What does a null value in DBMS represent?
Is it unassigned or inapplicable or zero or blank space?
views:
665answers:
1
+8
A:
Null is a special marker used in 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 RDBMS support a representation of missing information and/or inapplicable information
More info here
Learning
2009-02-09 06:27:45
Should "and" be "and/or"? If so, I'll +1.
paxdiablo
2009-02-09 06:31:22
I'm not an expert in db theory but you sound right. I'll check and edit.
Learning
2009-02-09 06:34:15
"Null values (distinct from an empty character string or a string of blank characters and distinct from zero or any other number) are supported in a fully relational DBMS for representing missing information and inapplicable information in a systematic way, independent of the data type" (cont..)
Learning
2009-02-09 06:37:05
It may be applicable but missing, for example, I decline to tell you my wage. Or it may be inapplicable, for example, I'm a contractor so my annual wage column is NULL but my hourly rate column is not.
paxdiablo
2009-02-09 06:37:22
So from the context , you are right. (btw : this is from Codd's 12 rules : http://www.scs.carleton.ca/~bertossi/integra08/material/Codd12Rules.txt)
Learning
2009-02-09 06:37:52
Misreading on my part - I thought you meant a column was NULL if it was both missing and inapplicable. Your clarification means NULL columns can be used for missing and inapplicable situations (i.e., one or both). My bad.
paxdiablo
2009-02-09 06:38:54
I simply think of NULLs as of 'unknown values'
Valentin Vasiliev
2009-02-09 07:24:39