Where Used the the condition for select the the particular filed

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SQL-Server

DECLARE @cols AS NVARCHAR(MAX),
    @query  AS NVARCHAR(MAX)

select @cols = STUFF((SELECT ',' + QUOTENAME(subjects) 
                    from mark_entry
                    group by subjects
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)') 
        ,1,1,'')

set @query = 'SELECT * from 
             (
               select student_name,roll_no,standard,section,subjects,cast(mark as int)as mark from mark_entry
            ) x
            pivot 
            (
                sum(mark)
                for subjects in (' + @cols + ')
            ) p '

execute(@query)

This query i have used to display the record of the all the student in school using pivot table
Now I have to display only one student record i do no know where to the WHERE condition
Please Help me