from @t1


想要的结果
===========================================
a b
749.00 1498.00
===========================================

改SQL语句中在查询分析器中提示：列名 'a' 无效。
请大家帮忙！

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SQL code:
select sum(数值)as a ,sum(数值)*2 as b from @t1

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select a, a*2 from (select sum(数值)as a    from @t1) m

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SQL code:
declare @t1 table(数值 decimal(10,2))
insert into @t1 values(200)
insert into @t1 values(150)
insert into @t1 values(199)
insert into @t1 values(200)

select sum(数值)as a ,sum(数值)*2 as b from @t1

/*
a b
---------------------------------------- ----------------------------------------
749.00 1498.00

（所影响的行数为 1 行）
*/

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SQL code

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