Checking for time range overlap, the watchman problem [SQL]

Question

I am running into a road block on a larger problem.

As part of a large query I need to solve a \"night watchman\" problem. I have a table with schedule shifts as suc

Answer 1

Here is a way to flatten date range like this

Start          | End
2009-1-1 06:00 | 2009-1-1 18:00
2009-2-1 20:00 | 2009-2-2 04:00
2009-2-2 06:00 | 2009-2-2 14:00

You have to compare previous and next dates in each row and see whether

• Current row's Start date falls between previous row's date range.
• Current row's End date falls between next row's date range.

Using above code, implementing UDF is as simple as followed.

create function fnThereIsWatchmenBetween(@from datetime, @to datetime)
returns bit
as
begin
    declare @_Result bit

    declare @FlattenedDateRange table (
        Start   datetime,
        [End]   datetime
    )

    insert  @FlattenedDateRange(Start, [End])
    select  distinct 
            Start = 
                case 
                    when Pv.Start is null then Curr.Start 
                    when Curr.Start between Pv.Start and Pv.[End] then Pv.Start
                    else Curr.Start 
                end,
            [End] = 
                case 
                    when Curr.[End] between Nx.Start and Nx.[End] then Nx.[End] 
                    else Curr.[End] 
                end
    from    shift Curr
            left join shift Pv on Pv.ID = Curr.ID - 1 --; prev
            left join shift Nx on Nx.ID = Curr.ID + 1 --; next

    if exists(  select  1
                from    FlattenedDateRange R
                where   @from between R.Start and R.[End]
                        and @to between R.Start and R.[End]) begin
        set @_Result = 1    --; There is/are watchman/men during specified date range
    end
    else begin
        set @_Result = 0    --; There is NO watchman
    end

    return @_Result
end