请问如何能嵌套使用select count(*) from tableName 所得到的值？

[benzite]

下面这个语句不能不执行，只是用来说明一下我的意图：
slect sum1*2 from (
select count(*) as sum1 from table1)

[伍子V5]

slect sum1*2 from (
select count(*) as sum1 from table1) a

[yujohny]

slect sum1*2 from ( select count(*) as sum1 from table1) a
或
select (select count(*) from table1)*2 from a

[LoveSQL]

select (select count(*) from table1)*2 as sum

[shuiniu]

slect sum1*2 from ( select count(*) as sum1 from table1) a
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"派生表"的应用

[qdubit]

slect sum1*2 from ( select count(*) as sum1 from table1) a

[txlicenhe]

slect sum1*2 from ( select count(*) as sum1 from table1) a
或
select (select count(*) from table1)*2 from a