Related
I would like to retrieve all rows with values except 1,2,3,4,5 in my COLUMNA in TABLEA .
SELECT * FROM TABLEA WHERE COLUMNA NOT IN (1,2,3,4,5)
But this eliminates the rows with NULL values in COLUMNA too.
I don't want to eliminate NULL values rows and would like to include those rows in the resultset.
Alternatively, I can try below query for the same
SELECT * FROM TABLEA WHERE COLUMNA NOT IN (1,2,3,4,5) OR COLUMNA IS NULL.
But I would like to know, why is it necessary to add this OR condition?
Why is the additional necessary?
NULL comparisons almost always results in NULL, rather than true or false. A WHERE clause filters out all non-true values, so they get filtered out.
This is how SQL defines NULL. Why is NULL defined like this?
NULL does not mean "missing" in SQL. It means "unknown". So, the result of <unknown> not in (1, 2, 3, 4, 5) is "unknown", because the value could be 1 or it might be 0. Hence it gets filtered.
I will also note that the SQL standard includes NULL-safe comparisons, IS NOT DISTINCT FROM and IS DISTINCT FROM corresponding to = and <> respectively. These treat NULL as just "any other value", so two NULL values are considered equal. However, there is no construct for NULL-safe IN and NOT IN, as far as I know.
Try the following:
SELECT * FROM TABLEA WHERE ISNULL(COLUMNA,0) NOT IN (1,2,3,4,5)
I have a table PATIENT with Column STATUS. When I queried to get STATUS not equal to 1, I was expecting the result as NULL and 2.
But I am only getting 2 as the result. Can someone help me with this?
CREATE TABLE #PATIENT
(STATUS INT)
INSERT INTO #PATIENT (STATUS)
SELECT 1
UNION
SELECT 2
UNION
SELECT NULL
SELECT * FROM #PATIENT WHERE STATUS <> 1
When I queried with
SELECT * FROM #PATIENT WHERE ISNULL(STATUS, 0) != 1
I am able to get NULL and 2 as the result.
This is SQL SERVER 2012.
You can use OR in WHERE with Condition STATUS IS NULL .
SELECT * FROM #PATIENT WHERE STATUS <> 1 OR STATUS IS NULL
This will do it.
EDIT:
Conceptually, NULL means “a missing unknown value” and it is
treated somewhat differently from other values.
You cannot use arithmetic comparison operators such as =, <, or <>
to test for NULL
Because the result of any arithmetic comparison with NULL is also
NULL, you cannot obtain any meaningful results from such comparisons
we can not equate or not equate anything with null, thats why IS NULL
SELECT NULL <> 1 ===> NULL
Even though it is supposed to be true, it will return `NULL`
Hope this helps.
When you compare NULL value to any value then the result is always NULL.
So if you wan to select the NULL value as well then try this:
SELECT * FROM #PATIENT WHERE STATUS <> 1 OR STATUS IS NULL
DEMO
Conceptually "NULL" means a missing value. To test for NULL IS NULL or IS NOT NULL condition is used. Arithmetic operators cannot be used for comparing NULL values.
NULL<>1 :: returns false because NULL can be 1 or any other value (an unknown value).
Null is a special marker used in Structured Query Language (SQL) to indicate that a data value does not exist in the database.
And reason because you got only 2 and NOT NULL is because in SQL nothing is equal to NULL but also nothing is NOT equal to NULL.
In Simple words you cannot equate and not equate any value with NULL.
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.
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 <> 1 and 3 <> 2 and 3 <> null
When ansi_nulls is on, 3 <> null is UNKNOWN, so the predicate evaluates to UNKNOWN, and you don't get any rows.
When ansi_nulls is off, 3 <> null is true, so the predicate evaluates to true, and you get a row.
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.
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 <> 1 and 3 <> 2 and 3 <> 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 <> null
select 'true' where not (3 <> 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.
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
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 )
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)!
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.
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⁽¹⁾ | col = 1 | col = NULL OR col = 1 | NOT (col = NULL OR col = 1) |
|-----|----------------|---------|-----------------------|-----------------------------|
| 1 | UNKNOWN | TRUE | TRUE | FALSE |
| 2 | UNKNOWN | FALSE | UNKNOWN⁽²⁾ | UNKNOWN⁽³⁾ |
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.
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.
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;
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
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
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.
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 <> 1 and 3 <> 2 and 3 <> null
When ansi_nulls is on, 3 <> null is UNKNOWN, so the predicate evaluates to UNKNOWN, and you don't get any rows.
When ansi_nulls is off, 3 <> null is true, so the predicate evaluates to true, and you get a row.
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.
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 <> 1 and 3 <> 2 and 3 <> 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 <> null
select 'true' where not (3 <> 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.
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
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 )
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)!
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.
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⁽¹⁾ | col = 1 | col = NULL OR col = 1 | NOT (col = NULL OR col = 1) |
|-----|----------------|---------|-----------------------|-----------------------------|
| 1 | UNKNOWN | TRUE | TRUE | FALSE |
| 2 | UNKNOWN | FALSE | UNKNOWN⁽²⁾ | UNKNOWN⁽³⁾ |
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.
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.
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;
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
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
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.
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 <> 1 and 3 <> 2 and 3 <> null
When ansi_nulls is on, 3 <> null is UNKNOWN, so the predicate evaluates to UNKNOWN, and you don't get any rows.
When ansi_nulls is off, 3 <> null is true, so the predicate evaluates to true, and you get a row.
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.
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 <> 1 and 3 <> 2 and 3 <> 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 <> null
select 'true' where not (3 <> 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.
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
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 )
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)!
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.
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⁽¹⁾ | col = 1 | col = NULL OR col = 1 | NOT (col = NULL OR col = 1) |
|-----|----------------|---------|-----------------------|-----------------------------|
| 1 | UNKNOWN | TRUE | TRUE | FALSE |
| 2 | UNKNOWN | FALSE | UNKNOWN⁽²⁾ | UNKNOWN⁽³⁾ |
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.
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.
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;
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
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