Does anyone know the equivalent of ISDATE() in LINQ to SQL query syntax? I've got a varchar field in SQL that contains dates and I need to filter out the non-date rows.
was hoping for something like this:
var query = SomeDataContext;
query = from p in query
where p.ISDATE(field1) == true;
select p;
also, how would one cast something for SQL in Linq syntax?
CAST(SomeDate AS SMALLDATETIME)
The trick to doing this is to create a user function in the database which merely calls ISDATE as returns the value
CREATE FUNCTION My_ISDATE(#maybeDate varchar(max))
returns bit
as return ISDATE(#maybeDate);
Then add My_IsDate to you database context, and use it in you query:
var db = SomeDataContext;
var query = from p in db.MyTable
where db.My_ISDATE(p.field1)
select p;
I don't think there is an extension method that maps to ISDATE when using LINQ to SQL.
If you are ok with loading all the data and then doing the filtering in the client space, then use TryParse on the field and compare to the date.
Otherwise, I would create a stored procedure which would return the data you want (so that you can use ISDATE), and then execute that through the context.
Also, your syntax for the use of ISDATE is incorrect. ISDATE just tells you if the expression is a valid date format. Your query would look like this:
var query = SomeDataContext;
query = from p in query
where ISDATE(field1) != 0 && CONVERT(datetime, field1) > some date
select p;
The syntax isn't valid, but it gives you an idea of how to form the query.
A problem with SO is that an increasing number of threads on answers of problems contain outdated answers, mostly because they are not touched in years. However in combination with a search engine (e.g. Google) who then returns as #1 a link (for instance this post) when you query "How should i use IsDate in Linq" it needs someone to then add one answer (at the bottom with 0 votes) of the latest correct answer.
:: https://github.com/dotnet/efcore/issues/8488 >
It now has been added to EF.Functions so you can simply do:
Where(x=> EF.Functions.IsDate(x.somefield))
Looks like you can do it. Links:
http://msdn.microsoft.com/en-us/library/bb386973.aspx
this might be of help but I haven't watched it:
http://mtaulty.com/videos/nuggets/l2s/15_mt_l2s_callingsqlfunctions.wmv
Full list here:
http://mtaulty.com/CommunityServer/blogs/mike_taultys_blog/archive/2007/05/10/9322.aspx
Related
Is it possible to use custom method In query for example:
var result = from u in context.MyTable where MyMethod(u) == 10 select u;
As Pranay explains, you cannot have a custom (C#) method as part of the LINQ to SQL query, because LINQ to SQL wouldn't be able to look at the expression tree of the method and so it cannot translate it to SQL.
One option that you have is to write your function in SQL and store it as a SQL function on the SQL Server (possibly, you could also use SQL CLR, but I have not tried that). Then you can add the function to your DataContext type and LINQ to SQL will translate it to calls to the function on SQL server. Something like:
var result = from u in context.MyTable
where context.MyMethod(u) == 10 select u;
The problem, of course, is that you'll need to write the function in SQL (I think SQL CLR could also work - not sure about the performance and other possible complications though)
I also wrote an article (some time ago) that shows how to do this when you write the "method" as an expression tree way (as a value of type Expression<Func<...>>), which is possible, because in this case, the code is compiled as an expression tree. However, there is some postprocessing that has to be done and you can still write just a single expression that can be easily inlined in the LINQ query.
Check this full article : What is and what isn't possible with linq
Following is not possible
// function used in filter
static bool MyFunc(Nwind.Product p)
{
return p.ProductName.StartsWith("B");
}
// query that uses MyFunc
var q =
from p in db.Products
where MyPriceFunc(p.UnitPrice) > 30m
select p
It compiles with no errors, but when you execute it LINQ to SQL throws an exception saying: "Static method System.Boolean MyTest(LINQTest.Nwind.Product) has no supported translation to SQL."
The exception is actually thrown when you try to fetch results from q (for example using the foreach statement), because LINQ to SQL attempts to convert the expression trees to T-SQL only when the results are needed and the query must be executed.
To fix the example you can simply copy the code that checks whether product name starts with "B" to the where clause of the query and it would work fine.
Yes, but if you are using Linq-to-Sql - your method has to have special code to handle to SQL conversion.
I'm trying to recreate the following SQL query in LINQ:
select * from table where column like 'RECORD_%01'
What I have is:
var data = from t in context.table
where t.column.StartsWith("RECORD")
&& t.column.EndsWith("01")
select t;
which is equivalent to:
select * from table where column like 'RECORD%01'
Can someone help me as to how I can add the condition for '_%', not '%'?
If you guys don't know what I'm talking about with the SQL issue, please check the following image, which explains SQL.
This is what you want (or something very similar):
https://learn.microsoft.com/en-us/dotnet/api/system.data.linq.sqlclient.sqlmethods.like
LINQ2SQL LIKE Command for Phrase
This allows you to specify a LIKE statement.
var data = from t in context.table
where SqlMethods.Like(t.column, "RECORD_%01")
select t;
From this it seems like you wanted to make sure that
It starts with RECORD
It ends with 01
And there is atleast one character after RECORD before 01
In this case you can check start with, end with and also the length.
var data = from t in context.table
where t.column.StartsWith("RECORD")
&& t.column.EndsWith("01")
&& t.column.Length>=9
select t;
Here's one way:
var data = from t in context.table
where t.column.StartsWith("RECORD")
&& t.column.EndsWith("01")
&& t.column.Length > 8
select t;
This makes sure that at there is at least one character between RECORD and 01.
Of course, depending on what Query Provider you're using, you may want to simply try use the LIKE operator using SqlMethods.Like, SqlFunctions.PatIndex or one of the methods describe here.
I'm aware there are alreay a lot of posts concerning this issue, but i can't seem to find a solution for this.
Here's my Ling to Entities query :
IEnumerable<Tblstamp> changes = (from c in userGSN.edb.Tblstamp
where (c.Ts_Date >= userGSN.DateLastCheck &&
TimeSpan.Parse(c.Ts_Time) >= userGSN.TimeLastCheck)
orderby c.Ts_Id ascending
select c);
I want to compare c.Ts_time to userGSN.TimeLastCheck, but for that I have to convert the c.Ts_Time to a timespan (it is a string, and comes from a database I can't modify, tried everything). I also can't do the converting before the query in an other variable since I can't access it outside of the query.
Obviously, I get an error for trying to use the TimeSpan.Parse method in my query, but I can't find any workaround to this. I have tried using LINQ to Object but since I am really not used to it I couldn't make the equivalent query that i have here.
I am aware of the problem, I'm just trying to find a workaround and need some help please !
EDIT :
So I tried the DateDiff function as suggested :
IEnumerable<Tblstamp> changes = (from c in userGSN.edb.Tblstamp
where (c.Ts_Date >= userGSN.DateLastCheck && SqlFunctions.DateDiff("second",userGSN.TimeLastCheck,c.Ts_Time).Value > 0 )
orderby c.Ts_Id ascending
select c
);
but it gives me the same error : "LINQ to Entities does not recognize the method 'System.Nullable1[System.Int32] DateDiff(System.String, System.Nullable1[System.TimeSpan], System.String)' method, and this method cannot be translated into a store expression."
Even though it clearly says here "You cannot call this function directly. This function can only appear within a LINQ to Entities query.", which is exactly what I'm doing ?!
There's no clean way to do this, so you'll have to think outside the box. Try using SqlFunctions.DateDiff to perform the check. You'll need to adapt it for your usage, but:
SqlFunctions.DateDiff("second", c.Ts_Time, userGSN.TimeLastCheck.ToString()) > 0
There are other methods that you can use, if DateDiff is not suitable. See EntityFunctions as well.
Depending on what you're using to query the data, you might want System.Data.Entity.SqlServer.SqlFunctions instead.
Every code you write in LinQ to get data from database must convert to a valid T-SQL query with valid statements. Not all C# methods have a correspondance in T-SQL so whenever you use them in your code while fatching data you will get errors.
You should either do modifications on the SQL side or use C# spesific functions after fething data from database, while working with entities in memory, or use fıunstions which comply with SQL statements.
If you can't get EF to translate your query to SQL, maybe you could do it yourself? See Writing SQL queries for entities
Linq to Entities does not support all functions since the query has to be translated to SQL:
from c in userGSN.edb.Tblstamp.AsEnumerable()
//now you are allready getting the data from the database
//so be carefull with that because you will have bad performance
I guess it should be really simple, but i cannot find how to do it.
I have a linq query, that selects one column, of type int, and i need it sorted.
var values = (from p in context.Products
where p.LockedSince == null
select Convert.ToInt32(p.SearchColumn3)).Distinct();
values = values.OrderBy(x => x);
SearchColumn3 is op type string, but i only contains integers. So i thought, converting to Int32 and ordering would definitely give me a nice 1,2,3 sorted list of values. But instead, the list stays ordered like it were strings.
199 20 201
Update:
I've done some tests with C# code and LinqPad.
LinqPad generates the following SQL:
SELECT [t2].[value]
FROM (
SELECT DISTINCT [t1].[value]
FROM (
SELECT CONVERT(Int,[t0].[SearchColumn3]) AS [value], [t0].[LockedSince], [t0].[SearchColumn3]
FROM [Product] AS [t0]
) AS [t1]
WHERE ([t1].[LockedSince] IS NULL)
) AS [t2]
ORDER BY [t2].[value]
And my SQL profiler says that my C# code generates this piece of SQL:
SELECT DISTINCT a.[SearchColumn3] AS COL1
FROM [Product] a
WHERE a.[LockedSince] IS NULL
ORDER BY a.[SearchColumn3]
So it look like C# Linq code just omits the Convert.ToInt32.
Can anyone say something useful about this?
[Disclaimer - I work at Telerik]
You can solve this problem with Telerik OpenAccess ORM too. Here is what i would suggest in this case.
var values = (from p in context.Products
where p.LockedSince == null
orderby "cast({0} as integer)".SQL<int>(p.SearchColumn3)
select "cast({0} as integer)".SQL<int>(p.SearchColumn3)).ToList().Distinct();
OpenAccess provides the SQL extension method, which gives you the ability to add some specific sql code to the generated sql statement.
We have started working on improving this behavior.
Thank you for pointing this out.
Regards
Ralph
Same answer as one my other questions, it turns out that the Linq provider i'm using, the one that comes with Telerik OpenAccess ORM does things different than the standard Linq to SQL provider! See the SQL i've posted in my opening post! I totally wasn't expecting something like this, but i seem that the Telerik OpenAccess thing still needs a lot of improvement. So be careful before you start using it. It looks nice, but it has some serious shortcomings.
I can't replicate this problem. But just make sure you're enumerating the collection when you inspect it. How are you checking the result?
values = values.OrderBy(x => x);
foreach (var v in values)
{
Console.WriteLine(v.ToString());
}
Remember, this won't change the order of the records in the database or anywhere else - only the order that you can retrieve them from the values enumeration.
Because your values variable is a result of a Linq expression, so that it doest not really have values until you calling a method such as ToList, ToArray, etc.
Get back to your example, the variable x in OrderBy method, will be treated as p.SearchColumn3 and therefore, it's a string.
To avoid that, you need to let p.SearchColumn3 become integer before OrderBy method.
You should add a let statement in to your code as below:
var values = (from p in context.Products
where p.LockedSince == null
let val = Convert.ToInt32(p.SearchColumn3)
select val).Distinct();
values = values.OrderBy(x => x);
In addition, you can combine order by statement with the first, it will be fine.
i need to get a difference of two dates ,one date from a table and one the current date, and the difference should be less than 9 0days. i need to use this as filter in where clause of the linq
i tried doing this
var list = from p in context.persons
where ((p.CreateDT).Subtract(DateTime.Now).Days < 90)
select p;
i get this excpetion :
LINQ to Entities does not recognize the method 'System.TimeSpan Subtract(System.DateTime)' method, and this method cannot be translated into a store expression.
I did research other articles but nothing helped..Any ideas
Trick here is that it can't translate all your fancy oo hoo-ha to plain old sql. Trick is to flip it on it's head:
var before = DateTime.Now.AddDays(-90);
var persons = context.Persons.Where(x => x.CreateDT > before);
EXPLANATION
Remember that everything in the WHERE bit of your LINQ statement must be translated from C# to SQL by the EF. It is very, very capable out of the box and handles most basic tasks, but it has no idea how to understand the most rudimentary method calls, such as DateTime.Subtract(). So the idea here is to let it do what it does best by precalculating a value and then passing that to the data tier.
The first line subtracts 90 days from the current time by adding negative 90 days. The second line passes it off to the database server.
The second line should translate to the SQL WHERE CreateDT > #BEFORETHIS
Update
It seems that EF doesn't support subtracting dates and returning a TimeSpan. Here's one way to solve the problem:
DateTime oldestDate = DateTime.Now.AddDays(-90);
var list = from p in context.persons
where p.CreateDT >= oldestDate
select p;
See this thread on Stackoverflow.
Try doing simply (p.CreateDate - DateTime.Now).Days < 90. Instead of calling DateTime.Subtract(). In some cases the operator overloads are implemented for Entity Framework even when the corresponding named methods are not.
If that doesn't work you could instead use ESQL or a stored procedure. As a final, dirty solution, you could call context.persons.ToList() and then call the DateTime.Subtract().