Adding a WHERE statement refering another table in Dynamic SQL - sql

Question

I currently have the following script which is pivoting results from rows into columns. It works a Great, apart from two issues;
I have a flag in another table which I want to filter by (basically: WHERE Table.Stats=YES). I'd normally do this in a basic query with an inner join followed by that WHERE statement. In the query below, I already have a WHERE FileSeq=25, which works, but this criteria I need to get working is calling on a different table.
The query is returning a lot of uncessary NULL fields.
DECLARE #cols AS NVARCHAR(MAX),
#query AS NVARCHAR(MAX)
SELECT #cols = STUFF((SELECT ',' + QUOTENAME(col+CAST(rn AS varchar(6)))
FROM
(
SELECT row_number() over(partition by UID ORDER BY ClassCode) rn
FROM dbo.StudentClasses
) d
CROSS APPLY
(
SELECT 'ClassCode', 1
) c (col, so)
GROUP BY col, rn, so
ORDER BY rn, so
FOR XML PATH(''), TYPE
).VALUE('.', 'NVARCHAR(MAX)')
,1,1,'')
set #query = 'SELECT UID,' + #cols + '
FROM
(
SELECT UID, col+CAST(rn AS varchar(10)) col, VALUE
FROM
(
SELECT UID, classcode,
row_number() over(partition by UID ORDER BY classcode) rn
FROM StudentClasses WHERE FileSeq=25
) t
CROSS APPLY
(
SELECT ''classcode'', CAST(classcode AS varchar(6))
) c (col, VALUE)
) x
PIVOT
(
MAX(VALUE)
for col in (' + #cols + ')
) p '
EXECUTE(#query)
Any assistance appreciated