Select使用另一个Select中As的值

Select使用另一个Select中As的值
Selelct As 的嵌套
Select使用As的值

代码示例：

SELECT rowcnt, 
    CASE 
        WHEN (rowcnt - 10) > 0 THEN (rowcnt - 10) 
        ELSE 0 
    END AS rowfirst
FROM 
    (SELECT Count(*) AS rowcnt FROM TestTable) AS subquery;