SQL Server: run function for each row based on the provided row value

Question

I am still learning SQL and I was wandering how can I pass a row value into a function in a cross apply query.

Assuming that this is my first table

SELECT 
    [Project], [emp_id],
    [Allocation_start], [Allocation_end]
FROM
    First_table

I have a function that takes 2 arguments - start and end date, and returns dates split by the week intervals...

Example :

select * from [udf_GetIntervals]('2017-01-01','2017-01-30')

Data:

dt_start    dt_end
----------------------
2016-12-26  2017-01-01 
2017-01-02  2017-01-08 
2017-01-09  2017-01-15 
2017-01-16  2017-01-22 
2017-01-23  2017-01-29 
2017-01-30  2017-02-05 

What I did was I run the mentioned function with dates starting from the 1st July to the 30th December and stored it in virtual table and then used cross apply with that table.

select * 
from [First_table] 
cross apply
    (select * from #temp) b

Which works, but it works independently of the first_tables start and end date always returning all weeks per 1 project record from first table.

I was wondering, how can i do this, using first_tables start_date and end_date values, so that my cross apply returns only records with the week_intervals contained in the rows start/end date.

I would appreciate any hints on the matter.

Answer 1

You can use APPLY do perform row-wise actions. CROSS APPLY behaves similar to an INNER JOIN, while OUTER APPLY is rather like a LEFT JOIN.

A Table-Valued-Function returns a table. You must provide an alias and inlcude this resultset into your column list:

SELECT [Project]
      ,[emp_id]
      ,[Allocation_start]
      ,[Allocation_end]
      ,WeekDates.*
From First_table
CROSS APPLY dbo.[udf_GetIntervals]([Allocation_start],[Allocation_end]) AS WeekDates

Answer 2

select * 
from   First_table cross apply [udf_GetIntervals]([Allocation_start],[Allocation_end])