如何使用Linq-to-SQL删除具有空外键的行？

Question

I have a table called tblOvrdImpVolsType with two foreign key columns that are both nullable. I'm having trouble deleting a row in this table using Linq-To-SQL when one of the foreign key columns in this row is null. Here's my code:

using (Databases.Global db = new Databases.Global())
{
    db.Log = Console.Out;
    var records = from iv in db.tblOvrdImpVolsType
                    join sm in db.tblSecurityMasterType 
                       on iv.FkSecurity equals sm.PkSecurity
                    where sm.Name == name
                    select iv; //returns only 1 record

    db.tblOvrdImpVolsType.DeleteAllOnSubmit<MyTableType>(records);

    int numDeletes = db.GetChangeSet().Deletes.Count; // = 1
    db.SubmitChanges(); //deletes 0 records
}

• Databases.Global inherits from DataContext and has the tables that I use.

• tblOvrdImpVolsType has two nullable, foreign key columns (key1 and key2)

• the row I am interested has these key values (key1 = 9898, key2 = null)

• In my C# code, the datatypes of key1 and key2 are both int? and CanBeNull = true

• The GetChangeSet() shows that one record will be deleted, but when I check the database, that row is clearly not deleted.

• No exceptions are thrown.

Here is the SQL that the DataContext generates:

DELETE FROM [tblOvrdImpVols] WHERE ([key1] = @p0) AND ([key2] = @p1)
-- @p0: Input Int (Size = -1; Prec = 0; Scale = 0) [9898]
-- @p1: Input Int (Size = -1; Prec = 0; Scale = 0) [Null]
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 4.0.30319.17929

I think the problem is the [key2] = @p1 part. Because the key2 value I'm interested in is null, the SQL is bad. These SQL queries below illustrate this. The only difference is the is vs. = in the key2 condition:

select * FROM [tblOvrdImpVols] WHERE ([key1] = 9898) AND ([key2] is null)
    --returns 1 row

select * FROM [tblOvrdImpVols] WHERE ([key1] = 9898) AND ([key2] = null)
    --returns 0 rows

What am I doing wrong? I assume that if I can get the DataContext object to replace the = with is for null values, the deletes will work. I don't know how to do that. I also assume this question has been asked before, but I can't find it.

EDIT: I originally called my keys "primary" rather than "foreign". I meant to say "foreign". I edited my post to say "foreign" instead. I am still having the same problem.

Answer 1

To make sure that you also catch nulls, you could modify your query to look like this :

select * FROM [tblOvrdImpVols] 
WHERE ([key1] = @p0) AND 
([key2] = @p1 OR 
    ([key2] is null and @p1 is null) )

Answer 2

The main problem here - in SQL, nothing is equal to NULL. NULL is not equal even to itself! "NULL = NULL" will evaluate to "false" in SQL query. So, when you need to delete rows where [key2] == null, you need to write LINQ query which will produce SQL like

"WHERE [key2] = @p1 AND [key2] IS NULL"

instead of

"WHERE [key2] = @p1 AND [key2] = @p2".

How to do it? It seems the right way will be to compare [key2] with null. For example, if you want to get all rows from tblSecurityMasterType where Name is NULL, this will be the wrong way:

string NameNull = null;
from sm in db.tblSecurityMasterType 
     where sm.Name == NameNull
     select sm;

You will need to use this query instead:

from sm in db.tblSecurityMasterType 
     where sm.Name == null
     select sm;

It's described here: http://smsohan.com/blog/2008/08/05/comparing-with-null-in-where-clause-in/