LINQ to SQL selecting records and converting dates - sql

Question

I'm trying to select records from a table based on a date using Linq to SQL. Unfortunately the date is split across two tables - the Hours table has the day and the related JobTime table has the month and year in two columns.
I have the following query:
Dim qry = From h As Hour In ctx.Hours Where Convert.ToDateTime(h.day & "/" & h.JobTime.month & "/" & h.JobTime.year & " 00:00:00") > Convert.ToDateTime("01/01/2012 00:00:00")
This gives me the error "Arithmetic overflow error converting expression to data type datetime."
Looking at the SQL query in SQL server profiler, I see:
exec sp_executesql N'SELECT [t0].[JobTimeID], [t0].[day], [t0].[hours]
FROM [dbo].[tbl_pm_hours] AS [t0]
INNER JOIN [dbo].[tbl_pm_jobtimes] AS [t1] ON [t1].[JobTimeID] = [t0].[JobTimeID]
WHERE (CONVERT(DateTime,(((((CONVERT(NVarChar,[t0].[day])) + #p0) + (CONVERT(NVarChar,COALESCE([t1].[month],NULL)))) + #p1) + (CONVERT(NVarChar,COALESCE([t1].[year],NULL)))) + #p2)) > #p3',N'#p0 nvarchar(4000),#p1 nvarchar(4000),#p2 nvarchar(4000),#p3 datetime',#p0=N'/',#p1=N'/',#p2=N' 00:00:00',#p3='2012-01-31 00:00:00'
I can see that it's not passing in the date to search for correctly but I'm not sure how to correct it.
Can anyone please help?
Thanks,
Emma

Answer 1

The direct cause of the error may have to do with this issue.
As said there, the conversions you use are a very inefficient way to build a query. On top of that, it is inefficient because the expressions are not sargable. I.e. you are using a computed value from database columns in a comparison which disables the query analyzer to use indexes to jump to individual column values. So, you could try to fix the error by doctoring the direct cause, but I think it's better to rewrite the query in a way that only the single column values are used in comparions.
I've worked this out in C#:
var cfg = new DateTime(12,6,12);
int year = 12, month = 6, day = 13; // Try some more values here.
// Date from components > datetime value?
bool gt = (
year > cfg.Year || (
(year == cfg.Year && month > cfg.Month) || (
year == cfg.Year && month == cfg.Month && day > cfg.Day)
)
);
You see that it's not as straightforward as it may look at first, but it works. There are much more comparisons to work out, but I'm sure that the ability to use indexes will easily outweigh this.
A more straightforward, but not sargable, way is to use sortable dates, like 20120101 and compare those (as integers).