I have a query that needs to exclude both Null and Blank Values, but for some reason I can't work out this simple logic in my head.
Currently, my code looks like this:
WHERE [Imported] = 0 AND ([Value] IS NOT NULL **OR** [Value] != '')
However, should my code look like this to exclude both condition:
WHERE [Imported] = 0 AND ([Value] IS NOT NULL **AND** [Value] != '')
For some reason I just can't sort this in my head properly. To me it seems like both would work.
In your question you wrote the following:
have a query that needs to exclude both Null and Blank Values
So you have answered yourself, the AND query is the right query:
WHERE [Imported] = 0 AND ([Value] IS NOT NULL AND [Value] != '')
Here is an extract from the ANSI SQL Draft 2003 that I borrowed from this question:
6.3.3.3 Rule evaluation order
[...]
Where the precedence is not determined by the Formats or by
parentheses, effective evaluation of expressions is generally
performed from left to right. However, it is
implementation-dependent whether expressions are actually evaluated left to right, particularly when operands or operators might
cause conditions to be raised or if the results of the expressions
can be determined without completely evaluating all parts of the
expression.
You don't specify what kind of database system you are using but the concept of short-circuit evaluation which is explained in the previous paragraph applies to all major SQL versions (T-SQL, PL/SQL etc...)
Short-circuit evaluation means that once an expression has been successfully evaluated it will immediately exit the condition and stop evaluating the other expressions, applied to your question:
If value is null you want to exit the condition, that's why it should be the first expression (from left to right) but if it isn't null it should also not be empty, so it has to be NOT NULL and NOT EMPTY.
This case is a bit tricky because you cannot have a non empty string that is also null so the OR condition will also work but you will do an extra evaluation because short-circuit evaluation will never exit in the first expression:
Value is null but we would always need to check that value is also not an empty string (value is null or value is not an empty string).
In this second case, you may get an exception because the expression [Value] != '' may be checked on a null object.
So I think AND is the right answer. Hope it helps.
If the value was numeric and you didn't want either 1 or 2, you would write that condition as
... WHERE value != 1 AND value != 2
An OR would always be true in this case. For instance a value of 1 would return true for the check against 2 - and then the OR-check would return true, as at least one of the conditions evaluated to true.
When yu also want to check against null values, the situation is a bit more complicated. A check against a null value always fails: value != '' is false when value is null. That is why there is a special IS NULL or IS NOT NULL test.
Related
I am currently learning SQL utilizing Codecademy and am curious if there is a difference between using "IS" or "=".
In the current lesson, I wrote this code:
SELECT *
FROM nomnom
WHERE neighborhood IS 'Midtown'
OR neighborhood IS 'Downtown'
OR neighborhood IS 'Chinatown';
Which ran perfectly fine. I always like to look at the answer after to see if there was something I did wrong or could improve on. The answer had this code:
SELECT *
FROM nomnom
WHERE neighborhood = 'Midtown'
OR neighborhood = 'Downtown'
OR neighborhood = 'Chinatown';
Do IS and = function the same?
All that you want to know you can find it here:
The IS and IS NOT operators work like = and != except when one or both
of the operands are NULL. In this case, if both operands are NULL,
then the IS operator evaluates to 1 (true) and the IS NOT operator
evaluates to 0 (false). If one operand is NULL and the other is not,
then the IS operator evaluates to 0 (false) and the IS NOT operator is
1 (true). It is not possible for an IS or IS NOT expression to
evaluate to NULL. Operators IS and IS NOT have the same precedence as
=.
taken from: SQL As Understood By SQLite.
The important part is: ...except when one or both of the operands are NULL... because when using = or != (<>) and 1 (or both) of the operands is NULL then the result is also NULL and this is the difference to IS and IS NOT.
They work the same but "IS" is a keyword in MySQL and is generally used while comparing NULL values. While comparing NULL values "=" does not work.
SELECT * FROM nomnom WHERE neighborhood IS NULL
The above statement would run perfectly fine but
SELECT * FROM nomnom WHERE neighborhood = NULL
would result in an error.
They are the same for these cases, but further down the line you will discover one nifty little value called NULL.
NULL is a pain because... it doesn't exist.
0 = NULL returns FALSE;
Date <> [Column] will not return lines with NULL, only those with a value that is different.
Hell, even NULL = NULL returns false. And NULL <> NULL also returns false. That is why "IS" exists. Because NULL IS NULL will return true.
So as a general rule, use = for values.
Keep "IS" for null.
[Column] IS NULL
or
[Column] IS NOT NULL
And remember to always check if your column is nullable that you need to plan for null values in your WHERE or ON clauses.
I've a scenrio which process many data in Oracle database. In some cases, the variable Sec_email will contain many values and in some cases Sec_email will contain null or ' '.
so can please any one tell me how to write a query for this?
I tried with
(C.SECONDARY_EMAIL IN ('?,?') OR '' = '' )
where C is the Client table.
When I use this i get the count as 0.
You can perform a not null check before the IN comparison like
Sec_email is not null and C.SECONDARY_EMAIL IN (...
One obvious problem is that Oracle (by default) treats empty strings as NULL. So: '' = '' is the same as NULL = NULL, which is never true.
Arrgh.
In any case, you are probably constructing the query, so use is null instead:
(C.SECONDARY_EMAIL IN ('?,?') OR '' IS NULL
I think the real problem, though, is the first comparison. The IN list has one element with a constant, not two (but perhaps that is your intention). If you want to put a variable number of values for comparison, one method uses regular expressions. For instance:
C.SECONDARY_EMAIL REGEXP_LIKE '^val1|val2|val3$' or '' IS NULL
If you would like to get a list of values when some of them is null you should use:
("some other conditions" OR C.SECONDARY_EMAIL IS NULL)
The question is if it is not null and not ' ' value what you are expecting, if it should be some king of
pattern you should use regular expression:
regexp_like(C.SECONDARY_EMAIL, '^(.+?[,]+?)+$')
Also, if you have a few conditions in where clause use should use brackets to group you conditions null check and another one.
All conditions i this case will be divided by OR.
(C.SECONDARY_EMAIL IS NULL OR regexp_like(C.SECONDARY_EMAIL, '^(.+?[,]+?)+$'))
or
(C.SECONDARY_EMAIL IS NULL OR regexp_like(C.SECONDARY_EMAIL, '^(.+?[,]+?)+$')
OR C.SECONDARY_EMAIL = ' ')
In SQL , why does 10/NULL evaluate to NULL (or unknown) ? Example :
if((10/NULL) is NULL)
DBMS_OUTPUT.PUT_LINE("Null.");
However , 1 = NULL being a COMPARISON is considered as FALSE. Shouldn't 10/NULL also be considered as FALSE ?
I am referring to SQL only . Not any DBMS in particular. And it might be a duplicate but I didn't know what keywords to put in search for this query.
Shouldn't 10/NULL also be considered as FALSE?
No, because:
Any arithmetic expression containing a null always evaluates to null. For example, null added to 10 is null. In fact, all operators (except concatenation) return null when given a null operand.
Emphasis mine, taken from the Oracle manual: http://docs.oracle.com/cd/E11882_01/server.112/e26088/sql_elements005.htm#i59110
And this is required by the SQL standard.
Edit, as the question was for RDBMS in general:
SQL Server
When SET ANSI_NULLS is ON, an operator that has one or two NULL expressions returns UNKNOWN
Link to the the manual:
MySQL
An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for a particular function or operator
Link to the manual
DB2
if either operand can be null, the result can be null, and if either is null, the result is the null value
Link to the manual:
PostgreSQL
Unfortunately I could not find such an explicit statement in the PostgreSQL manual, although I sure it behaves the same.
Warning: The "(except concatenation)" is an Oracle only and non-standard exception. (The empty string and NULL are almost identical in Oracle). Concatenating nulls gives null in all other DBMS.
1 = null is not null. It is actually unknown. As well as any other null operation.
The equality predicate 1 = NULL evaluates to NULL. But NULL in a boolean comparison is considered false.
If you do something like NOT( 1 = NULL ), 1 = NULL evaluates to NULL, NOT( NULL ) evaluates to NULL and so the condition as a whole ends up evaluating to false.
Oracle has a section in their documentation on handling NULL values in comparisons and conditional statements-- other databases will handle things in an very similar manner.
10/something means that you are counting how much "something" will be in 10
in this case you're counting how much "nothing" will be in 10 - that's infinity, unknown..
1 = NULL is false because one does not equal nothing
The NULLIF function accepts two parameters. If the first parameter is equal to the second parameter, NULLIF returns Null. Otherwise, the value of the first parameter is returned.
NULLIF(value1, value2)
NVL
The NVL function accepts two parameters. It returns the first non-NULL parameter or NULL if all parameters are NULL.
also check this conditional outcomes:
This "null equals UNKNOWN truth value" proposition introduces an inconsistency into SQL 3VL. One major problem is that it contradicts a basic property of nulls, the property of propagation. Nulls, by definition, propagate through all SQL expressions. The Boolean truth values do not have this property. Consider the following scenarios in SQL:1999, in which two Boolean truth values are combined into a compound predicate. According to the rules of SQL 3VL, and as shown in the 3VL truth table shown earlier in this article, the following statements hold:
( TRUE OR UNKNOWN ) → TRUE
( FALSE AND UNKNOWN ) → FALSE
However, because nulls propagate, treating null as UNKNOWN results in the following logical inconsistencies in SQL 3VL:
( TRUE OR NULL ) → NULL ( = UNKNOWN )
( FALSE AND NULL ) → NULL ( = UNKNOWN )
The SQL:1999 standard does not define how to deal with this inconsistency, and results could vary between implementations. Because of these inconsistencies and lack of support from vendors the SQL Boolean datatype did not gain widespread acceptance. Most SQL DBMS platforms now offer their own platform-specific recommendations for storing Boolean-type data.
Note that in the PostgreSQL implementation of SQL, the null value is used to represent all UNKNOWN results and the following evaluations occur:
( TRUE OR NULL ) → TRUE
( FALSE AND NULL ) → FALSE
( FALSE OR NULL ) IS NULL → TRUE
( TRUE AND NULL ) IS NULL → TRUE
Given:
The following Select statement:
select case NULL
when NULL then 0
else 1
end
Problem:
I'm expecting this to return 0 but instead it returns 1. What gives?
Generally speaking, NULL is not something you should attempt to compare for equality, which is what a case statement does. You can use "Is NULL" to test for it. There is no expectation that NULL != NULL or that NULL = NULL. It's an indeterminate, undefined value, not a hard constant.
-- To encompass questions in the comments --
If you need to retrieve a value when you may encounter a NULL column, try this instead:
Case
When SomeColumn IS NULL
Then 0
Else 1
End
I believe that should work. As far as your original post is concerned:
Select Case NULL
When NULL then 0 // Checks for NULL = NULL
else 1 // NULL = NULL is not true (technically, undefined), else happens
end
The trouble is that your Case select automatically attempts to use equality operations. That simply doesn't work with NULL.
I was going to add this as a comment to Aaron's answer, but it was getting too long, so I'll add it as another (part of the) answer.
The CASE statement actually has two distinct modes, simple and searched.
From BOL:
The CASE expression has two formats:
The simple CASE expression compares an expression to a set of simple expressions to determine the result.
The searched CASE expression evaluates a set of Boolean expressions to determine the result.
When the simple CASE (your example) does what it describes as comparison it does an equality comparison - i.e. =
This is clarified in the later documentation:
The simple CASE expression operates by comparing the first expression
to the expression in each WHEN clause for equivalency. If these
expressions are equivalent, the expression in the THEN clause will be
returned.
Allows only an equality check.
Because anything = NULL is always false in ANSI SQL (and if you didn't know this, you need to read up on NULLs in SQL more generally, particularly also with the behavior in the other searched comparison - WHERE x IN (a, b, c)), you cannot use NULL in a simple case and have it ever be compared to a value, with a NULL either in the initial expression or in the list of expressions to be compared against.
If you want to check for NULL, you will have to use an IF/ELSE construct or the searched CASE with a full expression.
I agree that it's kind of unfortunate there is no version which supports an IS comparison to make it easier to write:
select case colname
when IS NULL then 0
else 1
end
Which would make writing certain long CASE statements easier:
select case colname
when IS NULL then ''
when 1 then 'a'
when 2 then 'b'
when 3 then 'c'
when 4 then 'd'
else 'z'
end
But that's just wishful thinking...
An option is to use ISNULL or COALESCE:
select case COALESCE(colname, 999999) -- 999999 is some value never used
when 999999 then ''
when 1 then 'a'
when 2 then 'b'
when 3 then 'c'
when 4 then 'd'
else 'z'
end
But it isn't always a great option.
In addition to the other answers, you need to change the syntax for CASE slightly to do this:
SELECT CASE
WHEN NULL IS NULL THEN 0
ELSE 1
END;
Using the value in your syntax implicitly uses an equals comparison. NULL is unknown, and so is NULL = NULL, so with your current code you will always get zero 1 (geez I did it too).
To get the behavior you want, you can use SET ANSI_NULLS ON; however note that this can change other code in ways you may not be able to predict, and the setting is deprecated - so it will stop working at all in a future version of SQL Server (see this SQL Server 2008 doc).
You need to use the IS NULL operator. Standard comparison operators do not work with NULL.
Check out these MSDN articles about Null that may be useful:
IS [NOT] NULL (Transact-SQL)
Null Values
I was reading this article:
Get null == null in SQL
And the consensus is that when trying to test equality between two (nullable) sql columns, the right approach is:
where ((A=B) OR (A IS NULL AND B IS NULL))
When A and B are NULL, (A=B) still returns FALSE, since NULL is not equal to NULL. That is why the extra check is required.
What about when testing inequalities? Following from the above discussion, it made me think that to test inequality I would need to do something like:
WHERE ((A <> B) OR (A IS NOT NULL AND B IS NULL) OR (A IS NULL AND B IS NOT NULL))
However, I noticed that that is not necessary (at least not on informix 11.5), and I can just do:
where (A<>B)
If A and B are NULL, this returns FALSE. If NULL is not equal to NULL, then shouldn't this return TRUE?
EDIT
These are all good answers, but I think my question was a little vague. Allow me to rephrase:
Given that either A or B can be NULL, is it enough to check their inequality with
where (A<>B)
Or do I need to explicitly check it like this:
WHERE ((A <> B) OR (A IS NOT NULL AND B IS NULL) OR (A IS NULL AND B IS NOT NULL))
REFER to this thread for the answer to this question.
Because that behavior follows established ternary logic where NULL is considered an unknown value.
If you think of NULL as unknown, it becomes much more intuitive:
Is unknown a equal to unknown b? There's no way to know, so: unknown.
relational expressions involving NULL actually yield NULL again
edit
here, <> stands for arbitrary binary operator, NULL is the SQL placeholder, and value is any value (NULL is not a value):
NULL <> value -> NULL
NULL <> NULL -> NULL
the logic is: NULL means "no value" or "unknown value", and thus any comparison with any actual value makes no sense.
is X = 42 true, false, or unknown, given that you don't know what value (if any) X holds? SQL says it's unknown. is X = Y true, false, or unknown, given that both are unknown? SQL says the result is unknown. and it says so for any binary relational operation, which is only logical (even if having NULLs in the model is not in the first place).
SQL also provides two unary postfix operators, IS NULL and IS NOT NULL, these return TRUE or FALSE according to their operand.
NULL IS NULL -> TRUE
NULL IS NOT NULL -> FALSE
All comparisons involving null are undefined, and evaluate to false. This idea, which is what prevents null being evaluated as equivalent to null, also prevents null being evaluated as NOT equivalent to null.
The short answer is... NULLs are weird, they don't really behave like you'd expect.
Here's a great paper on how NULLs work in SQL. I think it will help improve your understanding of the topic. I think the sections on handling null values in expressions will be especially useful for you.
http://www.oracle.com/technology/oramag/oracle/05-jul/o45sql.html
The default (ANSI) behaviour of nulls within an expression will result in a null (there are enough other answers with the cases of that).
There are however some edge cases and caveats that I would place when dealing with MS Sql Server that are not being listed.
Nulls within a statement that is grouping values together will be considered equal and be grouped together.
Null values within a statement that is ordering them will be considered equal.
Null values selected within a statement that is using distinct will be considered equal when evaluating the distinct aspect of the query
It is possible in SQL Server to override the expression logic regarding the specific Null = Null test, using the SET ANSI_NULLS OFF, which will then give you equality between null values - this is not a recommended move, but does exist.
SET ANSI_NULLS OFF
select result =
case
when null=null then 'eq'
else 'ne'
end
SET ANSI_NULLS ON
select result =
case
when null=null then 'eq'
else 'ne'
end
Here is a Quick Fix
ISNULL(A,0)=ISNULL(B,0)
0 can be changed to something that can never happen in your data
"Is unknown a equal to unknown b? There's no way to know, so: unknown."
The question was : why does the comparison yield FALSE ?
Given three-valued logic, it would indeed be sensible for the comparison to yield UNKNOWN (not FALSE). But SQL does yield FALSE, and not UNKNOWN.
One of the myriads of perversities in the SQL language.
Furthermore, the following must be taken into account :
If "unkown" is a logical value in ternary logic, then it ought to be the case that an equality comparison between two logical values that both happen to be (the value for) "unknown", then that comparison ought to yield TRUE.
If the logical value is itself unknown, then obviously that cannot be represented by putting the value "unknown" there, because that would imply that the logical value is known (to be "unknown"). That is, a.o., how relational theory proves that implementing 3-valued logic raises the requirement for a 4-valued logic, that a 4 valued logic leads to the need for a 5-valued logic, etc. etc. ad infinitum.