How to add a flag column based on column values and COUNT() of records partitioned by customer in SQL Server - sql

Question

My SQL Server table looks like this:
DayNo. Customer AgentsInvolved CallID DesiredFlag
-------------------------------------------------------
0 AAA 1 1858 0
0 AAA 3 1859 0
2 AAA 1 1860 0
0 BBB 2 1862 0
0 CCC 1 1863 1
0 DDD 3 1864 0
9 DDD 1 1865 0
9 DDD 4 1866 0
How do I add a new column to say;
WHEN (DayNo. = 0, and AgentsInvolved = 1) AND COUNT(callID) = 1 (grouped by customer)
THEN 1
ELSE 0
The column values I understand can be wrapped into a case statement but not sure how to add the condition for the COUNT(CallID). See the DesiredFlag column to see the result I'm trying to achieve.
Any help would be great.

Answer 1

I would use NOT EXISTS :
SELECT t.*,
(CASE WHEN NOT EXISTS (SELECT 1
FROM table t1
WHERE t1.Customer = t.Customer AND t1.CallID <> t.CallID
) AND t.DayNo = 0 AND t.AgentsInvolved = 1
THEN 1
ELSE 0
END) AS DesiredFlag
FROM table t;

Answer 2

with SAMPLE DATA:
declare #T Table (DayNo int,Customer varchar(10),AgentsInvolved int,CallID int)
insert into #T values
(0,'AAA',1,1858),
(0,'AAA',3,1859),
(2,'AAA',1,1860),
(0,'BBB',2,1862),
(0,'CCC',1,1863),
(0,'DDD',3,1864),
(9,'DDD',1,1865),
(9,'DDD',4,1866)
you can get your result (as i understand correctly) by:
;with summation as
(
select Customer, count(1) as qty
from #T
group by Customer
)
select
T.*,
coalesce(F.flag, 0) as flag
from
#T T
inner join summation s on s.Customer = T.Customer
outer apply (select 1 as flag where DayNo = 0 and AgentsInvolved = 1 and qty = 1) F
RESULTS:
DayNo Customer AgentsInvolved CallID flag
----------- ---------- -------------- ----------- -----------
0 AAA 1 1858 0
0 AAA 3 1859 0
2 AAA 1 1860 0
0 BBB 2 1862 0
0 CCC 1 1863 1
0 DDD 3 1864 0
9 DDD 1 1865 0
9 DDD 4 1866 0