SQL Server: Comparing against next X in group

Question

I know I can compare previous/next rows with LAG and LEAD, but in the comparison that I am doing; I need to be able to skip rows occasionally (but still need to acknowledge them)

Specifics: I am trying to track consults, and warm and cold transfers in a call environment. Each call has a unique key, and an ascending employee number for each stage. It also has the start and end times for each segment of the call.
A consult is defined as a call that starts and finishes before the previous non consult segment ends.
A Warm transfer is one where the end of the previous segment is after the start of the previous segment, and ends after the end of the previous segment.
A Cold transfer is a segment that starts after the end of the previous segment.

Example Data:

topcallid   e_created       e_terminated    empnum
EG995GIFM   16:22:40.933    16:29:51.010    1
EG995GIFM   16:25:59.827    16:27:49.027    2
EG995GIFM   16:30:07.453    16:37:44.500    3
EG995GIFM   16:38:01.677    16:59:30.777    4
EG995GIFM   16:59:46.737    17:16:48.397    5
EG995GIFM   17:04:51.243    17:29:21.620    6

Desired output:

topcallid.......TransStatus  
EG995GIFM   Consult (Stage 1 To 2)  
EG995GIFM   Cold Transfer (Stage 1 To 3)  
EG995GIFM   Cold Transfer (Stage 3 To 4)  
EG995GIFM   Cold Transfer (Stage 4 To 5)  
EG995GIFM   Warm Transfer (Stage 5 To 6)  

Current Code, which gives every combination comparing every stage to all that follow:

SELECT ta.topcallid,
       CASE 
            WHEN ta.e_created < Trans.e_created AND ta.e_terminated > Trans.e_terminated THEN 'Consult (Stage ' + CAST(TA.EmpNum AS VARCHAR(3)) + ' To ' + CAST(Trans.EmpNum AS VARCHAR(3)) + ')'
            WHEN ta.e_terminated < Trans.e_created THEN 'Cold Transfer (Stage ' + CAST(TA.EmpNum AS VARCHAR(3)) + ' To ' + CAST(Trans.EmpNum AS VARCHAR(3)) + ')'
            WHEN ta.e_terminated > Trans.e_created THEN 'Warm Transfer (Stage ' + CAST(TA.EmpNum AS VARCHAR(3)) + ' To ' + CAST(Trans.EmpNum AS VARCHAR(3)) + ')'
       END TransStatus
FROM   [TransferTypeAnalysis] TA
       JOIN [TransferTypeAnalysis] Trans
            ON  TA.topcallid = Trans.topcallid
                AND ta.empnum < Trans.empnum
ORDER BY
       TA.topcallid,
       ta.empnum

Answer 1

It appears you need a subquery to compare to the previous terminated call. This logic assumes the terminated times are unique per call id.

select 
    t3.topcallid, 
    case when t3.e_created < t4.e_created and t3.e_terminated > t4.e_terminated then 'Consult (Stage ' +  cast(t3.EmpNum as varchar(3)) + ' To ' + cast(t4.EmpNum as varchar(3)) + ')'
        when t3.e_terminated < t4.e_created then 'Cold Transfer (Stage ' +  cast(t3.EmpNum as varchar(3)) + ' To ' +  cast(t4.EmpNum as varchar(3)) + ')'
        when t3.e_terminated > t4.e_created then 'Warm Transfer (Stage ' +  cast(t3.EmpNum as varchar(3)) + ' To ' +  cast(t4.EmpNum as varchar(3)) + ')'
        end TransStatus
from
    [TransferTypeAnalysis] t3
    INNER JOIN
    (
        select 
            t1.topcallid, t1.empnum, t1.e_created, t1.e_terminated, max(t2.e_terminated) previous_term_dt
        FROM 
            [TransferTypeAnalysis] t1
            left join [TransferTypeAnalysis] t2
            on t1.topcallid=t2.topcallid and t1.empnum>t2.empnum
        GROUP BY t1.topcallid, t1.empnum, t1.e_created, t1.e_terminated
    ) as t4 on t3.topcallid = t4.topcallid and t3.e_terminated = t4.previous_term_dt