Querying Missing rows in TSQL - tsql

Question

We have a table that is populated from information on multiple computers every day. The problem is sometimes it doesn't pull information from certain computers.
So for a rough example, the table columns would read computer_name, information_pulled, qty_pulled, date_pulled.
So Lets say it pulled every day in a week, except the 15th. A query will pull
Computer_name, Information_pulled, qty_pulled, date_pulled
computer1 infopulled 2 2014-06-14
computer2 infopulled 3 2014-06-14
computer3 infopulled 2 2014-06-14
computer1 infopulled 2 2014-06-15
computer3 infopulled 1 2014-06-15
computer1 infopulled 3 2014-06-16
computer2 infopulled 2 2014-06-16
computer3 infopulled 4 2014-06-16
As you can see, nothing pulled in for computer 2 on the 15th. I am looking to write a query that pulls up missing rows for a specific date.
For Example, after running it it says
computer 2 null null 20140615
or anything close to this. We're trying to catch it each morning when this table isn't populated that way we can be proactive and I am not positive I can even query for missing data w/o searching for null values.

Answer 1

You need to have a master list of all your computers somewhere, so that you know when a computer is not accounted for in your table. Say that you have a table called Computer that holds this.
Declare a variable to store the date you want to check:
declare #date date
set #date = '6/15/2014'
Then you can query for missing rows like this:
select c.Computer_name, null, null, #date
from Computer c
where not exists(select 1
from myTable t
where t.Computer_name = c.Computer_name
and t.date_pulled = #date)
SQL Fiddle
If you are certain that every computer_name already exists in your table at least once, you could skip creating a separate Computer table, and modify the query like this:
select c.Computer_name, null, null, #date
from (select distinct Computer_name from myTable) c
where not exists(select 1
from myTable t
where t.Computer_name = c.Computer_name
and t.date_pulled = #date)
This query isn't as robust because it will not show computers that do not already have a row in your table (e.g. a new computer, or a problematic computer that has never had its information pulled).

Answer 2

I think a cross-join will answer your problem.
In the query below, every computer will have to have successfully uploaded at least once and at least one every day.
This way you'll get every missing computer/date couple.
select
Compare.*
from Table_1 T1
right join (
select *
from
(select Computer_name from Table_1 group by Computer_name) CPUS,
(select date_pulled from Table_1 group by date_pulled) DAYs
) Compare
on T1.Computer_name=Compare.Computer_name
and T1.date_pulled=Compare.date_pulled
where T1.Computer_name is null
Hope this help.

Answer 3

If you join the table to itself by date and computer_name like the following, you should get a list of missing dates
SELECT t1.computer_name, null as information_pulled, null as qty_pulled,
DATEADD(day,1,t1.date_pulled) as missing_date
FROM computer_info t1
LEFT JOIN computer_info t2 ON t2.date_pulled = DATEADD(day,1,t1.date_pulled)
AND t2.computer_name = t1.computer_name
WHERE t1.date_pulled >= '2014-06-14'
AND t2.date_pulled IS NULL
This will also get the next date that hasn't been pulled yet, but that should be clear and you could add an additional condition to filter it out.
AND DATEADD(day,1,t1.date_pulled) < '2014-06-17'
Of course, this only works if you know each of the computer names already exist in the table for previous days. If not, #Jerrad's suggestion to create a separate computer table would help.
EDIT: if the gap is larger than a single day, you may want to see that
SELECT t1.computer_name, null as info, null as qty_pulled,
DATEADD(day,1,t1.date_pulled) as missing_date,
t3.date_pulled AS next_pulled_date
FROM computer_info t1
LEFT JOIN computer_info t2 ON t2.date_pulled = DATEADD(day,1,t1.date_pulled)
AND t2.computer_name = t1.computer_name
LEFT JOIN computer_info t3 ON t3.date_pulled > t1.date_pulled
AND t3.computer_name = t1.computer_name
LEFT JOIN computer_info t4 ON t4.date_pulled > t1.date_pulled
AND t4.date_pulled < t3.date_pulled
AND t4.computer_name = t1.computer_name
WHERE t1.date_pulled >= '2014-06-14'
AND t2.date_pulled IS NULL
AND t4.date_pulled IS NULL
AND DATEADD(day,1,t1.date_pulled) < '2014-06-17'
The 't3' join will join all dates over the first missing one and the 't4' join along with t4.pulled_date IS NULL will exclude all but the lowest of those dates.
You could do this with subqueries as well, but excluding joins have served me well in the past.