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What does a null value in DBMS represent?

Is it unassigned or inapplicable or zero or blank space?

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  • Can u specify which database particularly are u looking the answer for? Commented Feb 9, 2009 at 6:28
  • @goldenmean, it doesn't matter since it's DBMS-agnostic. NULL means the same on all DBMS' (or it should). Commented Feb 9, 2009 at 6:32

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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

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Should "and" be "and/or"? If so, I'll +1.
I'm not an expert in db theory but you sound right. I'll check and edit.
"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..)
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.
So from the context , you are right. (btw : this is from Codd's 12 rules : scs.carleton.ca/~bertossi/integra08/material/Codd12Rules.txt)
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I'm a student, also preparing myself for OCA now-days. As far as nulls are concerned the first thing to be kept in mind is that null is not a value. in the world of DBMS there are only two things either it has a value or it has a null. there are some points about null:-

  • null is not a value.
  • it is not compared with any value.
  • it doesn't occupy any space.
  • when we use null in our queries we have to associate it as "is & as".

For example:-

select * from employee where salary is null

note:- we can't write queries like,

select * from employee where salary=null:

(here "=" doesn't work)

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Special value that is supported by SQL is called as null which is used to represent values of attributes that are unknown or do not apply for that particular row

For example age of a particular student is not available in the age column of student table then it is represented as null but not as zero It is important to know that null values is always different from zero value A null value is used to represent the following different interpretations

Value unknown (value exists but is not known)

Value not available (exists but is purposely hidden)

Attribute not applicable (undefined for that row)

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