I have following table:
And following simple query:
SELECT * FROM dbo.Calendars WHERE calname NOT IN(NULL)
My question is why always NOT IN(NULL) return nothing?
PS:No matter what is your table,if you add NOT IN(NULL) to any column's condition,result is nothing.
Thank you in advance :)
Because value equals NULL is not defined, and will never evaluate to True.
Check This post on bad practices. Don't use IN and NULL, use EXISTS to test for at least one row where something exists.
NULL is treated differently to other values in most databases. For instance, this predicate will never be true:
WHERE foo = NULL
whereas this predicate is true if the value of foo is indeed NULL:
WHERE foo IS NULL
To your problem, if you only wish to check if calname is not null, use the following predicate:
WHERE calname IS NOT NULL
If on the other hand you have a set of values, of which NULL is one of those values (say [1,2,3,4,NULL]), and you still want to use the NOT IN syntax, you have to do it like this:
WHERE calname IS NOT NULL AND calname NOT IN(1, 2, 3, 4)
EDIT: A further way of doing this, if you are constrained to use the NOT IN syntax is to COALESCE the column calname into a value that you definitely know is not stored in that column. For instance, if calname can only take positive integer values, then we can do this instead:
WHERE COALESCE(calname, -1) NOT IN (-1)
Your query evaluates to calname <> null , which results in UNKNOWN.
Change your query to this ,
SELECT * FROM dbo.Calendars WHERE calname IS NOT NULL;
Related
I have table, in which a column has empty fields.when i try to run simple query like below it is not
returning the data.
select * from table1 where "colunm1" = ''
But below two queries returns data
1,
select * from table1
where coalesce("colunm1", '') = ''
2,
select * from table1
where "colunm1" is null
Can someone tell me the reason?
TIA
You have describe the behavior of a column that is NULL. NULL is not the same as an empty string.
It fails any equality comparison. However, you can use is null or (less preferentially) coalesce().
The only database that treats an empty string like a NULL value, is Oracle.
Logically, for a DBMS, that would be wrong.
Relational DBMSs are all about :
set theory
Boolean algebra
A NULL value is a value that we don't know. A string literal of '' is a string whose value we know. It is a string with no characters, with a length of 0. We don't know of a NULL as a string how long it is, how many and, if any, which, characters it contains.
So it is only logical that:
the comparison '' = '' will evaluate to TRUE
the comparison NULL = NULL will evaluate to FALSE , as any comparison with a NULL value will evaluate to FALSE.
And functions like COALESCE() or NVL(), IFNULL(), ISNULL() will return the first parameter if it does not contain a NULL value. That is consistent.
Except in Oracle
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.
In NAME table FIRST column is having null but no rows are selected. Please help me to understand.
SELECT * FROM NAME WHERE FIRST != '1'
Any comparison with null is false - = and <>, >, <, and so on. You cannot use null in an IN list as well - it would be ignored. Moreover, two nulls are not even equal to each other.
To get the nulls, you need to ask for them explicitly, like this:
SELECT * FROM NAME WHERE FIRST IS NULL OR FIRST != '1'
Any comparison to NULL returns NULL, which is equivalent to FALSE. This is true eve of not-equals.
If you want to include NULL values, do one of the following:
where first <> '1' or first is null
or
where coalesce(first, '<null>') <> '1'
In Oracle, null is not considered a legal value to select unless you explicitly ask for it:
select * from name where (first != '1') or first is null
You could also use NVL (similar to coalesce):
select * from name where nvl(first,'0') != '1'
That is correct because NULL can never be compared with anything else....
The only option that you have is to include a NULL check as an or in the command
SELECT * FROM NAME WHERE FIRST!=1 OR FIRST IS NULL
According to Oracle Documentation NULL is defined as a value not knownm or when the value is not meaningful. That is solely the reason why Oracle mentions not consider a value of ZERO as NULL. This is just an FYI, an addon. Thanks!
NULL is dumb. Period.
NULL is evil.
If X is NULL and Y is NULL, then X does in fact equal Y because they are both NULL.
It's also a PITA that I can't say
IF X IN ('a','B','C', null)
Because this condition happens. But now I have to say
IF ( X IN ('a','B','C') or X is NULL )
which is a waste of time and a risk of error if I forget the parentheses.
What irks me further is that NULL shouldn't happen in the first place. Fields (er... ok kids, I'll call them Columns) should always be initialized. Period. Stop the nulls. Don't allow them. Default values should always be zeroes or blanks so that those folks that are too lazy to initialize columns in their software will have them initialized for them automatically.
There are many instances where a failure to define default values of zeroes and blanks makes life more difficult than it has to be.
In a table containing five records where the Toppings value is "Chocolate", two of them have the value "Yes" in the MaraschinoCherry column, the other three contain nothing in that column (not "No" - nothing/blank).
This query works fine:
select definition from desserts
where (Toppings = 'Chocolate') and
(MaraschinoCherry <> 'Yes')
order by id
...returning the expected three records; but this one returns nothing at all, rather than the two records I expect:
select definition from desserts
where (Toppings = 'Chocolate') and
(MaraschinoCherry = 'Yes')
order by id
???
The answer to your question is simple. Any comparison to a NULL value, with two exceptions, produces NULL as the result. So,
MaraschinoCherry = 'Yes'
and
MaraschinoCherry <> 'Yes'
Both return NULL when MaraschinoCherry has a NULL value. NULL comparisons are treated the same as FALSE.
The two exceptions are: IS NULL and IS NOT NULL. Note that "= NULL" always returns NULL, which is interpreted as FALSE.
The normal way to fix this is by using COALESCE:
COALESCE(MaraschinoCherry, 'NO') = 'Yes'
(The function ISNULL() is kind of equivalent, but COALESCE allows more arguments and is standard SQL.)
There are other ways you can fix this, such as by specifying a default value for the column when you define the table, by adding an explicit comparison to NULL, by declaring the column to be "NOT NULL", or in some databases by overriding the behavior of NULLs in comparisons to violate the SQL standards (HIGHLY NOT RECOMMENDED!).
In a stored procedure (Oracle in my case), I want to add some values to an existing record. Problem is that both the existing value and the value to be added can be null. I only want the result to be NULL when both operands are null. If only one of them is null, I want the result to be the other operand. If both are non-null, I want the result to be "normal" addition.
Here's what I am using so far:
SELECT column INTO anz_old FROM aTable Where <someKeyCondition>;
IF anz_old IS NULL
THEN
anz_new := panzahl;
ELSE
anz_new := anz_new + NVL (panzahl, 0);
END IF;
UPATE aTabel set column = anz_new Where <someKeyCondition>;
Is there a more elegant way (pereferably completely in SQL, i.e. just in an update statement short of a long CASE-Statement with basically the same logic as the above code)?
If you want to add a and b and either may be null, you could use coalesce, which returns the first non-null parameter you pass it:
coalesce(a+b, a, b)
So in this case, if neither parameter is null, it will return the sum. If only b is null, it will skip a+b and return a. If a is null, it will skip a+b and a and return b, which will only be null if they are both null.
If you want the answer to be 0 rather than null if both a and b are null, you can pass 0 as the last parameter:
coalesce(a+b, a, b, 0)
Do consider #erwins answer - null might not be the right thing to be using.
I accomplished it this way:
coalesce("Column1",0.00) + coalesce("Column2",0.00)
I'm working with front end high level execs.... They don't understand why NULL and 0 aren't handled the same way.
In my case it works, just replacing NULLs with 0.00... may not in all though :)
You can also use ISNULL, so if you have 3 values
isnull(val1,0)+isnull(val2,0)+isnull(val3,0)
which ever column will have a NULL will use a 0, otherwise its original value.
In SQL, Null is supposed to be a state that says "I don't know".
If you don't know how much b is, then you also do not know how much a+b is, and it is misleading to pretend that a+b=a in that case.
In SQL terms, when adding numbers, a result of NULL means there were no non-null numbers added.
This suggests that a sensible answer in SQL terms would be
CASE WHEN A IS NULL AND B IS NULL THEN NULL ELSE ISNULL(A, 0) + ISNULL(B, 0) END