CASE不是语句而是表达式，它返回标量值。可以重构CASE表达式以用作 predicate ：

Select 
    * 
from 
    myTable
Where
    data = 'A'
    and 
        case  
            when @BothPendingAndApproved = 'Both' and ApprovalStatus <> 'Rejected' THEN 1
            when @BothPendingAndApproved = 'Specific' and ApprovalStatus like @ApprovalFilter THEN 1
        end = 1
    and 
    Data2='B'

恕我直言，使用OR条件会更清楚一些：

create procedure MyProcedure 
    @ApprovalFilter as nvarchar(50), 
    @BothPendingAndApproved as nvarchar(50)
AS
Select 
    * 
from 
    myTable
Where
    data = 'A'
    and (
            (@BothPendingAndApproved = 'Both' and ApprovalStatus <> 'Rejected')
            OR (@BothPendingAndApproved = 'Specific' and ApprovalStatus like @ApprovalFilter)
        )
    and 
    Data2='B'
    OPTION(RECOMPILE); --consider this option to optimize the query

请注意，为实现ANSI兼容性而对字符文字使用单引号，并使用SQL Server功能（如筛选索引、索引视图等）。

正如注解中所解释的，CASE并不是这样工作的，你可以使用“简单的”AND/OR条件：

Where
   data = "A"
   and 
   (
     (@BothPendingAndApproved  = 'Both' AND ApprovalStatus <> 'Rejected')
     OR 
     (@BothPendingAndApproved = 'Specific' AND ApprovalStatus like @ApprovalFilter)
   )
   and 
   Data2="B"