For some time, we've been running queries against a SQL Server database through an OLE DB connection. Recently, I came across some strange behaviour when retrieving datetime values.
In one location, the application populating the SQL database allows for an empty date value. However, SQL Server doesn't allow for an empty datetime value (though it does support a null datetime). The application handles this by using the maximum SQL Server datetime value of 9999-12-31 23:59:59.997 to represent an empty date. The data looks something like this:
This is maybe a bit unusual, but not too strange. However, when querying this data through an OLE DB connection in VFP, this is the result:
That's curious. Why is the date appearing as a blank? Doing some quick investigating on the retrieved record, both Empty(ONDATE) and IsNull(ONDATE) return false. Even more curious. What would make a datetime value display as blank, but still evaluate as not empty and not null?
Ok, let's try connecting to the database using an ODBC connection instead. Here are the results of the same query:
Curious again. The data is appearing correctly here. Something must be different in the way that OLE DB and ODBC are handling these datetime values. Let's add a couple more testing values, and then retrieve them both ways, to see if that sheds any light on the situation.
This shows what's going on. The OLE DB connection is using the milliseconds to round the value to the nearest second, while the ODBC connection is just truncating the milliseconds. The weirdness with the blank value is coming from the fact that 9999-12-31 23:59:59 is also VFP's maximum datetime value, and the rounding is forcing the value past this maximum.
I generated a VFP table using the results of the OLE DB query, and opened it with a hex editor. Sure enough, there is data in the "blank" datetime value. VFP stores a datetime value in two pieces: the date as a Julian day, and the number of milliseconds past midnight. The problem value has a number of milliseconds that evaluates to slightly more than 24 hours. The Empty and IsNull functions are correctly reporting false, based on the fact that there really is a value stored there.
All in all, this is not too much trouble to work around, since we can just use an expression with CASE or DATEADD/DATEPART to have SQL Server adjust the value for us before sending the query results. It's good to understand this behaviour though, since it will also appear in other situations where milliseconds are included in datetime values, and it will be much less noticeable that any rounding is happening.
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