Can anyone please point out what is wrong with the following SQL statement:
SELECT DiaryType
FROM tblDiaryTypes
WHERE DiaryType NOT IN (SELECT NextDiary
FROM tblActionLinks
WHERE HistoryType = 'Info Chased');
Now the nested SELECT statement currently returns NULL because there are initially no entries in tblActionLinks, and I am wondering if that is the issue.
The outer SELECT statement if executed on its own does return all the Diary Types from tblDiaryTypes as expected. But when I add the nested SELECT statement to exclusde certain values, then the overall SQL statement returns empty!
Does this have something to do withthe fact that tblActionLinks is currently empty? If so, how can I amend my SQL statement to handle that possibility.
For SQL SERVER (you didn't specified sql engine) try with:
SELECT ISNULL(NextDiary, 0) ...
When no rows found all value is null then it will return 0
Are you sure there are no entries currently in tblActionLinks? If there are no entries in tblActionLinks, then outer query should return all records
Does this have something to do withthe fact that tblActionLinks is currently empty?
Yes... NULL doesn't being handled so good in SQL, Comparing a value to NULL is undifned try give for null a flag value like -999:
SELECT DiaryType
FROM tblDiaryTypes
WHERE DiaryType NOT IN (SELECT NVL(NextDiary, -999) -- <===
FROM tblActionLinks
WHERE HistoryType = 'Info Chased');
NVL(NextDiary, -999) means that if NextDiary IS NULL, replace the value with -999
docs
I would rewrite your query the following way:
SELECT DiaryType
FROM tblDiaryTypes
WHERE NOT EXISTS (SELECT NextDiary
FROM tblActionLinks
WHERE HistoryType = 'Info Chased'
AND NextDiary = DiaryType)
This ensures proper behaviour irrespective of ANSI_NULLS setting and you don't have to worry about properly choosing the magic value returned by ISNULL(NextDiary, 0) (what if you have DiaryType equal to 0 in tblDiaryTypes?)
Related
I have this SQL query:
but I'm getting an error:
If I remove the comma-separated value from the variable, it is working fine. As well as if I remove the NULL checking feature it is working fine. Can I know the issue of this
It's because a CASE WHEN can only return 1 value.
And a STRING_SPLIT returns a resultset.
I assume something like this is what you want.
SELECT *
FROM Facility f
WHERE (#Facility IS NULL OR f.facilityCode IN (SELECT value FROM string_split(#Facility,',')))
This will get all records if the variable is null.
The function dbo.split will split the string in more than one value. This will confuse your subquery and the error you are receiving will be thrown.
In case you need what goes before the comma consider using:
select top 1 value
from dbo.split(#Facility, ','))
You want to say if the variable is NULL so ignore the WHERE statement, if so your query would be:
SELECT *
FROM Facility f
WHERE #Facility IN (select value from dbo.split(#Facility, ',')) OR #Facility IS NULL
I have a simple SQL query where a comparison is done between two tables for mismatching value.
Yesterday, we picked up an issue where one field was null and the other wasn't, but a mismatch was not detected.
As far as I can determine,the logic has been working all along until yesterday.
Here is the logic of the SQL:
CREATE TABLE Table1
(UserID INT,PlayDate DATETIME)
CREATE TABLE Table2
(UserID INT,PlayDate DATETIME)
INSERT INTO Table1 (UserID)
SELECT 5346
INSERT INTO Table2 (UserID,PlayDate)
SELECT 5346,'2012-11-01'
SELECT a.UserID
FROM Table1 a
INNER JOIN
Table2 b
ON a.UserID = b.UserID
WHERE a.PlayDate <> b.PlayDate
No values are returned even though the PlayDate values are different.
I have now updated the WHERE to read:
WHERE ISNULL(a.PlayDate,'') <> ISNULL(b.PlayDate,'')
Is there a setting in SQL which someone could have changed to cause the original code to no longer pick up the difference in fields?
Thanks
NULL <> anything
is unknown not true. SQL uses three valued logic (false/true/unknown) and the predicate needs to evaluate to true in a where clause for the row to be returned.
In fact in standard SQL any comparison with NULL except for IS [NOT] NULL yields unknown. Including WHERE NULL = NULL
You don't state RDBMS but if it supports IS DISTINCT FROM you could use that or if you are using MySQL it has a null safe equality operator <=> you could negate.
You say you think it previously behaved differently. If you are on SQL Server you might be using a different setting for ANSI_NULLS somehow but this setting is deprecated and you should rewrite any code that depends on it anyway.
You can simulate IS DISTINCT FROM in SQL Server with WHERE EXISTS (SELECT a.PlayDate EXCEPT SELECT b.PlayDate)
Not even a NULL can be equal to NULL.
Here are two common queries that just don’t work:
select * from table where column = null;
select * from table where column <> null;
there is no concept of equality or inequality, greater than or less
than with NULLs. Instead, one can only say “is” or “is not”
(without the word “equal”) when discussing NULLs.
- The correct way to write the queries
select * from table where column IS NULL;
select * from table where column IS NOT NULL;
I have a valid SQL select which returns an empty result, up and until a specific transaction has taken place in the environment.
Is there something available in SQL itself, that will allow me to return a 0 as opposed to an empty dataset? Similar to isNULL('', 0) functionality. Obviously I tried that and it didn't work.
PS. Sadly I don't have access to the database, or the environment, I have an agent installed that is executing these queries so I'm limited to solving this problem with just SQL.
FYI: Take any select and run it where the "condition" is not fulfilled (where LockCookie='777777777' for example.) If that condition is never met, the result is empty. But at some point the query will succeed based on a set of operations/tasks that happen. But I would like to return 0, up until that event has occurred.
You can store your result in a temp table and check ##rowcount.
select ID
into #T
from YourTable
where SomeColumn = #SomeValue
if ##rowcount = 0
select 0 as ID
else
select ID
from #T
drop table #T
If you want this as one query with no temp table you can wrap your query in an outer apply against a dummy table with only one row.
select isnull(T.ID, D.ID) as ID
from (values(0)) as D(ID)
outer apply
(
select ID
from YourTable
where SomeColumn = #SomeValue
) as T
alternet way is from code, you can check count of DataSet.
DsData.Tables[0].Rows.count > 0
make sure that your query matches your conditions
Debugging an app which queries SQL Server 05, can't change the query but need to optimise things.
Running all the selects seperately are quick <1sec, eg: select * from acscard, select id from employee... When joined together it takes 50 seconds.
Is it better to set uninteresting accesscardid fields to null or to '' when using EXISTS?
SELECT * FROM ACSCard
WHERE NOT EXISTS
( SELECT Id FROM Employee
WHERE Employee.AccessCardId = ACSCard.acs_card_number )
AND NOT EXISTS
( SELECT Id FROM Visit
WHERE Visit.AccessCardId = ACSCard.acs_card_number )
ORDER by acs_card_id
Do you have indexes on Employee.AccessCardId, Visit.AccessCardId, and ACSCard.acs_card_number?
The SELECT clause is not evaluated in an EXISTS clause. This:
WHERE EXISTS(SELECT 1/0
FROM EMPLOYEE)
...should raise an error for dividing by zero, but it won't. But you need to put something in the SELECT clause for it to be a valid query - it doesn't matter if it's NULL or a zero length string.
In SQL Server, NOT EXISTS (and NOT IN) are better than the LEFT JOIN/IS NULL approach if the columns being compared are not nullable (the values on either side can not be NULL). The columns compared should be indexed, if they aren't already.
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I'm using a stored procedure in MySQL, with a CASE statement.
In the ELSE clause of the CASE ( equivalent to default: ) I want to select and return an empty result set, thus avoiding to throw an SQL error by not handling the ELSE case, and instead return an empty result set as if a regular query would have returned no rows.
So far I've managed to do so using something like:
Select NULL From users Where False
But I have to name an existing table, like 'users' in this example.
It works, but I would prefer a way that doesn't break if eventually the table name used is renamed or dropped.
I've tried Select NULL Where False but it doesn't work.
Using Select NULL does not return an empty set, but one row with a column named NULL and with a NULL value.
There's a dummy-table in MySQL called 'dual', which you should be able to use.
select
1
from
dual
where
false
This will always give you an empty result.
This should work on most DBs, tested on Postgres and Netezza:
SELECT NULL LIMIT 0;
T-SQL (MSSQL):
SELECT Top 0 1;
How about
SELECT * FROM (SELECT 1) AS TBL WHERE 2=3
Checked in myphp, and it also works in sqlite and probably in any other db engine.
This will probably work across all databases.
SELECT * FROM (SELECT NULL AS col0) AS inner0 WHERE col0 IS NOT NULL;
SELECT TOP 0 * FROM [dbo].[TableName]
This is a reasonable approach to constant scan operator.
SELECT NULL WHERE FALSE;
it works in postgresql ,mysql, subquery in mysql.
How about this?
SELECT 'MyName' AS EmptyColumn
FROM dual
WHERE 'Me' = 'Funny'
SELECT * FROM (SELECT NULL) WHERE 0
In PostgreSQL a simple
SELECT;
works. You won't even get any columns labeled 'unknown'.
Note however, it still says 1 row retrieved.