I was wondering if anyone can suggest a suitable design pattern for achieving the following:
I have a payslip, each slip shows my previous pay and my current pay. Each payslip should not need to duplicate fields, but rather point the current value to the previous value in the next slip.
On top of all this, I also need to be able to retrieve any given payslip at any point in time (preferably O(1)).
Here's a visual to help understand my problem.
[key:"1"] [key:"2"] [key:"3"]
+------+ +------+ +------+
| | | | | |
| Curr | <--- | Prev | | Curr |
| Null | | Curr | <--- | Prev |
| | | | | |
+------+ +------+ +------+
Any help would be greatly appreciated
Give your PaySlip class a PreviousPaySlipKey property, which will be null for the very first pay slip. In your database, this should be a foreign key to the PaySlip's Key property.
This way, if you have a PaySlip, you can find the key of the previous pay slip, and query the database for the payslip with that key.
Disclaimer: I am not a VB programmer, and the syntax below may not apply. Assuming VB has a Map interface, you could use that for your lookups O(1)
Map<Integer, CompositeValue> = some hash based map
and declare a class (assuming VB lets you create classes) CompositeValue as such:
class CompositeValue {
Integer previousKey;
Value value;
}
Now, once you've retrieved a CompositeValue from the Map, you have the means to get it's real value (value), and to retrieve the previous value using previousKey.
Just a thought.
If we are talking DBs here, every payslip should be given a sequential ID (also called auto-increment in some RDBMS). This will be the primary key (so it's automatically indexed). Assuming you retrieved the payslip you wanted, here is how to get previous one from DB:
SELECT TOP 1 * FROM PaySlip WHERE PaySlipID < #PaySlipID ORDER BY PaySlipID Desc
And the next one:
SELECT TOP 1 * FROM PaySlip WHERE PaySlipID > #PaySlipID ORDER BY PaySlipID
You will probably have payslips for multiple employees stored in a single table, so just add to the WHERE condition.
It is counter productive to store references to previous/next payslip on each payslip.
Related
I apologise if this has been asked before. I'm still not certain how to phrase my question for the title, so wasn't sure what to search for.
I have a hundred or so databases in the same instance, one for each of my customers, named for the customer, and they all have the same structure. I want to select a single result set that includes the database name along with the most recent date entry in one of the tables. I can pull the database names from sys.databases, but then for each database I want to select the most recent date from Events.Date_Logged so that my result set looks something like this:
_______________________________
| | |
|Cust_Name |Latest_Event |
|_______________|_______________|
| | |
|Customer1 |01/02/2020 |
|_______________|_______________|
| | |
|Customer2 |02/02/2020 |
|_______________|_______________|
| | |
|Customer3 |03/02/2020 |
|_______________|_______________|
I'm really struggling with the syntax though. I either get just a single row returned or every single event for each customer. I think my joins are as rusty as hell.
Any help would be appreciated.
What I suggest you do:
Declare a result variable (of type table)
Use a cursor to go over every database
Inside the cursor: do a select top 1 ... order by date desc to get the most recent record. Save this result in the result variable.
After the cursor print the result variable.
That should do the trick.
I am having some trouble finding an answer for this one, so I apologize if it was somewhere else.
I have a table 'dbo.MileageImport' that has the following layout which I pulled to find duplicate entries:
|KEY | DATA |
---------------------
|V9864653 | 180288 |
|V9864653 | 22189 |
|V9864811 | 11464 |
|V9864811 | 12688 |
What I am having troubles with is when I run the following SQL in a DB2 environment:
SELECT KEY, MIN(DATA)
FROM dbo.MileageImport
GROUP BY KEY
HAVING (COUNT(KEY)>1);
It ends up pulling the following data:
|KEY | DATA |
---------------------
|V9864811 | 11464 |
|V9864653 | 180288 |
For some reason it's pulling the MIN value for V9864811, but not V9864653. If I inverse that and put MAX instead of MIN, it pulls the opposite values.
Is there something I am missing here so I can pull the MIN DATA value for only duplicate KEY records, or is there another way to do this? The report where this data comes from changes from month to month, so there could be different keys that end up being duplicated that I need to correct. Ultimately I am turning this into a DELETE statement to delete the lower of the two (or more) duplicated mileage entries.
Is your DATA column numerical? or a VARCHAR?
If you find its better to change it to a number if you can, maybe an integer if you aren't having any fractions and its just round numbers.
if not, then you could cast them to an integer value, but if there are lots of transactions or its a big table it will be slow and not ideal. Its bad practise to do that if you could just change the datatype!
SELECT KEY, MIN(CAST(DATA as Int))
FROM dbo.MileageImport
GROUP BY KEY
HAVING (COUNT(KEY)>1)
it is possible to store a function IN the table to automatically sum a group of columns and store the result in a final column?
ie:
+----+------------+-----------+-------------+------------+
| id | appleCount | pearCount | bananaCount | totalFruit |
+----+------------+-----------+-------------+------------+
| 1 | 300 | 60 | 120 | 480 |
+----+------------+-----------+-------------+------------+
where the column totalFruit is automatically calculated from the previous three columns and updated as the other columns update. in this specific application, there is ONLY going to be the one row. it would be spanky-handy to be able to just push the updated counts and then pull the calculated total out. i seem to recall reading about this ability somewhere, but for the life of me, i can't recall where... :poop:
if there is not way to do this, that's cool. but if there is... :smile:
TIA!
WR!
Yes, it is possible. But is it worth it? It is simple enough to do
SELECT ...
appleCount + pearCount + bananaCount AS totalFruit
...
See MariaDB Generated Columns for how to generate the extra column -- either as a real extra column or "virtual". What version of MariaDB?--There are a number of changes over time.
(MySQL users: 5.7.6 has a similar MySQL Generated Columns.)
While trying to build a data warehousing application using Talend, we are faced with the following scenario.
We have two tables tables that look like
Table master
ID | CUST_NAME | CUST_EMAIL
------------------------------------
1 | FOO | FOO_BAR#EXAMPLE.COM
Events Table
ID | CUST_ID | EVENT_NAME | EVENT_DATE
---------------------------------------
1 | 1 | ACC_APPLIED | 2014-01-01
2 | 1 | ACC_OPENED | 2014-01-02
3 | 1 | ACC_CLOSED | 2014-01-02
There is a one-to-many relationship between master and the events table.Since, given a limited number of event names I proposing that we denormalize this structure into something that looks like
ID | CUST_NAME | CUST_EMAIL | ACC_APP_DATE_ID | ACC_OPEN_DATE_ID |ACC_CLOSE_DATE_ID
-----------------------------------------------------------------------------------------
1 | FOO | FOO_BAR#EXAMPLE.COM | 20140101 | 20140102 | 20140103
THE DATE_ID columns refer to entries inside the time dimension table.
First question : Is this a good idea ? What are the other alternatives to this scheme ?
Second question : How do I implement this using Talend Open Studio ? I figured out a way in which I moved the data for each event name into it's own temporary table along with cust_id using the tMap component and later linked them together using another tMap. Is there another way to do this in talend ?
To do this in Talend you'll need to first sort your data so that it is reliably in the order of applied, opened and closed for each account and then denormalize it to a single row with a single delimited field for the dates using the tDenormalizeRows component.
After this you'll want to use tExtractDelimitedFields to split the single dates field.
Yeah, this is a good idea, this is called a cumulative snapshot fact. http://www.kimballgroup.com/2012/05/design-tip-145-time-stamping-accumulating-snapshot-fact-tables/
Not sure how to do this in Talend (dont know the tool) but it would be quite easy to implement in SQL using a Case or Pivot statement
Regarding only your first question, it's certainly a good idea -- unless there is any possibility of the same persons applying-opening-closing their account more than once AND you want to keep all this information in their history (so UPDATE wouldn't help).
Snowflaking is definitely not a good option if you are going to design a data warehouse. So, denormalizing will certainly be a good choice in this case. Following article almost fits perfectly to clear the air over such scenarios,
http://www.kimballgroup.com/2008/09/design-tip-105-snowflakes-outriggers-and-bridges/
I'm aware of IDENTITY fields but I have a feeling that I couldn't use one to solve my problem.
Let's say I have multiple clients. Each client has multiple orders. Each client needs to have their orders numbered sequentially, specific to them.
Example table structure:
Orders:
OrderID | ClientID | ClientOrderID | etc...
Some example rows for this table would be:
OrderID | ClientID | ClientOrderID | etc...
1 | 1 | 1 | ...
2 | 1 | 2 | ...
3 | 2 | 1 | ...
4 | 3 | 1 | ...
5 | 1 | 3 | ...
6 | 2 | 2 | ...
I know the naive way would be to take the MAX ClientOrderID for any client and use that value for INSERTs but that would be subject to concurrency issues. I was considering using a transaction but I'm not quite sure what the broadest isolation scope that can be used for this. I'll be using LINQ to SQL but I have feeling that isn't relevant.
Somebody correct me if I'm wrong, but as long as your MAX() call is in the same step as your insert, you won't have a problem with concurrency.
So, you could not do
select #newOrderID=max(ClientOrderID) + 1
from orders
where clientid=#myClientID;
insert into ( ClientID, ClientOrderID, ...)
values( #myClientID, #newOrderID, ...);
But you can do
insert into ( ClientID, ClientOrderID, ...)
select #myClientID, max(ClientOrderID) + 1, ...
from orders
where clientid=#myClientID;
I'm assuming OrderID is an identity column.
Again, if I'm incorrect on this, please let me know. Preferably with a URL
You could use a Repository pattern to handle your Orders and let it control the number of each specific clients order number. If you implement the OrderRepository correctly it could control the concurrency and number the order before saving it to the database (let the repository and not the db set the number).
Repository pattern: http://martinfowler.com/eaaCatalog/repository.html
One possibility (though I don't like to do this) is to have a lookup table that would tell you the greatest Order Number given for each vendor. Inside of a transaction, you'd fetch the most recent one from VendorOrderNumber, save your new order, increment the value in VendorOrderNumber, commit transaction.
This is an odd way to store data, but assuming you need it, there is nothing built-in that you can use.
Your suggestion of Max(ClientOrderID) is straight forward and pretty easy to implement (follow John MacIntyre's advice). It will probably work acceptably well on tables with a few thousand orders. As the table grows this approach will of course slow down.
Nick DeVore's suggestion of a lookup table is a little messier to implement but won't substantially be affected by data growth.
Depending on where/when you actually need the ClientOrderID, you could calculate the id when needed like this:
SELECT *,
ROW_NUMBER() OVER(ORDER BY OrderID) AS ClientOrderID
FROM Orders
WHERE ClientID = 1
This assumes that the ClientOrderIDs are in the same sequence as the OrderID. Without actually persisting the ID, it is awkward to use as a key to anything else. This approach should not be affected by data growth.