Why does NULL NULL evaluate to false in SQL server

Question

In SQL server if you have  nullParam NULL in a where clause  it always evaluates to false  This is counterintuitive and has caused me many errors  I do understand the IS NULL and IS NOT NULL keywords are the correct way to do it  But why does SQL server behave this way

User · Answer

Maybe it depends  but I thought NULL NULL evaluates to NULL like most operations with NULL as an operand

User · Answer

Think of the null as  unknown  in that case  or  does not exist     In either of those cases  you can t say that they are equal  because you don t know the value of either of them   So  null null evaluates to not true  false or null  depending on your system   because you don t know the values to say that they ARE equal   This behavior is defined in the ANSI SQL-92 standard   EDIT  This depends on your ansi nulls setting   if you have ANSI NULLS off  this WILL evaluate to true   Run the following code for an example     set ansi nulls off  if null   null     print  true  else     print  false    set ansi nulls ON  if null   null     print  true  else     print  false

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At technet there is a good explanation for how null values work   Null means unknown    Therefore the Boolean expression    value null   does not evaluate to false  it evaluates to null  but if that is the final result of a where clause  then nothing is returned  That is a practical way to do it  since returning null would be difficult to conceive   It is interesting and very important to understand the following     If in a query we have    where  value  param Or  param is null  And id  anotherParam   and     value 1    param is null   id 123    anotherParam 123   then     value  param  evaluates to null   param is null  evaluates to true  id  anotherParam  evaluates to true    So the expression to be evaluated becomes     null Or true  And true    We might be tempted to think that here  null Or true  will be evaluated to null and thus the whole expression becomes null and the row will not be returned     This is not so  Why     Because  null Or true  evaluates to true  which is very logical  since if one operand is true with the Or-operator  then no matter the value of the other operand  the operation will return true  Thus it does not matter that the other operand is unknown  null     So we finally have true true and thus the row will be returned   Note  with the same crystal clear logic that  null Or true  evaluates to true   null And true  evaluates to null   Update  Ok  just to make it complete I want to add the rest here too which turns out quite fun in relation to the above      null Or false  evaluates to null   null And false  evaluates to false      The logic is of course still as self-evident as before

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The concept of NULL is questionable  to say the least  Codd introduced the relational model and the concept of NULL in context  and went on to propose more than one kind of NULL   However  relational theory has evolved since Codd s original writings  some of his proposals have since been dropped  e g  primary key  and others never caught on  e g  theta operators   In modern relational theory  truly relational theory  I should stress  NULL simply does not exist  See The Third Manifesto  http   www thethirdmanifesto com   The SQL language suffers the problem of backwards compatibility  NULL found its way into SQL and we are stuck with it  Arguably  the implementation of NULL in SQL is flawed  SQL Server s implementation makes things even more complicated due to its ANSI NULLS option     I recommend avoiding the use of NULLable columns in base tables     Although perhaps I shouldn t be tempted  I just wanted to assert a corrections of my own about how NULL works in SQL   NULL   NULL evaluates to UNKNOWN   UNKNOWN is a logical value    NULL is a data value   This is easy to prove e g    SELECT NULL   NULL   correctly generates an error in SQL Server  If the result was a data value then we would expect to see NULL  as some answers here  wrongly  suggest we would   The logical value UNKNOWN is treated differently in SQL DML and SQL DDL respectively   In SQL DML  UNKNOWN causes rows to be removed from the resultset    For example   CREATE TABLE MyTable    key col INTEGER NOT NULL UNIQUE    data col INTEGER  CHECK  data col   55      INSERT INTO MyTable  key col  data col     VALUES  1  NULL     The INSERT succeeds for this row  even though the CHECK condition resolves to NULL   NULL  This is due defined in the SQL-92   ANSI   Standard      11 6 table constraint definition      3       If the table constraint is a check   constraint definition  then let SC be   the search condition immediately   contained in the check constraint   definition and let T be the table name   included in the corresponding table   constraint descriptor  the table   constraint is not satisfied if and   only if      EXISTS   SELECT   FROM T WHERE NOT     SC          is true    Read that again carefully  following the logic   In plain English  our new row above is given the  benefit of the doubt  about being UNKNOWN and allowed to pass   In SQL DML  the rule for the WHERE clause is much easier to follow      The search condition is applied to   each row of T  The result of the where   clause is a table of those rows of T   for which the result of the search   condition is true    In plain English  rows that evaluate to UNKNOWN are removed from the resultset

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Just an addition to other wonderful answers   AND  The result of true and unknown is unknown  false and unknown is false  while unknown and unknown is unknown   OR  The result of true or unknown is true  false or unknown is unknown  while unknown or unknown is unknown   NOT  The result of not unknown is unknown

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The answers here all seem to come from a CS perspective so I want to add one from a developer perspective   For a developer NULL is very useful  The answers here say NULL means unknown  and maybe in CS theory that s true  don t remember  it s been a while  In actual development though  at least in my experience  that happens about 1  of the time  The other 99  it is used for cases where the value is not UNKNOWN but it is KNOWN TO BE ABSENT   For example    Client LastPurchase  for a new client  It is not unknown  it is known that he hasn t made a purchase yet  When using an ORM with a Table per Class Hierarchy mapping  some values are just not mapped for certain classes  When mapping a tree structure a root will usually have Parent   NULL And many more      I m sure most developers at some point wrote WHERE value   NULL  didn t get any results  and that s how they learned about IS NULL syntax  Just look how many votes this question and the linked ones have   SQL Databases are a tool  and they should be designed the way which is easiest for their users to understand

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To quote the Christmas analogy again  In SQL  NULL basically means  quot closed box quot   unknown   So  the result of comparing two closed boxes will also be unknown  null   I understand  for a developer  this is counter-intuitive  because in programming languages  often NULL rather means  quot empty box quot   known   And comparing two empty boxes will naturally yield true   equal  This is why JavaScript for example distinguishes between null and undefined

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If you are looking for an expression returning true for two NULLs you can use   SELECT 1  WHERE EXISTS       SELECT NULL     INTERSECT     SELECT NULL     It is helpful if you want to replicate data from one table to another

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How old is Frank  I don t know  null    How old is Shirley  I don t know  null    Are Frank and Shirley the same age    Correct answer should be  I don t know   null   not  no   as Frank and Shirley might be the same age  we simply don t know

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The equality test  for example  in a case statement when clause  can be changed from   XYZ   NULL    to   XYZ IS NULL   If I want to treat blanks and empty string as equal to NULL I often also use an equality test like    NULLIF ltrim  XYZ       IS NULL

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You work for the government registering information about citizens  This includes the national ID for every person in the country  A child was left at the door of a church some 40 years ago  nobody knows who their parents are  This person s father ID is NULL  Two such people exist  Count people who share the same father ID with at least one other person  people who are siblings   Do you count those two too    The answer is no  you don   t  because we don   t know if they are siblings or not    Suppose you don   t have a NULL option  and instead use some pre-determined value to represent    the unknown     perhaps an empty string or the number 0 or a   character  etc  Then you would have in your queries that        0   0  and                  etc  This is not what you want  as per the example above   and as you might often forget about these cases  the example above is a clear fringe case outside ordinary everyday thinking   then you need the language to remember for you that NULL   NULL is not true   Necessity is the mother of invention

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Just because you don t know what two things are  does not mean they re equal  If when you think of NULL you think of    NULL     string  then you probably want a different test of equality like Postgresql s IS DISTINCT FROM AND IS NOT DISTINCT FROM From the PostgreSQL docs on  quot Comparison Functions and Operators quot   expression IS DISTINCT FROM expression expression IS NOT DISTINCT FROM expression For non-null inputs  IS DISTINCT FROM is the same as the  lt  gt  operator  However  if both inputs are null it returns false  and if only one input is null it returns true  Similarly  IS NOT DISTINCT FROM is identical to   for non-null inputs  but it returns true when both inputs are null  and false when only one input is null  Thus  these constructs effectively act as though null were a normal data value  rather than  quot unknown quot

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null is  unknown in sql so we cant expect two unknowns to be same   However you can get that behavior by setting ANSI NULLS to Off its On by Default  You will be able to use   operator for nulls  SET ANSI NULLS off if null null print 1 else  print 2 set ansi nulls on if null null print 1 else  print 2

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Because NULL means  unknown value  and two unknown values cannot be equal   So  if to our logic NULL N  1 is equal to NULL N  2  then we have to tell that somehow   SELECT 1 WHERE ISNULL nullParam1  -1    ISNULL nullParam2  -1    where known value -1 N  1 is equal to -1 N  2

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Here I will hopefully clarify my position   That NULL   NULL evaluate to FALSE is wrong  Hacker and Mister correctly answered NULL  Here is why  Dewayne Christensen wrote to me  in a comment to Scott Ivey      Since it s December  let s use a   seasonal example  I have two presents   under the tree  Now  you tell me if I   got two of the same thing or not    They can be different or they can be equal  you don t know until one open both presents  Who knows  You invited two people that don t know each other and both have done to you the same gift - rare  but not impossible      So the question  are these two UNKNOWN presents the same  equal      The correct answer is  UNKNOWN  i e  NULL    This example was intended to demonstrate that     false or null  depending on your system     is a correct answer - it is not  only NULL is correct in 3VL  or is ok for you to accept a system which gives wrong answers    A correct answer to this question must emphasize this two points    three-valued logic  3VL  is counterintuitive  see countless other questions on this subject on Stackoverflow and in other forum to make sure   SQL-based DBMSes often do not respect even 3VL  they give wrong answers sometimes  as  the original poster assert  SQL Server do in this case     So I reiterate  SQL does not any good forcing one to interpret the reflexive property of equality  which state that      for any x  x   x       in plain English  whatever the universe of discourse  a  thing  is always equal to itself        in a 3VL  TRUE  FALSE  NULL   The expectation of people would conform to 2VL  TRUE  FALSE  which even in SQL is valid for all other values   i e  x   x always evaluate to TRUE  for any possible value of x - no exceptions   Note also that NULLs are valid   non-values    as their apologists pretend them to be  which one can assign as attribute values     as part of relation variables  So they are acceptable values of every type  domain   not only of the type of logical expressions   And this was my point  NULL  as value  is a  strange beast   Without euphemism  I prefer to say  nonsense   I think that this formulation is much more clear and less debatable - sorry for my poor English proficiency   This is only one of the problems of NULLs  Better to avoid them entirely  when possible      we are concerned about values here  so the fact that the two presents are always two different physical objects are not a valid objection  if you are not convinced I m sorry  it is not this the place to explain the difference between value and  object  semantics  Relational Algebra has value semantics from the start - see Codd s information principle  I think that some SQL DBMS implementors don t even care about a common semantics         to my knowledge  this is an axiom accepted  in a form or another  but always interpreted in a 2VL  since antiquity and that exactly because is so intuitive  3VLs  is a family of logics in reality  is a much more recent development  but I m not sure when was first developed    Side note  if someone will introduce Bottom  Unit and Option Types as attempts to justify SQL NULLs  I will be convinced only after a quite detailed examination that will shows of how SQL implementations with NULLs have a sound type system and will clarify  finally  what NULLs  these  values-not-quite-values   really are     In what follow I will quote some authors  Any error or omission is probably mine and not of the original authors   Joe Celko on SQL NULLs  I see Joe Celko often cited on this forum  Apparently he is a much respected author here  So  I said to myself   what does he wrote about SQL NULLs  How does he explain NULLs numerous problems    One of my friend has an ebook version of Joe Celko s SQL for smarties  advanced SQL programming  3rd edition  Let s see   First  the table of contents  The thing that strikes me most is the number of times that NULL is mentioned and in the most varied contexts      3 4 Arithmetic and NULLs 109    3 5 Converting Values to and from NULL 110    3 5 1 NULLIF   Function 110    6 NULLs  Missing Data in SQL 185    6 4 Comparing NULLs 190    6 5 NULLs and Logic 190    6 5 1 NULLS in Subquery Predicates 191    6 5 2 Standard SQL Solutions 193    6 6 Math and NULLs 193    6 7 Functions and NULLs 193    6 8 NULLs and Host Languages 194    6 9 Design Advice for NULLs 195    6 9 1 Avoiding NULLs from the Host Programs 197    6 10 A Note on Multiple NULL Values 198    10 1 IS NULL Predicate 241    10 1 1 Sources of NULLs 242          and so on  It rings  nasty special case  to me   I will go into some of these cases with excerpts from this book  trying to limit myself to the essential  for copyright reasons  I think these quotes fall within  fair use  doctrine and they can even stimulate to buy the book - so I hope that no one will complain  otherwise I will need to delete most of it  if not all   Furthermore  I shall refrain from reporting code snippets for the same reason  Sorry about that  Buy the book to read about datailed reasoning   Page numbers between parenthesis in what follow      NOT NULL Constraint  11       The most important column constraint is the NOT NULL  which forbids   the use of NULLs in a column  Use this constraint routinely  and remove   it only when you have good reason  It will help you avoid the   complications of NULL values when you make queries against the data       It is not a value  it is a marker that holds a place where a value might go    Again this  value but not quite a value  nonsense  The rest seems quite sensible to me       12       In short  NULLs cause a lot of irregular features in SQL  which we will discuss   later  Your best bet is just to memorize the situations and the rules for NULLs   when you cannot avoid them    Apropos of SQL  NULLs and infinite       104  CHAPTER 3  NUMERIC DATA IN SQL      SQL has not accepted the IEEE model for mathematics for several reasons                If the IEEE rules for math were allowed in   SQL  then we would need type conversion rules for infinite and a way to   represent an infinite exact numeric value after the conversion  People   have enough trouble with NULLs  so let   s not go there    SQL implementations undecided on what NULL really means in particular contexts      3 6 2 Exponential Functions  116       The problem is that logarithms are undefined when  x  lt   0   Some SQL   implementations return an error message  some return a NULL and DB2    400  version 3 release 1 returned  NEGINF  short for    negative infinity       as its result    Joe Celko quoting David McGoveran and C  J  Date      6 NULLs  Missing Data in SQL  185       In their book A Guide to Sybase and SQL Server  David McGoveran   and C  J  Date said     It is this writer   s opinion than NULLs  at least as   currently defined and implemented in SQL  are far more trouble than   they are worth and should be avoided  they display very strange and   inconsistent behavior and can be a rich source of error and confusion     Please note that these comments and criticisms apply to any system   that supports SQL-style NULLs  not just to SQL Server specifically        NULLs as a drug addiction       186 187       In the rest of this book  I will be urging you not to use   them  which may seem contradictory  but it is not  Think of a NULL   as a drug  use it properly and it works for you  but abuse it and it can ruin   everything  Your best policy is to avoid NULLs when you can and use   them properly when you have to    My unique objection here is to  use them properly   which interacts badly with specific implementation behaviors      6 5 1 NULLS in Subquery Predicates  191 192       People forget that a subquery often hides a comparison with a NULL    Consider these two tables                The result will be empty  This is counterintuitive  but correct     separator      6 5 2 Standard SQL Solutions  193       SQL-92 solved some of the 3VL  three-valued logic  problems by adding   a new predicate of the form        lt search condition gt  IS  NOT  TRUE   FALSE   UNKNOWN   But UNKNOWN is a source of problems in itself  so that C  J  Date  in his book cited below  reccomends in chapter 4 5  Avoiding Nulls in SQL         Don t use the keyword UNKNOWN in any context whatsoever       Read  ASIDE  on UNKNOWN  also linked below      6 8 NULLs and Host Languages  194       However  you should know how NULLs are handled when they have   to be passed to a host program  No standard host language for   which an embedding is defined supports NULLs  which is another   good reason to avoid using them in your database schema     separator      6 9 Design Advice for NULLs  195       It is a good idea to declare all your base tables with NOT NULL   constraints on all columns whenever possible  NULLs confuse people   who do not know SQL  and NULLs are expensive    Objection  NULLs confuses even people that know SQL well  see below       195       NULLs should be avoided in FOREIGN KEYs  SQL allows this    benefit   of the doubt    relationship  but it can cause a loss of information in   queries that involve joins  For example  given a part number code in   Inventory that is referenced as a FOREIGN KEY by an Orders table  you   will have problems getting a listing of the parts that have a NULL  This is   a mandatory relationship  you cannot order a part that does not exist     separator      6 9 1 Avoiding NULLs from the Host Programs  197       You can avoid putting NULLs into the database from the Host Programs   with some programming discipline                   Determine impact of missing data on programming and reporting    Numeric columns with NULLs are a problem  because queries   using aggregate functions can provide misleading results        separator       227       The SUM   of an empty set is always NULL  One of the most common   programming errors made when using this trick is to write a query that   could return more than one row  If you did not think about it  you might   have written the last example as         separator      10 1 1 Sources of NULLs  242       It is important to remember where NULLs can occur  They are more than   just a possible value in a column  Aggregate functions on empty sets    OUTER JOINs  arithmetic expressions with NULLs  and OLAP operators   all return NULLs  These constructs often show up as columns in   VIEWs     separator       301       Another problem with NULLs is found when you attempt to convert   IN predicates to EXISTS predicates     separator      16 3 The ALL Predicate and Extrema Functions  313       It is counterintuitive at first that these two predicates are not the same in SQL                But you have to remember the rules for the extrema functions   they   drop out all the NULLs before returning the greater or least values  The   ALL predicate does not drop NULLs  so you can get them in the results     separator       315       However  the definition in the standard is worded in the   negative  so that NULLs get the benefit of the doubt             As you can see  it is a good idea to avoid NULLs in UNIQUE   constraints    Discussing GROUP BY      NULLs are treated as if they were all equal to each other  and   form their own group  Each group is then reduced to a single   row in a new result table that replaces the old one    This means that for GROUP BY clause NULL   NULL does not evaluate to NULL  as in 3VL  but it evaluate to TRUE   SQL standard is confusing      The ORDER BY and NULLs  329       Whether a sort key value that is NULL is considered greater or less than a   non-NULL value is implementation-defined  but             There are SQL products that do it either way       In March 1999  Chris Farrar brought up a question from one of his   developers that caused him to examine a part of the SQL Standard that   I thought I understood  Chris found some differences between the   general understanding and the actual wording of the specification    And so on  I think is enough by Celko   C  J  Date on SQL NULLs  C  J  Date is more radical about NULLs  avoid NULLs in SQL  period  In fact  chapter 4 of his SQL and Relational Theory  How to Write Accurate SQL Code is titled  NO DUPLICATES  NO NULLS   with subchapters  4 4 What s Wrong with Nulls   and  4 5 Avoiding Nulls in SQL   follow the link  thanks to Google Books  you can read some pages on-line    Fabian Pascal on SQL NULLs  From its Practical Issues in Database Management - A Reference for the Thinking Practitioner  no excerpts on-line  sorry       10 3 Pratical Implications      10 3 1 SQL NULLs          SQL suffers from the problems inherent in 3VL as well as from many   quirks  complications  counterintuitiveness  and outright errors  10  11     among them are the following          Aggregate functions  e g   SUM    AVG    ignore NULLs  except for COUNT       A scalar expression on a table without rows evaluates incorrectly to NULL  instead of 0    The expression  NULL   NULL  evaluates to NULL  but is actually invalid in SQL  yet ORDER BY treats NULLs as equal  whatever they precede or follow  regular  values is left to DBMS vendor     The expression  x IS NOT NULL  is not equal to  NOT x IS NULL    as is the case in 2VL                   All commercially implemented SQL dialects follow this 3VL approach  and  thus    not only do they exibits these problems  but they also have spefic implementation   problems  which vary across products

User · Answer

The question  Does one unknown equal another unknown   NULL   NULL  That question is something no one can answer so it defaults to true or false depending on your ansi nulls setting   However the question  Is this unknown variable unknown  This question is quite different and can be answered with true   nullVariable   null is comparing the values nullVariable is null is comparing the state of the variable

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NULL isn t equal to anything  not even itself   My personal solution to understanding the behavior of NULL is to avoid using it as much as possible

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The confusion arises from the level of indirection  abstraction  that comes about from using NULL    Going back to the  what s under the Christmas tree  analogy   Unknown  describes the state of knowledge about what is in Box A    So if you don t know what s in Box A  you say it s  Unknown   but that doesn t mean that  Unknown  is inside the box  Something other than unknown is in the box  possibly some kind of object  or possibly nothing is in the box    Similarly  if you don t know what s in Box B  you can label your state of knowledge about the contents as being  Unknown     So here s the kicker  Your state of knowledge about Box A is equal to your state of knowledge about Box B   Your state of knowledge in both cases is  Unknown  or  I don t know what s in the Box    But the contents of the boxes may or may not be equal    Going back to SQL  ideally you should only be able to compare values when you know what they are  Unfortunately  the label that describes a lack of knowledge is stored in the cell itself  so we re tempted to use it as a value  But we should not use that as a value  because it would lead to  the content of Box A equals the content of Box B when we don t know what s in Box A and or we don t know what s in Box B    Logically  the implication  if I don t know what s in Box A and if I don t know what s in Box B  then what s in Box A   What s in Box B  is false    Yay  Dead Horse

User · Answer

MSDN has a nice descriptive article on nulls and the three state logic that they engender  In short  the SQL92 spec defines NULL as unknown  and NULL used in the following operators causes unexpected results for the uninitiated    operator NULL   true   false  NULL       NULL   NULL   NULL true       NULL   true   false false      NULL   false  true  and op     NULL   true   false  NULL       NULL   NULL   false true       NULL   true   false false      false  false  false  or op      NULL   true   false  NULL       NULL   true   NULL true       true   true   true false      NULL   true   false

[sql] Why does NULL = NULL evaluate to false in SQL server

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