I run into trouble when learning more about the Entity Framework and LINQ.
I am trying to find values from a database that match a specific date.
I have 1.250.000 entries and 36300 of them are from a specific date.
I am using plain old SQL until now and want to practice LINQ to refactor my application to work with the EF.
Can you tell where my mistake is?
The second way works but takes a lot of time ~15 sec
Try:
Where(Function(d) d.ExportedDate.HasValue AndAlso d.ExportedDate.Value = exportDate)
On this way you select only rows where the nullable column ExportedDate is not null and where it equals the given date. ExportedDate.Value returns Date as opposed to d.ExportedDate which is a Date?. If you compare a Date? with a Date you get a Boolean? as result(not a Boolean), because Nothing means undefined.
This is an interesting difference to C#, related: Why is there a difference in checking null against a value in VB.NET and C#?
To fix the next error: append ToList to create a list.
Related
Folks
I am in the process of moving a decade old back-end from DB2 9.5 to Oracle 19c.
I frequently see in SQL queries and veiw definitions bizarre timestamp(nullif('','')) constructs used instead of a plain null.
What is the point of doing so? Why would anyone in their same mind would want to do so?
Disclaimer: my SQL skills are fairly mediocre. I might well miss something obvious.
It appears to create a NULL value with a TIMESTAMP data type.
The TIMESTAMP DB2 documentation states:
TIMESTAMP scalar function
The TIMESTAMP function returns a timestamp from a value or a pair of values.
TIMESTAMP(expression1, [expression2])
expression1 and expression2
The rules for the arguments depend on whether expression2 is specified and the data type of expression2.
If only one argument is specified it must be an expression that returns a value of one of the following built-in data types: a DATE, a TIMESTAMP, or a character string that is not a CLOB.
If you try to pass an untyped NULL to the TIMESTAMP function:
TIMESTAMP(NULL)
Then you get the error:
The invocation of routine "TIMESTAMP" is ambiguous. The argument in position "1" does not have a best fit.
To invoke the function, you need to pass one of the required DATE, TIMESTAMP or a non-CLOB string to the function which means that you need to coerce the NULL to have one of those types.
This could be:
TIMESTAMP(CAST(NULL AS VARCHAR(14)))
TIMESTAMP(NULLIF('',''))
Using NULLIF is more confusing but, if I have to try to make an excuse for using it, is slightly less to type than casting a NULL to a string.
The equivalent in Oracle would be:
CAST(NULL AS TIMESTAMP)
This also works in DB2 (and is even less to type).
It is not clear why - in any SQL dialect, no matter how old - one would use an argument like nullif('',''). Regardless of the result, that is a constant that can be calculated once and for all, and given as argument to timestamp(). Very likely, it should be null in any dialect and any version. So that should be the same as timestamp(null). The code you found suggests that whoever wrote it didn't know what they were doing.
One might need to write something like that - rather than a plain null - to get null of a specific data type. Even though "theoretical" SQL says null does not have a data type, you may need something like that, for example in a view, to define the data type of the column defined by an expression like that.
In Oracle you can use the cast() function, as MT0 demonstrated already - that is by far the most common and most elegant equivalent.
If you want something much closer in spirit to what you saw in that old code, to_timestamp(null) will have the same effect. No reason to write something more complicated for null given as argument, though - along the lines of that nullif() call.
I am using this query to generate a DataTable to display information for users to select from. The fill method I am using gives teh ff error.
Conversion failed when converting date and/or time from character
string
SELECT Time_Slot.TimeSlot, Proposed_Date.Date
FROM Time_Slot CROSS JOIN
Proposed_Date
WHERE (Time_Slot.TimeSlot_ID NOT IN
(SELECT Event_Time_ID
FROM Event_Booking
WHERE (Booking_Date = #Booking_Date)))
DTAvailableBookingsTableAdapter.FillAvailableBookings(DSEventBooking.DTAvailableBookings, MonthCalendar1.SelectionRange.Start.ToShortDateString)
Any help in resolving the error would be welcome
Many people make their own lives difficult when it comes to dates. VB.NET has a dedicated data type for dates so, if you are working with dates, use it. Don't convert anything that is not text to text, unless it is specifically for display/serialisation purposes, where only text is supported.
Presumably your Booking_Date is the appropriate data type for dates in your database, e.g. Date/Time in Access or date in SQL Server. In that case, the #Booking_Date parameter is expecting a value of that type, not text. In that case, why are you converting a value of the correct type to a value of the incorrect type here:
MonthCalendar1.SelectionRange.Start.ToShortDateString
The Start property is the correct type and you spoil it by converting it to a String. Don't. Just pass the DateTime value that you already have because that is what's expected.
MonthCalendar1.SelectionRange.Start
Coming from a MonthCalendar, it should already have the time portion zeroed so there's no need to do that yourself but, in cases where you did need to, you can get the Date property of a DateTime value to get another DateTime value with the same date and the time zeroed.
MonthCalendar1.SelectionRange.Start.Date
Note that DateTime is a .NET type and VB has a Date type that is simply an alias for that, i.e. a Date and a DateTime are the same thing. If this is confusing, it shouldn't be. Int32 is a .NET type too and Integer is a VB data type that maps to it. Do you use Integer all the time without any confusion? If so then you should use Date all the time without confusion too.
I have a database field, Field260, that stores date representations as text. For instance a value for 3/29/2018 would be stored as a string "03/29/2018"
Dim Db = New DataClasses1DataContext
Return From cf In Db.FileDatas
Where cf.Field260 <= System.DateTime.Today
Returns error
System.Data.SqlClient.SqlException: 'Explicit conversion from data type text to datetime is not allowed.'
When I tried to Parse the date
Where DateTime.Parse(cf.Field260) <= System.DateTime.Today
I receive
System.NotSupportedException: 'Method 'System.DateTime Parse(System.String)' has no supported translation to SQL.'
I'm stumped.
I encountered a similar issue - have an app that I used to have a DatePicker for a field, and my database field defined as DateTime, but the users absolutely wanted to copy/paste whatever garbage they wanted into the field. Problem is, I'm allowing them to search for records by that "Date" field.
As a workaround, I do some basic validations on the garbage they enter to attempt to only allow dates (IsDate function captures most errors, but still allows them to enter years like 208 instead of 2018, so I also check that Year(DateValue(txtDate.Text))>2000, which works for me because it's an application-received date, so should never be older than this year). Then I store it as varchar in the database.
When searching for the records, you have two options - either grab all records, then perform a date conversion on the list returned to pare down the results, or hope and pray that your faulty LINQ WHERE clause will get everything you want.
Figured it out cuz I'm a genius.
Return From cf In Db.FileDatas
Where cf.Field260.ToString() <= System.DateTime.Today.ToString("MM/dd/yyyy")
Somehow comparing string to string works, who knew!
I have a simple select statement like this:
SELECT [dok__Dokument].[dok_Id],
[dok__Dokument].[dok_WartUsNetto],
[dok__Dokument].[dok_WartUsBrutto],
[dok__Dokument].[dok_WartTwNetto],
[dok__Dokument].[dok_WartTwBrutto],
[dok__Dokument].[dok_WartNetto],
[dok__Dokument].[dok_WartVat],
[dok__Dokument].[dok_WartBrutto],
[dok__Dokument].[dok_KwWartosc]
FROM [dok__Dokument]
WHERE [dok_NrPelnyOryg] = 2753
AND [dok_PlatnikId] = 174
AND [dok_OdbiorcaId] = 174
AND [dok_PlatnikAdreshId] = 625
AND [dok_OdbiorcaAdreshId] = 624
Column dok_NrPelnyOryg is of type varchar(30), and not null.
The table contained both integer and string values in this column and this select statement was fired millions of times.
However recently this started crashing with message:
Conversion failed when converting the varchar value 'garbi czerwiec B' to data type int.
Little explanation: the table contains multiple "document" records and the mentioned column contains document original number (which comes from multiple different sources).
I know I can fix this by adding '' around the the number, but I'm rather looking for an explanation why this used to work and while not changing anything now it crashes.
It's possible that a plan change (due to changed statistics, recompile etc) led to this data being evaluated earlier (full scan for example), or that this particular data was not in the table previously (maybe before this started happening, there wasn't bad data in there). If it is supposed to be a number, then make it a numeric column. If it needs to allow strings as well, then stop treating it like a number. If you properly parameterize your statements and always pass a varchar you shouldn't need to worry about whether the value is enclosed in single quotes.
All those equality comparison operations are subject to the Data Type Precedence rules of SQL Server:
When an operator combines two
expressions of different data types,
the rules for data type precedence
specify that the data type with the
lower precedence is converted to the
data type with the higher precedence.
Since character types have lower precedence than int types, the query is basically the same as:
SELECT ...
FROM [dok__Dokument]
WHERE cast([dok_NrPelnyOryg] as int) = 2753
...
This has two effects:
it makes all indexes on columns involved in the WHERE clause useless
it can cause conversion errors.
You're not the first to have this problem, in fact several CSS cases I faced had me eventually write an article about this: On SQL Server boolean operator short-circuit.
The correct solution to your problem is that if the field value is numeric then the column type should be numeric. since you say that the data come from a 3rd party application you cannot change, the best solution is to abandon the vendor of this application and pick one that knows what is doing. Short of that, you need to search for character types on character columns:
SELECT ...
FROM [dok__Dokument]
WHERE [dok_NrPelnyOryg] = '2753'
...
In .Net managed ADO.Net parlance this means you use a SqlCommand like follows:
SqlCommand cmd = new SqlCommand (#" SELECT ...
FROM [dok__Dokument]
WHERE [dok_NrPelnyOryg] = #nrPelnyOryg
... ");
cmd.Parameters.Add("#nrPelnyOryg", SqlDbType.Varchar).Value = "2754";
...
Just make sure you don't fall into he easy trap of passing in a NVARCHAR parameter (Unicode) for comparing with a VARCHAR column, since the same data type precendence rules quoted before will coerce the comparison to occur on the NVARCHAR type, thus rendering indexes, again, useless. the easiest way to fall for this trap is to use the dredded AddWithValue and pass in a string value.
Your query stopped working because someone inserted the text string in to the field you are querying using INT. Up until that time it was possible to implicitly convert the data but now that's no longer the case.
I'd go check your data and, more importantly, the model; as Aaron said do you need to allow strings in that field? If not, change the data type to prevent this happening in the future.
A cohort of mine is building a somewhat long search query based on various input from the user. We've got NHibernate mapped up using Fluent NHibernate, and aside from some noob mistakes, all seems to be going well.
One issue we can't resolve in the case of this search is that for a particular parameter, NHibernate is creating sql that treats the input as int when we explicitly need it to be a string. We have a string property mapped to an nvarchar(255) column which mostly contains integer numbers, excluding some arbitrary inputs like "temporary" or long numbers like 4444444444444444 which is beyond the int limit.
In the course of testing, I've seen a couple things: 1) If I prepend a 0 to the incoming value, NH generates the sql param as a string, appropriately so; 2) If the value can realistically be converted to an int, the resulting sql treats it as so. In case #2, if I run the generated sql directly through sql server, I get an exception when the query comes across an non-integer value (such as the examples I listed above). For some reason, when I just let NH do it's thing, I'm getting appropriate records back, but it doesn't make sense; I would expect it to fail or at least tell me that something is wrong with some records that can't be evaluated by SqlServer.
The mapping is simple, the data store is simple; I would be ok leaving well enough alone if I at least understood why/how NHibernate is making this work when running the same state manually fails... Any thoughts?
Are you running the exact same code directly into SQL Server?
NHibernate parameterises all of its queries, and will in doing so define what value is passed through to SQL in the parameters. Which is probably what you're asking about, the reason SQL my fail, is that by default it will only know the difference if you input:
select * from table_name
where col_name = 5
in comparison with
select * from table_name
where col_name = '5'
If you do not define it as a string with the 's it will search for an int, and try to convert all the varchar's to ints, which will obviously fail in some cases with strings.