Want to improve this post? Provide detailed answers to this question, including citations and an explanation of why your answer is correct. Answers without enough detail may be edited or deleted.
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.
Related
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
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?)
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.
I have a stored procedure that will query and return records based on if items are available or not. That part is easy, Im simply passing in a variable to check where available is yes or no.
But what I want to know is how do I throw a everything clause in there (i.e available, not available, or everything)?
The where clause right now is
where availability = #availability
The values of availabitility are either 1 or 0, nothing else.
You can use NULL to represent everything.
WHERE (#availability IS NULL OR availability = #availability)
SELECT *
FROM mytable
WHERE #availability IS NULL
UNION ALL
SELECT *
FROM mytable
WHERE availability = #availability
Passing a NULL value will select everything.
This query will use an index on availability if any.
Don't know the type of #availability, but assuming -1 = everything then you could simply do a
where #availability = -1 OR availability = #availability
multiple ways of doing it. Simpliest way is is to set the default value of the #availability to null and then your where clause would look like this
WHERE (#availability IS NULL OR availability = #availability)
Also what will be the scenarios where this query is used
select * from TableA where exists
(select null from TableB where TableB.Col1=TableA.Col1)
As the query is in an EXISTS then you can return anything. It is not even evaluated.
In fact, you can replace the null with (1/0) and it will not even produce a divide by zero error.
The NULL makes no sense. It's simply bad SQL.
The exists clause is supposed to use SELECT *.
People make up stories about the cost of SELECT *. They claim it does an "extra" metadata query. It doesn't. They claim it's a "macro expansion" and requires lots of extra parse time. It doesn't.
The EXISTS condition is considered "to be met" if the subquery returns at least one row.
The syntax for the EXISTS condition is:
SELECT columns
FROM tables
WHERE EXISTS ( subquery );
Please note that "Select Null from mytable" will return number of rows in mytable but all will contain only one column with null in the cell as the requirement of outer query is just to check whether any row fall in the given given condition like in your case it is "TableB.Col1=TableA.Col1"
you can change null to 1, 0 or any column name available in the table. 1/0 may not be a good idea :)
It's a tacky way of selecting all records in TableA, which have a matching record (Col1=Col1) in TableB. They might equally well have selected '1', or '*', for instance.
A more human-readable way of achieving the same would be
SELECT * FROM TableA WHERE Col1 IN ( SELECT Col1 IN TableB )
Please, please, all ....
EXISTS returns a BOOLEAN i.e. TRUE or FALSE. If the result set is non empty then return TRUE. Correlation of the sub-query is important as in the case above.
i.e Give me all the rows in A where AT LEAST one col1 exists in B.
It does not matter what is in the select list. Its just a matter of style.