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Question

Why is it that when i query in Entity Framework Core for string Not Equals it will give return NULL values (by appending "Or ISNULL(field)"), but when i query for example "not Contains" or "not StartWith" It will not

I know that i can achieve the same result by adding manually to the query to include NULL, my question is why are the 2 behaving differently when it comes to NULL? is there any obvious reason for that?

Code Example:

C# - dbContext.Employee.Where(x => x.jobNotes != "abc").Select(x => x.firstName).ToList();

Generated Sql - exec sp_executesql N'SELECT [x].[firstName] FROM [Employee] AS [x] WHERE (([x].[jobNotes] <> N''abc'') OR [x].[jobNotes] IS NULL)'

C# - dbContext.Employee.Where(x => !x.jobNotes.Contains("abc")).Select(x => x.firstName).ToList();

Generated Sql - exec sp_executesql N'SELECT [x].[firstName] FROM [Employee] AS [x] WHERE (CHARINDEX(N''abc'', [x].[jobNotes]) <= 0)'

Answer 1

The reason is not "obviously", but it can be explained somehow:

Having the C# expression:

x.jobNotes.Contains("abc")

only "works", when the value of x.JobNotes is not NULL , otherwise you would get a NullReferenceException . To ensure that you don't, only NOT NULL values are checked. The generated SQL statement

(CHARINDEX(N''abc'', [x].[jobNotes]) <= 0)

does that because for any NULL values in the jobNotes table, the result of CHARINDEX will be NULL , as defined in the documentation of CHARINDEX :

If either the expressionToFind or expressionToSearch expression has a NULL value, CHARINDEX returns NULL.

Comparing NULL values in this case ( NULL <= 0 ) results in UNKNOWN , as defined in the documentations of comparison operators :

When SET ANSI_NULLS is ON, an operator that has one or two NULL expressions returns UNKNOWN. When SET ANSI_NULLS is OFF, the same rules apply, except for the equals (=) and not equals (<>) operators. When SET ANSI_NULLS is OFF, these operators treat NULL as a known value, equivalent to any other NULL, and only return TRUE or FALSE (never UNKNOWN).

I'm not sure what WHERE UNKNOWN would do as you cannot run this expression directly, but as an example the expression WHERE NULL <= 0 results in WHERE FALSE , which means the row with the jobNotes column set to NULL is not returned.

When you look at the query from the C# or Entity Framework standpoint, it does make sense that these NULL values are not returned. When you have a query like

dbContext.Employee
    .Where(x => !x.jobNotes.Contains("abc"))
    .Select(x => x.firstName)
    .ToList()

and you get entities where the jobNotes column is NULL , the question arise:

"Wait, why is jobNotes NULL? It should have thrown a NullReferenceException because of the Contains() method call."

So, that might be the reason why it does not return rows where the jobNotes column has the value NULL (and works "differently" with x.jobNotes != "abc" ).