Count previous row only if > 2 days later - sql

Question

In SQL Server 2012 using Studio: I need results displayed count of distinct clientnumbers (CN) for re-entry, grouped by Type like this:
Type CountOfCN
5 1
10 3
Only a RE-entry counts (ENTRY_NO 1 never counts) and it has to be more than 2 days after the end of the previous entry for that clientnumber. So basically ENTRY_NO 1 doesn't count. ENTRY_NO 2 counts if it's startdate is more than 2 days after the enddate of ENTRY_NO 1, and so on with ENTRY_NO 3, 4, 5.
I got ENTRY_NO by doing a ROW_NUMBER function when I created the table. I have no idea how to go about creating a datediff or dateadd function (?) to look at the previous row's enddate and calculate it with my startdate for each CN?
Here is my table:
CN STARTDATE ENDDATE TYPE ENTRY_NO
1 1/1/2018 1/20/2018 10 1
1 1/21/2018 1/30/2018 5 2
1 2/3/2018 NULL 10 3
2 1/1/2018 1/20/2018 10 1
2 1/27/2018 1/30/2018 10 2
3 1/1/2018 1/20/2018 5 1
3 1/27/2018 1/30/2018 10 2
3 2/10/2018 2/20/2018 5 3
4 1/7/2018 1/30/2018 5 1
5 1/27/2018 1/30/2018 5 1
5 1/31/2018 NULL 5 2
So the rows that should be in the results are ENTRY_NO 2 for CN 1, ENTRY_NO 2 for CN 2, ENTRY_NO 2 & 3 for CN 3.
Only the last Entry may/may not have a NULL enddate

Answer 1

Using the LAG window function you can get the previous enddate.
SELECT *
FROM
(
SELECT * ,
LAG(ENDDATE) OVER (PARTITION BY CN ORDER BY STARTDATE) AS prevEndDate
FROM yourtable
) q
WHERE DATEDIFF(d, prevEndDate, STARTDATE) > 2
AND ENDDATE IS NOT NULL

Answer 2

Inner join the table to itself on the conditions you want to enforce:
Can't be Entry_No 1
The Entry_No on one side is one greater than on the other side
Previous Entry must be more than 2 days earlier
Both sides of the join have the same CN
Use that join to create a CTE or derived table, and then SELECT from it, grouping by Type and getting the COUNT(*)

Answer 3

So this ended up being more involved than I first thought, but here it goes...
You can run this example in SSMS.
Create a table variable matching your definition above:
DECLARE #data TABLE ( CN INT, STARTDATE DATETIME, ENDDATE DATETIME, [TYPE] INT, ENTRY_NO INT );
Insert data given:
INSERT INTO #data ( CN, STARTDATE, ENDDATE, [TYPE], ENTRY_NO ) VALUES
( 1, '1/1/2018', '1/20/2018', 10, 1 )
, ( 1, '1/21/2018', '1/30/2018', 5, 2 )
, ( 1, '2/3/2018', NULL, 10, 3 )
, ( 2, '1/1/2018', '1/20/2018', 10, 1 )
, ( 2, '1/27/2018', '1/30/2018', 10, 2 )
, ( 3, '1/1/2018', '1/20/2018', 5, 1 )
, ( 3, '1/27/2018', '1/30/2018', 10, 2 )
, ( 3, '2/10/2018', '2/20/2018', 5, 3 )
, ( 4, '1/7/2018', '1/30/2018', 5, 1 )
, ( 5, '1/27/2018', '1/30/2018', 5, 1 )
, ( 5, '1/31/2018', NULL, 5, 2 );
Confirm inserted data:
+----+-------------------------+-------------------------+------+----------+
| CN | STARTDATE | ENDDATE | TYPE | ENTRY_NO |
+----+-------------------------+-------------------------+------+----------+
| 1 | 2018-01-01 00:00:00.000 | 2018-01-20 00:00:00.000 | 10 | 1 |
| 1 | 2018-01-21 00:00:00.000 | 2018-01-30 00:00:00.000 | 5 | 2 |
| 1 | 2018-02-03 00:00:00.000 | NULL | 10 | 3 |
| 2 | 2018-01-01 00:00:00.000 | 2018-01-20 00:00:00.000 | 10 | 1 |
| 2 | 2018-01-27 00:00:00.000 | 2018-01-30 00:00:00.000 | 10 | 2 |
| 3 | 2018-01-01 00:00:00.000 | 2018-01-20 00:00:00.000 | 5 | 1 |
| 3 | 2018-01-27 00:00:00.000 | 2018-01-30 00:00:00.000 | 10 | 2 |
| 3 | 2018-02-10 00:00:00.000 | 2018-02-20 00:00:00.000 | 5 | 3 |
| 4 | 2018-01-07 00:00:00.000 | 2018-01-30 00:00:00.000 | 5 | 1 |
| 5 | 2018-01-27 00:00:00.000 | 2018-01-30 00:00:00.000 | 5 | 1 |
| 5 | 2018-01-31 00:00:00.000 | NULL | 5 | 2 |
+----+-------------------------+-------------------------+------+----------+
Run SQL to get type count given your business rules:
ENTRY_NO must be greater than 1
Current CN ENDDATE must be greater than 2 days from previous ENDDATE
T-SQL:
SELECT
[TYPE], COUNT( DISTINCT CN ) AS ClientCount
FROM #data
WHERE
CN IN (
SELECT DISTINCT CN FROM (
SELECT
dat.CN
, dat.ENTRY_NO
, dat.[TYPE]
, DATEDIFF( DD
, LAG( ENDDATE, 1, NULL ) OVER ( PARTITION BY CN ORDER BY CN, ENDDATE ) -- gets enddate for previous CN entry
, ENDDATE
) AS DayDiff
FROM #data dat
) AS Clients
WHERE
Clients.ENTRY_NO >= 2
AND Clients.DayDiff > 2
)
GROUP BY
[TYPE]
ORDER BY
[TYPE];
Returns:
+------+-------------+
| TYPE | ClientCount |
+------+-------------+
| 5 | 2 |
| 10 | 3 |
+------+-------------+
A quick look at the IN subquery shows us that CNs 1, 2, and 3 will be included during the "TYPE" count.
SELECT
dat.CN
, dat.ENTRY_NO
, dat.[TYPE]
, DATEDIFF( DD
, LAG( ENDDATE, 1, NULL ) OVER ( PARTITION BY CN ORDER BY CN, ENDDATE ) -- gets enddate for previous CN entry
, ENDDATE
) AS DayDiff
FROM #data dat
ORDER BY
dat.CN, dat.ENTRY_NO;
+----+----------+------+---------+
| CN | ENTRY_NO | TYPE | DayDiff |
+----+----------+------+---------+
| 1 | 1 | 10 | NULL |
| 1 | 2 | 5 | 10 |
| 1 | 3 | 10 | NULL |
| 2 | 1 | 10 | NULL |
| 2 | 2 | 10 | 10 |
| 3 | 1 | 5 | NULL |
| 3 | 2 | 10 | 10 |
| 3 | 3 | 5 | 21 |
| 4 | 1 | 5 | NULL |
| 5 | 1 | 5 | NULL |
| 5 | 2 | 5 | NULL |
+----+----------+------+---------+