Complex recursive SQL to produce hierarchical data

Question

I am trying to evaluate the impact of store visitors on the spread of COVID-19.

Here is a simple scenario:

1. VisitorA walks into store and meets Employee1 @ Time = 0.
2. VisitorA then meets Employee2 @ Time = 1.
3. VisitorB walks into store and meets Employee1 @ Time = 1.
4. VisitorB then meets Employee3 @ Time = 2.
5. VisitorA leaves store.

When I collect all visitor data and who they met over time the data set looks something like this:

Table visitorByEmployee:

| VisitorID | EmployeeID | Contact           |
+-----------+------------+-------------------+
| 100       |   X123     | 3/11/2020 1:00    |
| 100       |   X124     | 3/11/2020 1:10    |
| 101       |   X123     | 3/12/2020 1:11    |
| 101       |   X125     | 3/11/2020 1:20    |
| 102       |   X126     | 3/12/2020 10:00   |
| 102       |   X124     | 3/12/2020 10:00   |
| 103       |   X123     | 3/12/2020 11:00   |
| 104       |   X124     | 3/12/2020 12:00   |
| 104       |   X126     | 3/12/2020 12:00   |
| 105       |   X126     | 3/12/2020 12:00   |

I want to build a hierarchy off of this data that can eventually be expressed as follows:

Each Tree represents the impact of the Visitors on the spread of the virus:

100
  --> X123
    --> 101
      --> X125
    --> 103
  --> X124
    --> 104

102
  --> X126
    --> 104
    --> 105
  --> X124
    --> 104
      --> X126

I attempted to do this by first finding the root nodes (root visitors who were no impacted by previous visitors and/or employees they saw). These were 100 and 102.

SELECT 
    *,
    ROW_NUMBER() OVER (PARTITION BY EmployeeID ORDER BY Contact) AS SeenOrder
INTO 
    #SeenOrder
FROM
    visitorByEmployee

SELECT *
INTO #RootVisitors
FROM #SeenOrder
WHERE SeenOrder = 1

From #RootVisitors and #SeenOrder, I want to build a table that can tell me that hierarchy of impact and maybe result in something like this:

| InitVisitorID | HLevel     | EmployeeID        |   VisitorID |
+---------------+------------+-------------------+-------------+
| 100           |   0        |  X123             |     100     |
| 100           |   0        |  X124             |     100     |
| 100           |   1        |  X123             |     101     |
| 100           |   1        |  X123             |     103     |
| 100           |   1        |  X124             |     104     |
| 100           |   2        |  X125             |     101     |
| 102           |   0        |  X126             |     102     |
| 102           |   0        |  X124             |     102     |
| 102           |   1        |  X126             |     104     |
| 102           |   1        |  X126             |     105     |
| 102           |   1        |  X124             |     104     |
| 102           |   2        |  X126             |     104     |

Is this something that can be done using a recursive CTEs? I attempted to do this but due to the shifting hierarchy from visitor to employee to visitor to employee, I am having a hard time creating that recursive CTE.

UPDATE Here is the recursive CTE I am working on. It doesn't work yet but the approach is what I am sharing:

; WITH exposure_tree AS (
/* == Anchor with the root visitors == */
/* == You can think of this: The Employees who were exposed by the Visior == */
SELECT re.VisitorID InitVisitor,
    1 as Level, 
    CASE WHEN 1%2=1 THEN 'Visitor' ELSE 'Employee' END ExposerType,
    re.VisitorID Exposer,
    re.EmployeeID Exposee,
    re.SeenOrder,
    re.InitialContact
FROM #SeenOrder re
WHERE re.SeenOrder = 1

/* == Recursive Part #1 ==
Get the visitors who were exposed next by the exposed employees
*/
UNION ALL

SELECT et.VisitorID InitVisitor,
    Level + 1,
    CASE WHEN (Level+1)%2=1 THEN 'Visitor' ELSE 'Employee' END ExposerType,
    re.EmployeeID,
    re.VisitorID, -- These are switched from the anchor.
    re.SeenOrder,
    re.InitialContact
FROM #SeenOrder re
JOIN exposure_tree et ON et.Exposee = re.EmployeeID AND re.SeenOrder > 1 AND re.InitialContact > et.InitialContact

UNION ALL

/* == Recursive Part #2 ==
Get the next employees who were exposed the second level exposed visitors
*/
SELECT et.VisitorID InitVisitor,
    Level + 2,
    CASE WHEN (Level+2)%2=1 THEN 'Visitor' ELSE 'Employee' END ExposerType,
    re.VisitorID,
    re.EmployeeID,
    re.SeenOrder,
    re.InitialContact
FROM #ROOT_EXPOSURES re
JOIN exposure_tree et ON re.VisitorID = et.Exposer and re.SeenOrder > 1 AND re.InitialContact > et.InitialContact
)
select top 1000 * from exposure_tree ORDER BY InitVisitor, Level

Answer 1

You can still write a recursive CTE using those tables. The coding gets tricky, though.

Here's the CTE. You probably will need to tweak it to get exactly what you want. I changed the column names fo simplicity:

with
c as (
  select 'v' as type, vid as id, contact, 0 as lvl, cast(concat('/', vid, '/') as varchar(255)) as path
  from (select *, row_number() over(partition by vid order by contact) as rn from v) x where rn = 1
 union all
  select
   case when type = 'v' then 'e' else 'v' end, -- type
   case when type = 'v' then v.eid else v.vid end, -- id
   v.contact,
   c.lvl + 1,
   cast(concat(c.path, case when type = 'v' then v.eid else v.vid end, '/') as varchar(255))
  from c 
  join v on c.lvl <= 10 and v.contact >= c.contact and (c.type = 'v' and v.vid = c.id or c.type = 'e' and v.eid = c.id)
        and c.path not like concat('%', case when type = 'v' then v.eid else v.vid end, '%')
)
select * from c order by path

Result:

type  id    contact                lvl  path                   
----  ----  ---------------------  ---  -----------------------
v     100   2020-03-11 01:00:00.0    0  /100/                  
e     X123  2020-03-11 01:00:00.0    1  /100/X123/             
v     101   2020-03-12 01:11:00.0    2  /100/X123/101/         
v     103   2020-03-12 11:00:00.0    2  /100/X123/103/         
e     X124  2020-03-11 01:10:00.0    1  /100/X124/             
v     102   2020-03-12 10:00:00.0    2  /100/X124/102/         
e     X126  2020-03-12 10:00:00.0    3  /100/X124/102/X126/    
v     104   2020-03-12 12:00:00.0    4  /100/X124/102/X126/104/
v     105   2020-03-12 12:00:00.0    4  /100/X124/102/X126/105/
v     104   2020-03-12 12:00:00.0    2  /100/X124/104/         
e     X126  2020-03-12 12:00:00.0    3  /100/X124/104/X126/    
v     105   2020-03-12 12:00:00.0    4  /100/X124/104/X126/105/
v     101   2020-03-11 01:20:00.0    0  /101/                  
e     X123  2020-03-12 01:11:00.0    1  /101/X123/             
v     103   2020-03-12 11:00:00.0    2  /101/X123/103/         
e     X125  2020-03-11 01:20:00.0    1  /101/X125/             
v     102   2020-03-12 10:00:00.0    0  /102/                  
e     X124  2020-03-12 10:00:00.0    1  /102/X124/             
v     104   2020-03-12 12:00:00.0    2  /102/X124/104/         
e     X126  2020-03-12 12:00:00.0    3  /102/X124/104/X126/    
v     105   2020-03-12 12:00:00.0    4  /102/X124/104/X126/105/
e     X126  2020-03-12 10:00:00.0    1  /102/X126/             
v     104   2020-03-12 12:00:00.0    2  /102/X126/104/         
e     X124  2020-03-12 12:00:00.0    3  /102/X126/104/X124/    
v     105   2020-03-12 12:00:00.0    2  /102/X126/105/         
v     103   2020-03-12 11:00:00.0    0  /103/                  
e     X123  2020-03-12 11:00:00.0    1  /103/X123/             
v     104   2020-03-12 12:00:00.0    0  /104/                  
e     X124  2020-03-12 12:00:00.0    1  /104/X124/             
e     X126  2020-03-12 12:00:00.0    1  /104/X126/             
v     105   2020-03-12 12:00:00.0    2  /104/X126/105/         
v     105   2020-03-12 12:00:00.0    0  /105/                  
e     X126  2020-03-12 12:00:00.0    1  /105/X126/             
v     104   2020-03-12 12:00:00.0    2  /105/X126/104/         
e     X124  2020-03-12 12:00:00.0    3  /105/X126/104/X124/    

For reference, here's the data script I used to test, if you need to create a SQL Fiddle to run it:

create table v (
  vid varchar(6),
  eid varchar(6),
  contact datetime
);

insert into v (vid, eid, contact) values ('100', 'X123', '2020-03-11 01:00:00');
insert into v (vid, eid, contact) values ('100', 'X124', '2020-03-11 01:10:00');
insert into v (vid, eid, contact) values ('101', 'X123', '2020-03-12 01:11:00');
insert into v (vid, eid, contact) values ('101', 'X125', '2020-03-11 01:20:00');
insert into v (vid, eid, contact) values ('102', 'X126', '2020-03-12 10:00:00');
insert into v (vid, eid, contact) values ('102', 'X124', '2020-03-12 10:00:00');
insert into v (vid, eid, contact) values ('103', 'X123', '2020-03-12 11:00:00');
insert into v (vid, eid, contact) values ('104', 'X124', '2020-03-12 12:00:00');
insert into v (vid, eid, contact) values ('104', 'X126', '2020-03-12 12:00:00');
insert into v (vid, eid, contact) values ('105', 'X126', '2020-03-12 12:00:00');