NULL values inside NOT IN clause

Question

This issue came up when I got different records counts for what I thought were identical queries one using a not in where constraint and the other a left join  The table in the not in constraint had one null value  bad data  which caused that query to return a count of 0 records  I sort of understand why but I could use some help fully grasping the concept   To state it simply  why does query A return a result but B doesn t   A  select  true  where 3 in  1  2  3  null  B  select  true  where 3 not in  1  2  null    This was on SQL Server 2005  I also found that calling set ansi nulls off causes B to return a result

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this is for Boy   select party code  from abc as a where party code not in  select party code                           from xyz                           where party code   a party code     this works regardless of ansi settings

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It may be concluded from answers here that NOT IN  subquery  doesn t handle nulls correctly and should be avoided in favour of NOT EXISTS  However  such a conclusion may be premature  In the following scenario  credited to Chris Date  Database Programming and Design  Vol 2 No 9  September 1989   it is NOT IN that handles nulls correctly and returns the correct result  rather than NOT EXISTS   Consider a table sp to represent suppliers  sno  who are known to supply parts  pno  in quantity  qty   The table currently holds the following values         VALUES   S1    P1   NULL                   S2    P1   200                  S3    P1   1000    Note that quantity is nullable i e  to be able to record the fact a supplier is known to supply parts even if it is not known in what quantity    The task is to find the suppliers who are known supply part number  P1  but not in quantities of 1000   The following uses NOT IN to correctly identify supplier  S2  only   WITH sp AS         SELECT             FROM   VALUES    S1    P1   NULL                               S2    P1   200                              S3    P1   1000                   AS T   sno  pno  qty          SELECT DISTINCT spx sno   FROM sp spx  WHERE spx pno    P1         AND 1000 NOT IN                           SELECT spy qty                           FROM sp spy                          WHERE spy sno   spx sno                                AND spy pno    P1                              However  the below query uses the same general structure but with NOT EXISTS but incorrectly includes supplier  S1  in the result  i e  for which the quantity is null    WITH sp AS         SELECT             FROM   VALUES    S1    P1   NULL                               S2    P1   200                              S3    P1   1000                   AS T   sno  pno  qty          SELECT DISTINCT spx sno   FROM sp spx  WHERE spx pno    P1         AND NOT EXISTS                          SELECT                            FROM sp spy                         WHERE spy sno   spx sno                               AND spy pno    P1                                AND spy qty   1000                            So NOT EXISTS is not the silver bullet it may have appeared   Of course  source of the problem is the presence of nulls  therefore the  real  solution is to eliminate those nulls   This can be achieved  among other possible designs  using two tables    sp suppliers known to supply parts spq suppliers known to supply parts in known quantities   noting there should probably be a foreign key constraint where spq references sp   The result can then be obtained using the  minus  relational operator  being the EXCEPT keyword in Standard SQL  e g    WITH sp AS         SELECT             FROM   VALUES    S1    P1                                S2    P1                               S3    P1                    AS T   sno  pno                spq AS         SELECT             FROM   VALUES    S2    P1   200                              S3    P1   1000                   AS T   sno  pno  qty          SELECT sno   FROM spq  WHERE pno    P1  EXCEPT  SELECT sno   FROM spq  WHERE pno    P1         AND qty   1000

User · Answer

IF you want to filter with NOT IN for a subquery containg NULLs justcheck for not null  SELECT blah FROM t WHERE blah NOT IN          SELECT someotherBlah FROM t2 WHERE someotherBlah IS NOT NULL

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NOT IN returns 0 records when compared against an unknown value  Since NULL is an unknown  a NOT IN query containing a NULL or NULLs in the list of possible values will always return 0 records since there is no way to be sure that the NULL value is not the value being tested

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The title of this question at the time of writing is      SQL NOT IN constraint and NULL values   From the text of the question it appears that the problem was occurring in a SQL DML SELECT query  rather than a SQL DDL CONSTRAINT   However  especially given the wording of the title  I want to point out that some statements made here are  potentially misleading statements  those along the lines of  paraphrasing       When the predicate evaluates to UNKNOWN you don t get any rows    Although this is the case for SQL DML  when considering constraints the effect is different   Consider this very simple table with two constraints taken directly from the predicates in the question  and addressed in an excellent answer by  Brannon    DECLARE  T TABLE     true CHAR 4  DEFAULT  true  NOT NULL    CHECK   3 IN  1  2  3  NULL       CHECK   3 NOT IN  1  2  NULL        INSERT INTO  T VALUES   true     SELECT COUNT    AS tally FROM  T    As per  Brannon s answer  the first constraint  using IN  evaluates to TRUE and the second constraint  using NOT IN  evaluates to UNKNOWN  However  the insert succeeds  Therefore  in this case it is not strictly correct to say   you don t get any rows  because we have indeed got a row inserted as a result   The above effect is indeed the correct one as regards the SQL-92 Standard  Compare and contrast the following section from the SQL-92 spec     7 6 where clause      The result of the  is a table of those rows of T for   which the result of the search condition is true       4 10 Integrity constraints      A table check constraint is satisfied if and only if the specified   search condition is not false for any row of a table    In other words   In SQL DML  rows are removed from the result when the WHERE evaluates to UNKNOWN because it does not satisfy the condition  is true    In SQL DDL  i e  constraints   rows are not removed from the result when they evaluate to UNKNOWN because it  does satisfy the condition  is not false    Although the effects in SQL DML and SQL DDL respectively may seem contradictory  there is practical reason for giving UNKNOWN results the  benefit of the doubt  by allowing them to satisfy a constraint  more correctly  allowing them to not fail to satisfy a constraint   without this behaviour  every constraints would have to explicitly handle nulls and that would be very unsatisfactory from a language design perspective  not to mention  a right pain for coders    p s  if you are finding it as challenging to follow such logic as  unknown does not fail to satisfy a constraint  as I am to write it  then consider you can dispense with all this simply by avoiding nullable columns in SQL DDL and anything in SQL DML that produces nulls  e g  outer joins

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Compare to null is undefined  unless you use IS NULL   So  when comparing 3 to NULL  query A   it returns undefined   I e  SELECT  true  where 3 in  1 2 null   and  SELECT  true  where 3 not in  1 2 null   will produce the same result  as NOT  UNDEFINED  is still undefined  but not TRUE

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Null signifies and absence of data  that is it is unknown  not a data value of nothing   It s very easy for people from a programming background to confuse this because in C type languages when using pointers null is indeed nothing     Hence in the first case 3 is indeed in the set of  1 2 3 null  so true is returned  In the second however you can reduce it to   select  true  where 3 not in  null   So nothing is returned because the parser knows nothing about the set to which you are comparing it - it s not an empty set but an unknown set   Using  1  2  null  doesn t help because the  1 2  set is obviously false  but then you re and ing that against unknown  which is unknown

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Query A is the same as   select  true  where 3   1 or 3   2 or 3   3 or 3   null   Since 3   3 is true  you get a result   Query B is the same as   select  true  where 3  lt  gt  1 and 3  lt  gt  2 and 3  lt  gt  null   When ansi nulls is on  3  lt  gt  null is UNKNOWN  so the predicate evaluates to UNKNOWN  and you don t get any rows   When ansi nulls is off  3  lt  gt  null is true  so the predicate evaluates to true  and you get a row

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In A  3 is tested for equality against each member of the set  yielding  FALSE  FALSE  TRUE  UNKNOWN    Since one of the elements is TRUE  the condition is TRUE    It s also possible that some short-circuiting takes place here  so it actually stops as soon as it hits the first TRUE and never evaluates 3 NULL    In B  I think it is evaluating the condition as NOT  3 in  1 2 null     Testing 3 for equality against the set yields  FALSE  FALSE  UNKNOWN   which is aggregated to UNKNOWN   NOT   UNKNOWN   yields UNKNOWN   So overall the truth of the condition is unknown  which at the end is essentially treated as FALSE

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also this might be of use to know the logical difference between join  exists and in http   weblogs sqlteam com mladenp archive 2007 05 18 60210 aspx

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SQL uses three-valued logic for truth values  The IN query produces the expected result  SELECT   FROM  VALUES  1    2   AS tbl col  WHERE col IN  NULL  1  -- returns first row  But adding a NOT does not invert the results  SELECT   FROM  VALUES  1    2   AS tbl col  WHERE NOT col IN  NULL  1  -- returns zero rows  This is because the above query is equivalent of the following  SELECT   FROM  VALUES  1    2   AS tbl col  WHERE NOT  col   NULL OR col   1   Here is how the where clause is evaluated    col   col   NULL        col   1   col   NULL OR col   1   NOT  col   NULL OR col   1     ----- ---------------- --------- ----------------------- -----------------------------    1     UNKNOWN          TRUE      TRUE                    FALSE                           2     UNKNOWN          FALSE     UNKNOWN                  UNKNOWN                         Notice that   The comparison involving NULL yields UNKNOWN The OR expression where none of the operands are TRUE and at least one operand is UNKNOWN yields UNKNOWN  ref  The NOT of UNKNOWN yields UNKNOWN  ref   You can extend the above example to more than two values  e g  NULL  1 and 2  but the result will be same  if one of the values is NULL then no row will match

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Whenever you use NULL you are really dealing with a Three-Valued logic   Your first query returns results as the WHERE clause evaluates to       3   1 or 3   2 or 3   3 or 3   null which is      FALSE or FALSE or TRUE or UNKNOWN which evaluates to      TRUE   The second one       3  lt  gt  1 and 3  lt  gt  2 and 3  lt  gt  null which evaluates to      TRUE and TRUE and UNKNOWN which evaluates to      UNKNOWN   The UNKNOWN is not the same as FALSE you can easily test it by calling   select  true  where 3  lt  gt  null select  true  where not  3  lt  gt  null    Both queries will give you no results  If the UNKNOWN was the same as FALSE then assuming that the first query would give you FALSE the second would have to evaluate to TRUE as it would have been the same as NOT FALSE   That is not the case   There is a very good article on this subject on SqlServerCentral   The whole issue of NULLs and Three-Valued Logic can be a bit confusing at first but it is essential to understand in order to write correct queries in TSQL   Another article I would recommend is SQL Aggregate Functions and NULL

[sql] NULL values inside NOT IN clause

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