(SQL) Assembling multiple tables of deltas into a single populated view? - sql

Question

Lets say I have 3 different tables. Foo, Bar, and Baz. Each tables has the same structure; a timestamp and a data value. We can also assume that each table is synchronized at the top row.
Foo Bar Baz
________________ ________________ _________________
|Time |Value| |Time |Value| |Time |Value |
|1:00 |0 | |1:00 |10 | |1:00 |100 |
|1:15 |1 | |1:10 |11 | |1:20 |101 |
|1:30 |2 | |1:40 |12 | |1:50 |102 |
|1:45 |3 | |1:50 |13 | |1:55 |103 |
Is there a simple way to to assemble these records into a single view where the value of each column is assumed to be the last known value for each populates the unprovided times?
________________________________________
|Time |Foo.Value|Bar.Value|Baz.Value|
|1:00 | 1| 10| 100|
|1:10 | 1| 11| 100|
|1:15 | 2| 11| 100|
|1:20 | 2| 11| 101|
|1:30 | 3| 11| 101|
|1:40 | 3| 12| 101|
|1:45 | 4| 12| 101|
|1:50 | 4| 13| 102|
|1:55 | 4| 13| 103|
Edit:
What if I wanted to select a time range, but wished to have the last known value of each column brought forward? Is there a simple way to do so without producing the entire table then filtering it down?
e.g. if I wanted records from 1:17 to 1:48, I would want the following...
________________________________________
|Time |Foo.Value|Bar.Value|Baz.Value|
|1:20 | 2| 11| 101|
|1:30 | 3| 11| 101|
|1:40 | 3| 12| 101|
|1:45 | 4| 12| 101|

Answer 1

SQL Server 2008 doesn't support lag(), much less lag() with ignore nulls. So, I think the easiest way may be with correlated subqueries. Get all the times from the three tables and then populate the values:
select fbb.time,
(select top 1 value from foo t where t.time <= fbb.time order by t.time desc
) as foo,
(select top 1 value from bar t where t.time <= fbb.time order by t.time desc
) as bar,
(select top 1 value from baz t where t.time <= fbb.time order by t.time desc
) as baz
from (select time from foo union
select time from bar union
select time from baz
) fbb;
EDIT:
An alternative approach uses aggregation:
select time, max(foo) as foo, max(bar) as bar, max(baz) as baz
from (select time, value as foo, NULL as bar, NULL as baz from foo union all
select time, NULL, value, NULL from bar union all
select time, NULL, NULL baz from baz
) fbb
group by time
order by time;
This probably has better performance than the first method.

Answer 2

Here is an another alternative solution as you are using SQL SERVER 2008:
SELECT *
FROM (
SELECT t, [time], value
FROM ( SELECT 'Foo' as t, *
FROM #Foo
UNION
SELECT 'Bar' as t, *
FROM #Bar
UNION
SELECT 'Baz' as t, *
FROM #Baz
) un
WHERE [time] BETWEEN '1:17' AND '1:48'
) AS fbb
PIVOT (MAX(value) FOR fbb.[t] IN (Foo, Bar, Baz)) pvt