What is null value?

[Swati]

Hello friends!

Can you guys tell me what is Null value.

Regards,

[Geo3213]

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.

[Leena]

null is a value that is unknown , unavailable or
inapplicable. Null is not same as zero or blank space.

[Matellis]

Many argue that it should be allowed to have NULL values in a database. The reality is; all database systems I know of, allow NULL values for columns. But is it right; or even better; are NULL values at all allowed at all according to the relational model?

I should make you aware of the fact that the term "NULL values" is actually incorrect: NULL means "unknown", and as such, it can not be considered a "value" at all.

The relational model states that every attribute has a value for a given occurence (row/tuple). (Remember, NULL is not a value, it is something unknown). NULL means something is unknown. It does NOT mean null (the digit 0). Null (or zero, the digit) is a known value: You can include 0 in calculations, whilst NULL stands for "value is unknown". There is a world of difference between the two.

Please refer this webpage for further details:
http://www.databasedesign-resource.c...-database.html