TSQL Keyword Previous or Last or something similar

Question

This question is geared for those who have more SQL experience than me.

I am writing a query(that will eventually be a Stored Procedure but this should be irrelevant) where I want to select the count of rows if the most recent entry's is equivalent to the one that was just entered before. And i want to continue to do this until it hits an entry that has a different value. (Poorly explained so I will show the example)

In my table I have a column 'Product_Id' and when this query is run i want it take the product_id and compare it to the previously entered product Id, if its the same I want to add one, and I want it to keep checking the previously entered product_id until it runs into a different product_id

I'm hoping it sounds more complicated than it is, and the query would look something like

Select count(Product_ID)
FROM dbo.myTable
Where Product_Id = previous(Product_Id)

Now, i know that previous isn't a keyword in TSQL, and neither was Last, but I'm hoping of someone who knows a keyword that does what I am asking.

• Edit for Sam

   USE DbName;
   GO
  WITH OrderedCount as
  (
  select ROW_NUMBER() OVER (Order by dbo.Line_Production.Run_Date DESC) as RowNumber,
  Line_Production.Product_ID
  From dbo.Line_Production
  )
  Select RowNumber, COUNT(OrderedCount.Product_ID) as PalletCount
  From OrderedCount
  WHERE OrderedCount.RowNumber + 1 = RowNumber
  and Product_ID = Product_ID
  Group by RowNumber
  

The OrderedCount portion works, and it returns the data back how I want it, I'm now having trouble comparing the Product_ID's for different RowNumbers

my Where Clause is wrong

Answer 1

There's no keyword. That would be a nice magic solution, but it doesn't exist, at least in part because there is no guaranteed ordering (okay, you could have the keyword only if there is an ORDER BY...). I can write you a query, but that'll take time, so for now I'll give you a few steps and I'll come back and see if you still need help in a bit.

1. Figure out an ORDER BY, otherwise no order is guaranteed. If there is a time entered field, that's a good choice, or an index, that works too.

2. Learn to use Row_Number.

3. Compare the table (with Row_Number) to itself where instance1.row - 1 = instance2.row.

Answer 2

If product_id is an identity column, couldn't you just do product_id - 1? In other words, if it's sequential, it's the same as using ROW_NUMBER mentioned in the previous comment.