First of all, check if nulls are allowed in that filed,
second, do you want it to be done in C# or sql ?
in C# that’ll be:
(blah-blah-db.table.field.or.storedProcParameter).Value = DBNull.Value;
slq - you have an example above, just replace “0.0” with “NULL”
As above, make sure the database is setup to allow null values, and when sending your data, if your textbox.length != 0 then send the textbox value otherwise send DBNull.Value;