What is NULL value?

Answer 1

A NULL value in a table is a value in a field that appears to be blank which means A field with a NULL value is a field with no value.

It is very important to understand that a NULL value is different than a zero value or a field that contains spaces. A field with a NULL value is one that has been left blank during record creation.

Answer 2

The nullability of a column determines if the rows in the table can contain a null value for that column. A null value, or NULL, is not the same as zero (0), blank, or a zero-length character string such as ""; NULL means that no entry has been made.

A value of NULL indicates the value is unknown. A value of NULL is different from an empty or zero value. No two null values are equal. Comparisons between two null values, or between a NULL and any other value, return unknown because the value of each NULL is unknown.

In general, avoid permitting null values because they incur more complexity in queries and updates and because there are other column options, such as PRIMARY KEY constraints, that cannot be used with nullable columns.

If it is possible that null values may be stored in your data, it is a good idea to create queries and data-modification statements that either eliminate NULLs or transform NULLs into some other value (if you do not want null values appearing in your data).

If a row is inserted but no value is included for a column that allows null values, the database engine supplies the value NULL (unless a DEFAULT definition or object exists). A column defined with the keyword NULL also accepts an explicit entry of NULL from the user, no matter what data type it is or if it has a default associated with it. The value NULL should not be placed within quotation marks because it will be interpreted as the character string 'NULL', rather than the null value.

Specifying a column as not permitting null values can help maintain data integrity by ensuring that a column in a row always contains data. If null values are not allowed, the user entering data in the table must enter a value in the column or the table row cannot be accepted into the database.

Note:  Columns defined with a PRIMARY KEY constraint or IDENTITY property cannot allow null values.