How are the NULL and Empty Varchar values stored in SQL Server. And in case I have no user entry for a string field on my UI, should I store a NULL or a '' ?
There's a nice article here which discusses this point. Key things to take away are that there is no difference in table size, however some users prefer to use an empty string as it can make queries easier as there is not a NULL check to do. You just check if the string is empty. Another thing to note is what NULL means in the context of a relational database. It means that the pointer to the character field is set to 0x00 in the row's header, therefore no data to access.
Update
There's a detailed article here which talks about what is actually happening on a row basis
Each row has a null bitmap for columns that allow nulls. If the row in
that column is null then a bit in the bitmap is 1 else it's 0.
For variable size datatypes the acctual size is 0 bytes.
For fixed size datatype the acctual size is the default datatype size
in bytes set to default value (0 for numbers, '' for chars).
the result of DBCC PAGE shows that both NULL and empty strings both take up zero bytes.
Be careful with nulls and checking for inequality in sql server.
For example
select * from foo where bla <> 'something'
will NOT return records where bla is null. Even though logically it should.
So the right way to check would be
select * from foo where isnull(bla,'') <> 'something'
Which of course people often forget and then get weird bugs.
The conceptual differences between NULL and "empty-string" are real and very important in database design, but often misunderstood and improperly applied - here's a short description of the two:
NULL - means that we do NOT know what the value is, it may exist, but it may not exist, we just don't know.
Empty-String - means we know what the value is and that it is nothing.
Here's a simple example:
Suppose you have a table with people's names including separate columns for first_name, middle_name, and last_name. In the scenario where first_name = 'John', last_name = 'Doe', and middle_name IS NULL, it means that we do not know what the middle name is, or if it even exists. Change that scenario such that middle_name = '' (i.e. empty-string), and it now means that we know that there is no middle name.
I once heard a SQL Server instructor promote making every character type column in a database required, and then assigning a DEFAULT VALUE to each of either '' (empty-string), or 'unknown'. In stating this, the instructor demonstrated he did not have a clear understanding of the difference between NULLs and empty-strings. Admittedly, the differences can seem confusing, but for me the above example helps to clarify the difference. Also, it is important to understand the difference when writing SQL code, and properly handle for NULLs as well as empty-strings.
An empty string is a string with zero length or no character.
Null is absence of data.
NULL values are stored separately in a special bitmap space for all the columns.
If you do not distinguish between NULL and '' in your application, then I would recommend you to store '' in your tables (unless the string column is a foreign key, in which case it would probably be better to prohibit the column from storing empty strings and allow the NULLs, if that is compatible with the logic of your application).
NULL is a non value, like undefined. '' is a empty string with 0 characters.
The value of a string in database depends of your value in your UI, but generally, it's an empty string '' if you specify the parameter in your query or stored procedure.
if it's not a foreign key field, not using empty strings could save you some trouble. only allow nulls if you'll take null to mean something different than an empty string. for example if you have a password field, a null value could indicate that a new user has not created his password yet while an empty varchar could indicate a blank password. for a field like "address2" allowing nulls can only make life difficult. things to watch out for include null references and unexpected results of = and <> operators mentioned by Vagif Verdi, and watching out for these things is often unnecessary programmer overhead.
edit: if performance is an issue see this related question: Nullable vs. non-null varchar data types - which is faster for queries?
In terms of having something tell you, whether a value in a VARCHAR column has something or nothing, I've written a function which I use to decide for me.
CREATE FUNCTION [dbo].[ISNULLEMPTY](#X VARCHAR(MAX))
RETURNS BIT AS
BEGIN
DECLARE #result AS BIT
IF #X IS NOT NULL AND LEN(#X) > 0
SET #result = 0
ELSE
SET #result = 1
RETURN #result
END
Now there is no doubt.
How are the "NULL" and "empty varchar" values stored in SQL Server.
Why would you want to know that? Or in other words, if you knew the answer, how would you use that information?
And in case I have no user entry for a string field on my UI, should I store a NULL or a ''?
It depends on the nature of your field. Ask yourself whether the empty string is a valid value for your field.
If it is (for example, house name in an address) then that might be what you want to store (depending on whether or not you know that the address has no house name).
If it's not (for example, a person's name), then you should store a null, because people don't have blank names (in any culture, so far as I know).
Related
I'm not asking if it does. I know that it doesn't.
I'm curious as to the reason. I've read support docs such as as this one on Working With Nulls in MySQL but they don't really give any reason. They only repeat the mantra that you have to use "is null" instead.
This has always bothered me. When doing dynamic SQL (those rare times when it has to be done) it would be so much easier to pass "null" into where clause like this:
#where = "where GroupId = null"
Which would be a simple replacement for a regular variable. Instead we have to use if/else blocks to do stuff like:
if #groupId is null then
#where = "where GroupId is null"
else
#where = "where GroupId = #groupId"
end
In larger more-complicated queries, this is a huge pain in the neck. Is there a specific reason that SQL and all the major RDBMS vendors don't allow this? Some kind of keyword conflict or value conflict that it would create?
Edit:
The problem with a lot of the answers (in my opinion) is that everyone is setting up an equivalency between null and "I don't know what the value is". There's a huge difference between those two things. If null meant "there's a value but it's unknown" I would 100% agree that nulls couldn't be equal. But SQL null doesn't mean that. It means that there is no value. Any two SQL results that are null both have no value. No value does not equal unknown value. Two different things. That's an important distinction.
Edit 2:
The other problem I have is that other HLLs allow null=null perfectly fine and resolve it appropriately. In C# for instance, null=null returns true.
The reason why it's off by default is that null is really not equal to null in a business sense. For example, if you were joining orders and customers:
select * from orders o join customers c on c.name = o.customer_name
It wouldn't make a lot of sense to match orders with an unknown customer with customers with an unknown name.
Most databases allow you to customize this behaviour. For example, in SQL Server:
set ansi_nulls on
if null = null
print 'this will not print'
set ansi_nulls off
if null = null
print 'this should print'
Equality is something that can be absolutely determined. The trouble with null is that it's inherently unknown. If you follow the truth table for three-value logic, null combined with any other value is null - unknown. Asking SQL "Is my value equal to null?" would be unknown every single time, even if the input is null. I think the implementation of IS NULL makes it clear.
It's a language semantic.
Null is the lack of a value.
is null makes sense to me. It says, "is lacking a value" or "is unknown". Personally I've never asked somebody if something is, "equal to lacking a value".
I can't help but feel that you're still not satisfied with the answers that have been given so far, so I thought I'd try another tack. Let's have an example (no, I've no idea why this specific example has come into my head).
We have a table for employees, EMP:
EMP
---
EMPNO GIVENNAME
E0001 Boris
E0002 Chris
E0003 Dave
E0004 Steve
E0005 Tony
And, for whatever bizarre reason, we're tracking what colour trousers each employee chooses to wear on a particular day (TROUS):
TROUS
-----
EMPNO DATE COLOUR
E0001 20110806 Brown
E0002 20110806 Blue
E0003 20110806 Black
E0004 20110806 Brown
E0005 20110806 Black
E0001 20110807 Black
E0003 20110807 Black
E0004 20110807 Grey
I could go on. We write a query, where we want to know the name of every employee, and what colour trousers they had on on the 7th August:
SELECT e.GIVENNAME,t.COLOUR
FROM
EMP e
LEFT JOIN
TROUS t
ON
e.EMPNO = t.EMPNO and
t.DATE = '20110807'
And we get the result set:
GIVENNAME COLOUR
Chris NULL
Steve Grey
Dave Black
Boris Black
Tony NULL
Now, this result set could be in a view, or CTE, or whatever, and we might want to continue asking questions about these results, using SQL. What might some of these questions be?
Were Dave and Boris wearing the same colour trousers on that day? (Yes, Black==Black)
Were Dave and Steve wearing the same colour trousers on that day? (No, Black!=Grey)
Were Boris and Tony wearing the same colour trousers on that day? (Unknown - we're trying to compare with NULL, and we're following the SQL rules)
Were Boris and Tony not wearing the same colour trousers on that day? (Unknown - we're again comparing to NULL, and we're following SQL rules)
Were Chris and Tony wearing the same colour trousers on that day? (Unknown)
Note, that you're already aware of specific mechanisms (e.g. IS NULL) to force the outcomes you want, if you've designed your database to never use NULL as a marker for missing information.
But in SQL, NULL has been given two roles (at least) - to mark inapplicable information (maybe we have complete information in the database, and Chris and Tony didn't turn up for work that day, or did but weren't wearing trousers), and to mark missing information (Chris did turn up that day, we just don't have the information recorded in the database at this time)
If you're using NULL purely as a marker of inapplicable information, I assume you're avoiding such constructs as outer joins.
I find it interesting that you've brought up NaN in comments to other answers, without seeing that NaN and (SQL) NULL have a lot in common. The biggest difference between them is that NULL is intended for use across the system, no matter what data type is involved.
You're biggest issue seems to be that you've decided that NULL has a single meaning across all programming languages, and you seem to feel that SQL has broken that meaning. In fact, null in different languages frequently has subtly different meanings. In some languages, it's a synonym for 0. In others, not, so the comparison 0==null will succeed in some, and fail in others. You mentioned VB, but VB (assuming you're talking .NET versions) does not have null. It has Nothing, which again is subtly different (it's the equivalent in most respects of the C# construct default(T)).
The concept is that NULL is not an equitable value. It denotes the absence of a value.
Therefore, a variable or a column can only be checked if it IS NULL, but not if it IS EQUAL TO NULL.
Once you open up arithmetic comparisions, you may have to contend with IS GREATER THAN NULL, or IS LESS THAN OR EQUAL TO NULL
NULL is unknown. It is neither true nor false so when you are comparing anything to unknown, the only answer is "unknown" Much better article on wikipedia http://en.wikipedia.org/wiki/Null_(SQL)
Because in ANSI SQL, null means "unknown", which is not a value. As such, it doesn't equal anything; you can just evaluate the value's state (known or unknown).
a. Null is not the "lack of a value"
b. Null is not "empty"
c. Null is not an "unset value"
It's all of the above and none of the above.
By technical rights, NULL is an "unknown value". However, like uninitialized pointers in C/C++, you don't really know what your pointing at. With databases, they allocate the space but do not initialize the value in that space.
So, it is an "empty" space in the sense that it's not initialized. If you set a value to NULL, the original value stays in that storage location. If it was originally an empty string (for example), it will remain that.
It's a "lack of a value" in the fact that it hasn't been set to what the database deems a valid value.
It's an "unset value" in that if the space was just allocated, the value that is there has never been set.
"Unknown" is the closest that we can truly come to knowing what to expect when we examine a NULL.
Because of that, if we try to compare this "unknown" value, we will get a comparison that
a) may or may not be valid
b) may or may not have the result we expect
c) may or may not crash the database.
So, the DBMS systems (long ago) decided that it doesn't even make sense to use equality when it comes to NULL.
Therefore, "= null" makes no sense.
In addition to all that has already been said, I wish to stress that what you write in your first line is wrong. SQL does support the “= NULL” syntax, but it has a different semantic than “IS NULL” – as can be seen in the very piece of documentation you linked to.
I agree with the OP that
where column_name = null
should be syntactic sugar for
where column_name is null
However, I do understand why the creators of SQL wanted to make the distinction. In three-valued logic (IMO this is a misnomer), a predicate can return two values (true or false) OR unknown which is technically not a value but just a way to say "we don't know which of the two values this is". Think about the following predicate in terms of three-valued logic:
A == B
This predicate tests whether A is equal to B. Here's what the truth table looks like:
T U F
-----
T | T U F
U | U U U
F | F U T
If either A or B is unknown, the predicate itself always returns unknown, regardless of whether the other one is true or false or unknown.
In SQL, null is a synonym for unknown. So, the SQL predicate
column_name = null
tests whether the value of column_name is equal to something whose value is unknown, and returns unknown regardless of whether column_name is true or false or unknown or anything else, just like in three-valued logic above. SQL DML operations are restricted to operating on rows for which the predicate in the where clause returns true, ignoring rows for which the predicate returns false or unknown. That's why "where column_name = null" doesn't operate on any rows.
NULL doesn't equal NULL. It can't equal NULL. It doesn't make sense for them to be equal.
A few ways to think about it:
Imagine a contacts database, containing fields like FirstName, LastName, DateOfBirth and HairColor. If I looked for records WHERE DateOfBirth = HairColor, should it ever match anything? What if someone's DateOfBirth was NULL, and their HairColor was too? An unknown hair color isn't equal to an unknown anything else.
Let's join the contacts table with purchases and product tables. Let's say I want to find all the instances where a customer bought a wig that was the same color as their own hair. So I query WHERE contacts.HairColor = product.WigColor. Should I get matches between every customer I don't know the hair color of and products that don't have a WigColor? No, they're a different thing.
Let's consider that NULL is another word for unknown. What's the result of ('Smith' = NULL)? The answer is not false, it's unknown. Unknown is not true, therefore it behaves like false. What's the result of (NULL = NULL)? The answer is also unknown, therefore also effectively false. (This is also why concatenating a string with a NULL value makes the whole string become NULL -- the result really is unknown.)
Why don't you use the isnull function?
#where = "where GroupId = "+ isnull(#groupId,"null")
I want to find out using a select statement what columns in a table share similar information.
Example: Classes table with ClassID, ClassName, ClassCode, ClassDescription columns.
This was part of my SQL class that I already turned in. The question asked "What classes are part of the English department?"
I used this Select statement:
SELECT *
FROM Classes
WHERE ClassName LIKE "English%" OR ClassCode LIKE "ENG%"
Granted we have only input one actual English course in this database, the end result was it executed fine and displayed everything for just the English class. Which I thought was a success since we did populate other non English courses in the database.
Anyways, I was told I should have used a BETWEEN statement.
I am just sitting here thinking they would both do what I needed them to do right?
I'm using SQL Server 2014
No, BETWEEN would probably be a bad idea here. BETWEEN doesn't allow wildcards and doesn't do any pattern matching in any RDBMS I've used. So you'd have to say BETWEEN 'ENG' AND 'English'. Except that doesn't return things like 'English I' (which would be after 'English' in a sorted list).
It would also potentially include something like 'Engineering' or 'Engaging Artistry', but that's a weakness of your existing query, too, since LIKE 'ENG%' matches those.
If you happen to be using a case-sensitive collation you add a whole new dimension of complexity. Your BETWEEN statement gets even more confusing. Just know that capital letters generally come before lower case letters, so 'ENGRAVING I' would be included but 'Engraving I' would not. Additionally, 'eng' would not be included. Note that case-insensitive collation is the default.
Also whats the difference when searching for null values in one table
and one column
column_name =''
or
column_name IS NULL
You're not understanding the difference between an empty string and null.
An empty string is explicit. It says "This field has a known value and it is a string of zero length."
A null string is imprecise. It means "unknown". It could mean "This value wasn't asked for," or "This value was not available," or "This value has not yet been determined," or "This values does not make sense for this record."
"What is this person's middle name?"
"He doesn't have one. See, his birth certificate has no middle name listed." --> Empty string
"I don't know. He never told me and I don't have any birth or identity record." --> NULL
Note that Oracle, due to backwards compatibility, treats empty strings as NULLs. This is explicitly against ANSI SQL, but since Oracle is that old and that's how it's always worked that's how it will continue to work.
Another way to look at it is the example I tend to use with numbers. The difference between 0 and NULL is the difference between having a bank account with $0 balance and not having a bank account at all.
Nothing can be said unless we see table and its data.Though don't use between.
Secondly first find which of the column is not null by design.Say for example ClassName cannot be null then there is no use using ClassCode LIKE "ENG%",just ClassName LIKE "English%" is enough,similarly vice versa is also true.
Thirdly you should use same parameter in both column.for example
ClassName LIKE "English%" OR ClassCode LIKE "English%"
see the difference.
Select * FROM Classes
Where ClassName LIKE "%English%"
I've just come across an interesting scenario on how NULL is handled in T-SQL (and possibly other forms of SQL). The issue is pretty well described and answered by this question and I've illustrated the issue below;
-- SET ANSI_NULLS ON -- Toggle this between ON/OFF to see how it changes behaviour
DECLARE #VAR1 DATETIME
DECLARE #VAR2 DATETIME
SET #VAR1 = (SELECT CURRENT_TIMESTAMP)
SET #VAR2 = (SELECT NULL)
-- This will return 1 when ansi_nulls is off and nothing when ansi_nulls is on
SELECT 1 WHERE #VAR1 != #VAR2
DECLARE #TstTable TABLE (
COL1 DATETIME,
COL2 DATETIME)
INSERT INTO #TstTable
SELECT #VAR1, #VAR1
UNION
SELECT #VAR1, NULL
-- This won't ever return a value irrespective of the ansi_nulls setting
SELECT * FROM #TstTable WHERE COL1 != COL2
This situation led me to question my understanding of null representations specifically within SQL. I've always understood null to mean that it has no value. This seems to be an incorrect assumption given the first paragraph of this page. It states (my emphasis...I could quite easily just highlight the whole paragraph though);
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.
Does this hold true for T-SQL variable conditions also? It certainly does for my SELECT 1 WHERE #VAR1 != #VAR2 example above, but I don't understand why NULL in this instance is considered "UNKNOWN" and not empty/uninitialised/nothing etc. I know ANSI_NULLS changes how this works, but it is deprecated and will be removed from some future version.
Can someone offer a good explanation as to why NULL in T-SQL refers to an unknown value rather than an uninitialised value? If so, can you extend your answer to show why T-SQL variables with a NULL value are also considered to be unknown?
In SQL, we're interested in storing facts in tables (a.k.a relations).
What Codd asked for was:
Rule 3: Systematic treatment of null values:
The DBMS must allow each field to remain null (or empty). Specifically, it must support a representation of "missing information and inapplicable information" that is systematic, distinct from all regular values (for example, "distinct from zero or any other number", in the case of numeric values), and independent of data type. It is also implied that such representations must be manipulated by the DBMS in a systematic way.
What we've ended up with is three-valued logic (as #zmbq stated). Why is it this way?
We have two items that we're trying to compare for equality. Are they equal? Well, it turns out that we don't (yet) know what item 1 is, and we don't (yet) know what item 2 is (both are NULL). They might be equal. They might be unequal. It would be equally wrong to answer the equality comparison with either TRUE or FALSE. So we answer UNKNOWN.
In other languages, null is usually used with pointers (or references in languages without pointers, but notably not C++), to indicate that the pointer does not, at this time, point to anything.
Welcome to Three Valued Logic, where everything can be true, false or unknown.
The value of the null==null is not true, and it's not false, it's unknown...
but I don't understand why NULL in this instance is considered "UNKNOWN" and not
empty/uninitialised/nothing
?? What is there not to understand. It is like that BECAUSE IT WAS DEFINED LIKE THAT. Someone had the idea it is like that. It was put into the standard.
Yes, this is a little recursive, but quite often design decisions run like that.
This has more to do with arithmetics. Sum of 20 rows with one Null is Null - how would you treat it as unknown? C# etc. react with an exception, but that gets in your way when doing statistical analysis. Uknonwn values have tto move all they come in contact with into unknown, and no unknown is ever the same.
Yes, another NULL vs empty string question.
I agree with the idea that NULL means not set, while empty string means "a value that is empty". Here's my problem: If the default value for a column is NULL, how do I allow the user to enter that NULL.
Let's say a new user is created on a system. There is a first and last name field; last name is required while first name is not. When creating the user, the person will see 2 text inputs, one for first and one for last. The person chooses to only enter the last name. The first name is technically not set. During the insert I check the length of each field, setting all fields that are empty to NULL.
When looking at the database, I see that the first name is not set. The question that immediately comes to mind is that maybe they never saw the first name field (ie, because of an error). But this is not the case; they left if blank.
So, my question is, how do you decide when a field should be set to NULL or an empty string when receiving user input? How do you know that the user wants the field to be not set without detecting focus or if they deleted a value...or...or...?
Related Question: Should I use NULL or an empty string to represent no data in table column?
I'll break the pattern, and say that I would always use NULL for zero-length strings, for the following reasons.
If you start fine-slicing the implications of blanks, then you must ensure somehow that every other developer reads and writes it the same way.
How do you alphabetize it?
Can you unambiguously determine when a user omitted entering a value, compared with intentionally leaving it blank?
How would you unambiguously query for the difference? Can a query screen user indicate NULL vs. blank using standard input form syntax?
In practice, I've never been prohibited from reading and writing data using default, unsurprising behavior using this rule. If I've needed to know the difference, I've used a boolean field (which is easier to map to unambiguous UI devices). In one case, I used a trigger to enforce True => null value, but never saw it invoked because the BR layer filtered out the condition effectively.
If the user provides an empty string, I always treat it as null from the database perspective. In addition, I will typically trim my string inputs to remove leading/trailing spaces and then check for empty. It's a small win in the database with varchar() types and it also reduces the cases for search since I only need to check for name is null instead of name is null or name = '' You could also go the other way, converting null to ''. Either way, choose a way and be consistent.
What you need to do is figure out what behavior you want. There is no one fixed algebra of how name strings are interpreted.
Think about the state machine here: you have fields which have several states: it slounds like you're thinking about a state of "unitialized", another of "purposefully empty" and a third with some set value. ANYTHING you do that makes that assignment and is consistent with the rest of your program will be find; it sounds like the easy mapping is
NULL → uninitialized
"" → purposefully unset
a name → initialized.
I almost never use NULL when referring to actual data. When used for foreign keys, I would say that NULL is valid, but it is almost never valid for user entered data. The one exception that would probably come up quite regularly is for dates that don't exist, such as an employee database with a "termination_date" field. In that case, all current employees should have a value of NULL in that field. As for getting them to actually enter a null value, for the values that truly require a null value, I would put a checkbox next to the input field so that the user can check it on and off to see the corresponding value to null (or in a more user friendly manner, none). When enabling the checkbox to set the field to null, the corresponding text box should be disabled, and if a null value is already associated, it should start out as disabled, and only become enabled once the user unchecks the null checkbox.
I try to keep things simple. In this case, I'd make the first-name column not-nullable and allow blanks. Otherwise, you'll have three cases to deal with anywhere you refer to this field:
Blank first name
Null first name
Non-blank first name
If you go with 'blank is null' or 'null is blank' then you're down to two cases. Two cases are better than three.
To further answer your question: the user entering data probably doesn't (and shouldn't) know anything about what a "null" is and how it compares to an "empty". This issue should be resolved cleanly and consistently in the system--not the UI.
While your example is mostly for strings, I like to say that I use null for numerical and boolean fields.
An accountbalance of 0 is very different to me as one that is null. Same for booleans, if people take a multiple choice test with true and false answers, it is very important to know whether someone answered true or false or didn't answer at all.
Not using null in these cases would require me to have an extra table or a different setup to see whether someone answered a question. You could use for instance -1 for not filled in 0 for false and 1 for true, but then you're using a numerical field for something that's essentially a boolean.
I've never, ever had a use for a NULL value in production code. An empty string is a fine sentinel value for a blank name field, phone number, or annual income for any application. That said, I'm sure you could find some use for it, but I just think it's overused. If I were to use a NULL value, however, I imagine I'd use it anywhere I want to represent an empty value.
I have always used NULL for uninitialized values, empty for purposely empty values and 0 for off indicators.
By doing this all the time, it is there even if I am not using it, but I don't have to do anything different if I need that distinction.
I am usually testing for empty(), but sometimes I check for isset() which evaluates false on NULL. This is useful for reminders to answer certain questions. If it is empty, false or 0 then the question is answered.
I'm curious to know how NULLs are stored into a database ?
It surely depends on the database server but I would like to have an general idea about it.
First try:
Suppose that the server put a undefined value (could be anything) into the field for a NULL value.
Could you be very lucky and retrieve the NULL value with
...WHERE field = 'the undefined value (remember, could be anything...)'
Second try:
Does the server have a flag or any meta-data somewhere to indicate this field is NULL ?
Then the server must read this meta data to verify the field.
If the meta-data indicates a NULL value and if the query doesn't have "field IS NULL",
then the record is ignored.
It seems too easy...
MySql uses the second method. It stores an array of bits (one per column) with the data for each row to indicate which columns are null and then leaves the data for that field blank. I'm pretty sure this is true for all other databases as well.
The problem with the first method is, are you sure that whatever value you select for your data won't show up as valid data? For some values (like dates, or floating point numbers) this is true. For others (like integers) this is false.
On PostgreSQL, it uses an optional bitmap with one bit per column (0 is null, 1 is not null). If the bitmap is not present, all columns are not null.
This is completely separate from the storage of the data itself, but is on the same page as the row (so both the row and the bitmap are read together).
References:
http://www.postgresql.org/docs/8.3/interactive/storage-page-layout.html
The server typically uses meta information rather than a magic value. So there's a bit off someplace that specifies whether the field is null.
-Adam
IBM Informix Dynamic Server uses special values to indicate nulls. For example, the valid range of values for a SMALLINT (16-bit, signed) is -32767..+32767. The other value, -32768, is reserved to indicate NULL. Similarly for INTEGER (4-byte, signed) and BIGINT (8-byte, signed). For other types, it uses other special representations (for example, all bits 1 for SQL FLOAT and SMALLFLOAT - aka C double and float, respectively). This means that it doesn't have to use extra space.
IBM DB2 for Linux, Unix, Windows uses extra bytes to store the null indicators; AFAIK, it uses a separate byte for each nullable field, but I could be wrong on that detail.
So, as was pointed out, the mechanisms differ depending on the DBMS.
The problem with special values to indicate NULL is that sooner or later that special value will be inserted. For example, it will be inserted into a table specifying the special NULL indicators for different database servers
| DBServer | SpecialValue |
+--------------+--------------+
| 'Oracle' | 'Glyph' |
| 'SQL Server' | 'Redmond' |
;-)