Question

I have a database with about 3 milion records, each record has id (int, primary key), uniqueNumber(int) and updateTime(string).

uniqueNumber can belong to several records.
updateTime represents the time of the day the record was updated such as 14:33:42

EDIT: the records are ordered by update time.

I want to let the user define a uniqueNumber and an interval. Then I want to query the DB to get all the records belongs to that uniqueNumber where their updateTime is equal to a variable which it’s value is increasing by interval each time the where condition is true.

For example, I want something like this: (there is no DB used in this example)

    string currentTime = "12:25:00";
    int interval = 2;

    public void LinqExample()
    {
        string[] arr = new string[4] { "12:23:00", "12:26:00", "12:27:00", "12:28:00"};

        var query = from c in arr
                    where c.Replace(":", "").CompareTo(currentTime.Replace(":", "")) > 0 && Inc()
                    select c;

        // query's output is: "12:26:00", "12:28:00"
    }

    private bool Inc()
    {
        currentTime = DateTime.Parse(currentTime).AddMinutes(interval).ToLongTimeString();
        return true;
    }

is my code is the proper way to do this?

thanks!

Answer 1

It seems that code you want if it exists require generating SQL query with CURSOR over your table by EF. However as I quess, EF never produces code with cursors, therefore you cannot solve your problem using LINQ. IMHO the only way to do this is to write table-valued function like this

CREATE FUNCTION GetTimesByUniqueNumberWithIntervalAndCondition
(
    @interval int, @uniqueNumber int,@currentTime time
)
RETURNS 
@Times TABLE 
(
    t char(10)
)
AS
BEGIN
    DECLARE timeCursor CURSOR LOCAL FAST_FORWARD READ_ONLY FOR
    SELECT updateTime FROM Table2 WHERE uniqueNumber = @uniqueNumber
    ORDER BY updateTime 

    DECLARE @t char(10);

    OPEN timeCursor
    FETCH NEXT FROM timeCursor INTO @t;

    WHILE @@FETCH_STATUS = 0
    BEGIN
        IF 
            1 = 1 -- place your condition here
        BEGIN
            INSERT @Times(t) VALUES(@t)
            SET @currentTime = DATEADD(MINUTE,@interval,CAST(@t AS time))
        END

        FETCH NEXT FROM timeCursor INTO @t;
    END

    CLOSE timeCursor
    DEALLOCATE timeCursor

    RETURN 
END
GO

and query against this function every time you need the selection.