Creating a week_ref number to pass into a stored procedure - sql

Question

Currently one of the reports reads the month_ref which is created like this:
((DATEPART(yy, month_date) - 1200) * 100) + DATEPART(mm, month_date) AS month_ref,
month_date, DATENAME(MONTH, month_date) + ' ' + DATENAME(YEAR, month_date) AS month_name
I would like to create something like a week_ref?
The month ref uniqely identifies the month for the year. When its passed into a particular stored procedure, the sp will filter the results based on the month ref.
I am now wanting to create a weekly report - (mon - sun) and need to uniquely identify the week_ref so i can filter the results using the sp.
How can i do this?

Answer 1

This will give you a unique number for each week from monday to sunday and it doesn't depend on datefirst.
SELECT datediff(day, 0, month_date) / 7 week_ref FROM <table>
This will return an integer always as tested here:
SELECT SQL_VARIANT_PROPERTY(datediff(day, 0, getdate()) / 7, 'BaseType')
Returns:
int

Answer 2

Although I'd consider this technique very dirty, for months it does work. This is because there are always 12 months in a year.
For weeks it's both dirty and flawed. Some years have more weeks than others, because some weeks span more than one year.
I would recommend instead just using DATEDIFF().
Depending on the day-of-week that you want your weeks to start on, you need to do a little bit of calculation...
- Sun as start of week = DATEDIFF(WEEK, -1, yourDate )
- Mon as start of week = DATEDIFF(WEEK, -1, yourDate-1)
- Tue as start of week = DATEDIFF(WEEK, -1, yourDate-2)
- Wed as start of week = DATEDIFF(WEEK, -1, yourDate-3)
- Thu as start of week = DATEDIFF(WEEK, -1, yourDate-4)
- Fri as start of week = DATEDIFF(WEEK, -1, yourDate-5)
- Sat as start of week = DATEDIFF(WEEK, -1, yourDate-6)
I also recommend using this for your month calculations. DATEDIFF(MONTH, <a base date>, <your date>), the current one is really clunky.
EDIT Examples to turn the values above back into DATETIME
- Sun as start of week = DATEADD(WEEK, DATEDIFF(WEEK, -1, yourDate ), -1 )
- Mon as start of week = DATEADD(WEEK, DATEDIFF(WEEK, -1, yourDate-1), -1+1)
- Tue as start of week = DATEADD(WEEK, DATEDIFF(WEEK, -1, yourDate-2), -1+2)
- Wed as start of week = DATEADD(WEEK, DATEDIFF(WEEK, -1, yourDate-3), -1+3)
- Thu as start of week = DATEADD(WEEK, DATEDIFF(WEEK, -1, yourDate-4), -1+4)
- Fri as start of week = DATEADD(WEEK, DATEDIFF(WEEK, -1, yourDate-5), -1+5)
- Sat as start of week = DATEADD(WEEK, DATEDIFF(WEEK, -1, yourDate-6), -1+6)

Answer 3

Use
Select DATEPART(wk, GetDate()) --In your case month_date and not GetDate()
to get the week of the year back, and then you can use any formula you like to create a week_ref, but from what I can see from your edit, this whould the only thing that you need. However, you would change your month_ref to something like this.
((DATEPART(yy, month_date) - 1200) * 10000) + (DATEPART(mm, month_date) * 100) + DATEPART(wk, month_date) AS month_ref
Then you don't have to add an extra column

Answer 4

Use a calendar table with (at least) 3 columns: base_date, month_ref and week_ref. Then to get the month and week references (whatever those are) for a certain date, you can query them directly from the calendar table without using hard-to-read nested functions or other 'non-obvious' code:
select week_ref, month_ref
from dbo.calendar
where base_date = #my_date
You haven't explained what week_ref and month_ref actually mean, but if they are just numbers then a calendar table has the significant advantage that it can support mutliple different definitions. For example, you might want to support both the ISO week number and your own, internal week number scheme. You can also easily change values in the table if your logic changes, without changing any code.
As a general observation, a calendar table is usually a better solution than inline functions, especially if your requirements involve reporting.