Null value means No value Inapplicable,unassigned, unknown, or unavailable Which is true?

rdbms

antony

Reputation:

Null value in Database

Null value means

No value
Inapplicable,unassigned, unknown, or unavailable

Which is true?

Upvotes: 3

Answers (12)

Steven Huwig

Reputation: 20784

The only answer supported by SQL semantics is "unknown." If it meant "no value," then

'Hi there' = NULL

would return FALSE, but it returns NULL. This is because the NULL value in the expression means an unknown value, and the unknown value could very well be 'Hi there' as far as the system knows.

Upvotes: 1

user117936

Reputation: 29

null indicates that a data value does not exist in the database, thus representing missing information.

Also allows for three-way truth value; true, false and unknown.

Upvotes: 1

finnw

Reputation: 48629

It can mean any of those things (and it is not always obvious which), which is one argument against using nulls at all.

See: http://en.wikipedia.org/wiki/Null_(SQL)#Controversy

Upvotes: 2

RichardOD

Reputation: 29157

Obviously you have the DB definition of what null means, however to an application it can mean anything. I once worked on a strange application (disclaimer- I didn't design it), that used null in a junction table to represent all of the options (allegedly it was designed this way to "save space"). This was a DB design for user and role management.

So null in this case meant the user was in all roles. That's one for daily WTF. :-)

Like many people I tend to avoid using nulls where realistically possible.

Upvotes: 1

Chris W

Reputation: 3314

It's all about the context in which it's used. A null means there is no value but the reason for this will depend on the domain in which it is being used. In many cases the items you've listed are all valid uses of a null.

Upvotes: 3

Pierre

Reputation: 35246

all :-) if you want to add a semantic meaning to your field, add an ENUM

create TABLE myTable
  (
  myfield varchar(50)
  myfieldType enum ('OK','NoValue','InApplicable','Unassigned','Unknown','Unavailable') NOT NULL
  )

Upvotes: 0

Stuart

Reputation: 12215

The NULL SQL keyword is used to represent either a missing value or a value that is not applicable in a relational table

Upvotes: 0

Jeff Meatball Yang

Reputation: 39027

Null is a testable state of a column in a row, but it has no value itself.

By example:

An int can be only ...,0,1,2,3,... and also NULL.

An datetime can be only a valid date... and also NULL.

A bit can be only 0 or 1... and also NULL.

An varchar can be a string... and also NULL.

see the pattern?

You can make a column NOT NULL-able so that you can force a column to take a value.

Upvotes: 0

Johnno Nolan

Reputation: 29659

Null means nothing, unknown and no value.

It does not mean unavailable or in applicable.

Upvotes: 0

Barry Gallagher

Reputation: 6246

NULL, in the relational model, means Unknown. It's a mark that appears instead of a value wherever a value can appear in SQL.

Upvotes: 0

1800 INFORMATION

Reputation: 135285

From Wikipedia

Null is a special marker used in Structured Query Language (SQL) to indicate that a data value does not exist in the database. Introduced by the creator of the relational database model, E. F. Codd, SQL Null serves to fulfill the requirement that all true relational database management systems (RDBMS) support a representation of "missing information and inapplicable information". Codd also introduced the use of the lowercase Greek omega (ω) symbol to represent Null in database theory. NULL is also an SQL reserved keyword used to identify the Null special marker.

Upvotes: 1

Alex Taylor

Reputation: 7168

NULL is a representation that a field has not had a value set, or has been re-set to NULL. It is not unknown or unavailable.

Note, that when looking for NULL values, do not use '=' in a where clause, use 'is', e.g.:

select * from User where username is NULL;

Not:

select * from User where username = NULL;

Upvotes: 0

Null value in Database

Answers (12)

Related Questions