在第一个SELECT中分配的变量不在第二个SELECT中检索

Question

在执行以下语句时，会为Start_Index和End_index返回非空/空值的混合值：

SELECT 
    CHARINDEX('_',  dbo.Components.Cast, 1) AS Start_Index,
    CHARINDEX('_', dbo.Components.Cast, CHARINDEX('_', dbo.Components.Cast, 1) + 1) AS End_Index
FROM dbo.Components

由于我希望同时执行赋值和数据检索，因此我将SELECT语句分为两部分。但是，Start_Index和End_index现在为所有行返回NULL：

DECLARE @start_index INT
DECLARE @end_index INT

SELECT 
    @start_index = CHARINDEX('_',  dbo.Components.Cast, 1),
    @end_index = CHARINDEX('_', dbo.Components.Cast, CHARINDEX('_', dbo.Components.Cast, 1) + 1)
FROM dbo.Components

SELECT
    @start_index AS Start_Index,
    @end_index AS End_Index
FROM dbo.Components

我在做什么错了？