INSERT INTO atable (x,y,z) VALUES ( NULL,NULL,NULL)

If you were writing a tsql with literals, you would need to detect the null scenario and append "null" rather than the value, so the SQL looks like

1,2,null,4,5

If you see what I mean. However, this is a bad way to do it - your SQL right now is really really dangerous. You should parameterize instead - this solves a range of problems:

• sql injection(your code is a security risk right now)
• formatting (dates etc)
• null values
• query plan reuse (or the lack of it)

For example:

query = "insert into Assignment Values(@eventId, @branchId, @isPremiere, @isNew, ...)"

where you add a parameter with value for each of the placeholders. Note that due to how ado.net you need to represent nulls with DBNull:

cmd.Parameters.AddWithValue("foo",
    foo == null ? (object)DBNull.Value : (object)foo);

(For each parameter)

Note that orms and micro-orms will help make this simple. For example with dapper:

DateTime foo = ...
int? bar = ...
connection.Execute(
     @"insert ... values (@foo, @bar)",
    new { foo, bar });

If a value is NULL, then adding it to a string will produce a NULL. This allows us to add the quotes in the ISNULL check and just produce NULL in the true value of the check, producing the correct syntax for nulls or not nulls as necessary.

select 'Insert into Organizations(Name, IndustryId, ContactPerson, Email, Website, LocationId, ContactNumber, Mobilenumber) values(' + 
       IsNull(''''+Nameofthecompany+'''', 'NULL') + ', ' + 
       Isnull(''''+IndustryType+'''', 'NULL') + ', ' +
       Isnull(''''+Nameofthepersonresponsibleforrecruitment+'''', 'NULL') + ', ' +
       Isnull(''''+EmailId+'''', 'NULL') + ', ' +
       Isnull(''''+websiteaddress+'''', 'NULL') + ', ' +
       Isnull(''''+Location+'''', 'NULL') + ', ' +
       Isnull(PhoneNumber, 'NULL') + ', ' +
       Isnull(MobileNumber, 'NULL') + ')' 
from Organization