TSQL Distinct Counts - sql

Question

I have a table that looks like this:
ID SuppressionTypeID PersonID
------------------------------
1 1 123
2 1 456
3 2 456
I want to get a rolling count (distinct people) rather than a normal group by count.
e.g. not this:
SuppressionTypeID Count
---------------------------
1 2
2 1
This:
SuppressionTypeID RecordsLost
----------------------------------
1 2
2 0
The latter being zero as we lost person 456 on suppresiontypeid 1.
Thanks in advance.

Answer 1

You may need to use a temporary table or a table variable as shown below
DECLARE #t TABLE (
ID INT
,SuppressionTypeID INT
,PersonID INT
)
INSERT INTO #t
SELECT 1
,1
,123
UNION ALL
SELECT 2
,1
,456
UNION ALL
SELECT 3
,2
,456
DECLARE #t1 TABLE (
ID INT
,SuppressionTypeID INT
,PersonID INT
,firstid INT
)
INSERT INTO #t1
SELECT *
,NULL
FROM #t
UPDATE t1
SET t1.firstid = t2.firstid
FROM #t1 AS t1
INNER JOIN (
SELECT personid
,min(SuppressionTypeID) AS firstid
FROM #t1
GROUP BY personid
) AS t2 ON t1.PersonID = t2.PersonID
SELECT coalesce(t2.firstid, t1.SuppressionTypeID) AS SuppressionTypeID
,count(DISTINCT t2.personid) AS count
FROM #t1 AS t1
LEFT JOIN #t1 AS t2 ON t1.personid = t2.personid
AND t1.SuppressionTypeID = t2.firstid
GROUP BY coalesce(t2.firstid, t1.SuppressionTypeID)
The result is
SuppressionTypeID count
----------------- -----------
1 2
2 0

Answer 2

You can try;
with tmp_tbl as (
select
x.SuppressionTypeID, count(x.PersonID) as RecordsLost
from (
select
min(SuppressionTypeID) as SuppressionTypeID,
PersonID
from tbl
group by PersonID
) as x
group by x.PersonID
order by x.SuppressionTypeID
)
select
distict t.SuppressionTypeID, coalesce(tmp.RecordsLost, 0) as RecordsLost
from tbl t
left join tmp_tbl tmp on tmp.SuppressionTypeID = t.SuppressionTypeID