Sql-server – SQL server DB status runnable, sleeping, suspended, running and background meanings

What you're seeing is the threads with CXPACKET waits are actually done with whatever work they had to do and are now waiting for the other active threads (the ones with NULL as the wait type) to complete.

Brent O had a great analogy using a classroom. The teacher hands out a bunch of different stacks of paper to the class and has them find a word on them. What we need to take into consideration is the fact that 1) the stacks of paper could be different sizes 2) different students read faster/slower than others 3) one student may find the word 1 time on the first page, while the next could find it 400 times across 3000 pages.

You're seeing a natural and intended behavior when you deal with parralelism, some threads finish faster than others and are forced to wait until the other threads complete, regather all the threads and give you an output.

http://www.brentozar.com/archive/2013/08/what-is-the-cxpacket-wait-type-and-how-do-you-reduce-it/

This is what I use to monitor sleeping SPIDs causing blocking issues:

SELECT 
     s.session_id
    ,s.status
    ,s.login_time
    ,s.host_name
    ,s.program_name
    ,s.host_process_id
    ,s.original_login_name
    ,s.last_request_end_time
    ,CAST(t.text AS nvarchar(4000)) AS [text]
FROM sys.dm_exec_sessions AS s
INNER JOIN sys.dm_exec_connections AS c
    ON s.session_id = c.session_id
CROSS APPLY (
    SELECT MAX(DB_NAME(dt.database_id)) AS database_name
    FROM sys.dm_tran_session_transactions AS st
    INNER JOIN sys.dm_tran_database_transactions AS dt
        ON st.transaction_id = dt.transaction_id
    WHERE is_user_transaction = 1
    GROUP BY st.session_id
    HAVING s.session_id = st.session_id 
) AS trans
CROSS APPLY sys.dm_exec_sql_text(most_recent_sql_handle) AS t
WHERE s.session_id NOT IN (
        SELECT session_id
        FROM sys.dm_exec_requests
    )
    AND s.session_id IN (
        SELECT request_session_id
        FROM sys.dm_tran_locks
        WHERE request_status = 'GRANT'
    )
    AND STATUS = 'sleeping'
    AND is_user_process = 1;

Hope this helps.

To answer your question specifically, sys.dm_tran_database_transactions holds the information you're looking for.