Create a Timeline Query with date periods in SQL ACCESS - sql

Question

I would like to write a Query in SQL Access (no choice with the software...) in order to create a timeline from a Table of records.
Hereunder a simple example in order to explain what I want to do.
I have a table with 2 records. Each record is defined by its ID, a Value, a date of begin, a date of end and a Level of confidence.
Id | vValue | dtBegin | dtEnd | lLevel
-------------------------------------------
1 | a |20/06/2016|28/06/2016| Low
2 | b |23/06/2016|25/06/2016| High
The query should return a timeline with the highest level of information (and its value) available for each period of time. In the example the result of the query should be:
vValue| dtBegin | dtEnd |lLevel
------------------------------------
a |20/06/2016|23/06/2016| Low
b |23/06/2016|25/06/2016| High
a |25/06/2016|28/06/2016| Low
From the 20/06/2016 to the 23/06/2016 the highest level of confidence available in the table is 'Low' and the associated value is 'a'/
From the 23/06 to the 25/06, the period is covered by two records but the highest level of confidence available is 'High', therefore the value is 'b'
Thanks a lot for your help
Thibaud

Answer 1

There may be an easier way of doing it, you could start with a base query to get all dates necessary:
SELECT dtBegin
FROM Table1
GROUP BY dtBegin
UNION
SELECT dtEnd AS dtBegin
FROM Table1
With this query you can get the non-overlapping date ranges:
SELECT Timeline.dtBegin, Min(Timeline_1.dtBegin) AS dtEnd
FROM Query1 AS Timeline, Query1 AS Timeline_1
WHERE (((Timeline_1.dtBegin) > Timeline.dtBegin))
GROUP BY Timeline.dtBegin
From this you could cross join back to your original table, and get the max level and vValue for any table entry that overlaps the date range using a subquery:
SELECT Query2.dtBegin, Query2.dtEnd, Table1.lLevel, Table1.vValue
FROM Query2, Table1
WHERE (((Table1.lLevel) =
(SELECT MAX(lLevel)
FROM Table1 T
WHERE T.dtBegin <=Query2.dtBegin AND
T.dtEnd >=Query2.dtEnd))) AND
((Table1.dtBegin)<=Query2.dtBegin) AND
((Table1.dtEnd)>=Query2.dtEnd)
You'll have to do some extra logic to get a max level, of course, depending on how your choices are configured.

Answer 2

It's hard to help without seeing the data structure. If the begin / end dates don't overlap in your data this could possibly work:
/* Finds distinct dates which only have a low record */
SELECT
vValue,dtBegin,dtEnd,'Low' AS lLevel
FROM
(
SELECT DISTINCT vValue,dtBegin,dtEnd
FROM tbl
WHERE lLevel = "Low"
EXCEPT
SELECT DISTINCT vValue,dtBegin,dtEnd
FROM tbl
WHERE lLevel = "High"
) AS X
/* Appends distinct date sets */
UNION
SELECT DISTINCT vValue,dtBegin,dtEnd,lLevel
FROM tbl
WHERE lLevel = "High"