Values are inserted in a row according to what is specified in the INSERT statement. If no value is specified for a column, the inserted value depends on column settings such as whether to allow NULLs, whether to use a DEFAULT, and so on. Sometimes the insert operation fails and an error is returned. The following table shows the possible outcomes depending on the value being inserted (if any) and the column settings:

By default, columns allow NULL values unless you explicitly state NOT NULL in the column definition when creating a table. You can alter this default using the allow_nulls_by_default option. You can also alter whether a specific column allows NULLs using the ALTER TABLE statement. See allow_nulls_by_default option and ALTER TABLE statement.

You can create constraints for a column or domain. Constraints govern the kind of data you can or cannot add. See Using table and column constraints.

You can explicitly insert NULL into a column by entering NULL. Do not enclose this in quotes, or it will be taken as a string. For example, the following statement explicitly inserts NULL into the DepartmentHeadID column:

INSERT INTO Departments
VALUES ( 703, 'Western Sales', NULL );

You can define a column so that, even though the column receives no value, a default value automatically appears whenever a row is inserted. You do this by supplying a default for the column. See Using column defaults.