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Because you are using the same column and table in the subquery, then you simply make the subquery redundant.
SELECT SomeField, AccountName FROM AccountInfo WHERE AccountID=123456 will get you the accurate result.
The issue in your query is that you may have more than 1 record in the accountInfo table with a given CustomerID so you will throw an error in that the subquery can have multiple results when it can only have one. If you actually want more than one result then you could do something like this, but I'm not entirely sure the inner join will be valid like this. Something you can try, though.
SELECT t1.SomeField,
t2.AccountName
FROM tblAccountInfo t1
INNER JOIN tblAcocuntInfo t2 ON t2.CustomerID = t1.CustomerID
WHERE t1.AccountID = 123456
I wasn't, now I am, then I won't be anymore.
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I think i need the construct i initially posed, but there may be the case where the result may return more than one result - in which case i could handle with MAX() or a simple aggregate.
When i first call tblAccountInfo with AccountID, it gives me 1 record in tblAccountInfo with a CustomerID.
When i call tblAccountInfo using CustomerID, it will return AccountName, different than if I used AccountID, but I need both calls to retrieve different sets of data from same table.
I need first call to tblAccountInfo to get the CustomerID to use in the second call. 
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I have to agree with Marcus. The correlated subquery is redundant. I can't imagine any structure and/or data that would make this subquery useful.
tb2.CustomerID = tb1.CustomerID will always return both columns from the same row. If CustomerID is duplicated within the table, then it will throw an error.
It also seems strange that AccountName is not dependant upon AccountId as thier names imply. Are you sure your structure is designed correctly?
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To answer your question, there will be no "circular problem". If you run this query, you should see that it works fine. What allows you to have the same table multiple times in a single query is aliasing, which you are doing (you are aliasing tblAccountInfo as tb1 and tb2).
Like others have mentioned, your query doesn't make sense as is. If, however, there can be multiple records with the same CustomerID, this would make sense (notice the TOP 1):
SELECT
tb1.SomeField,
(
SELECT TOP 1
tb2.AccountName
FROM tblAccountInfo AS tb2
WHERE
tb2.CustomerID = tb1.CustomerID
) AS strAccountName
FROM tblAccountInfo AS tb1
WHERE
tb1.AccountID = 123456
But then, you also mentioned an aggregate function (such as MAX) in a reply above, which would accomplish much the same. If CustomerID is a unique field, then an aggregate or TOP 1 would be unnecessary, but then so would the subquery.
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First thing is u should avoid sub query as a column and make a inner join and take a column from sub table as it improves your performance when u fire your query with large data.
Second is it works fine till sub query produces single record for parameter, if there is more than one row returned by sub query, it throws errors.
Third for same u can use distinct clause in sub query.
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Thanks for all the replies and suggestions. as it stands the table structure does reveal different AccountNames when queried on different columns (AccountID, CustomerID). And yes, that seems redundant because for case of one record having unique set of AccountID, CustomerID, and AccountName it is obvious.
For case of many records with mixed combinations of those 3 columns, I am not guaranteed the AccountName is what I want.
I think I'm only concerned if AccountName is not NULL.
Thanks!
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We're seeing a large number of lock requests / sec on our SQL 2008 db, the current average is ~240,000.
They seem to be either key locks or page locks. There's no time-outs, deadlocks and the average wait time is 0.
The only other stat that seems high is the context switches, which is around 1k-3k on our 2cpu system.
Is this something to be worried about?
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cjb110 wrote: Is this something to be worried about?
Only if you have time to spare. It might hint that additional optimization is possible, but it's not guaranteed that it can be optimized further - it'd be a fun exercise though. You might find this[^] an interesting read to get you started.
Bastard Programmer from Hell 
if you can't read my code, try converting it here[^]
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Hi all,
Firstly, I must confess to being a very amateur programmer with a lot to learn! However, I'm battling away and am slowly getting my head around things.
I have a query I have written which as follows:
SELECT File_Name
WHERE (File_Name = @File_Name1)
File_Name1 is a user input parameter. My problem is that, even if the record does not exist, it returns the record, resulting in an error on the client side.
Is it possible to edit that query to return a file_name field value of say, 1, if the searched record doesn't exist?
Any help appreciated! (Also for note most of my queries are generated by VStudio, so my understanding of them leaves a lot to be desired...)
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Using which database-dialect? If you're using Sql Server, then the IsNull[^]-function might be handy.
SELECT ISNULL(File_Name, '1')
FROM Files
WHERE File_Name = @FileName1
If we're talking about more than a single record, you'd need a join and a dummy-table.
Bastard Programmer from Hell
if you can't read my code, try converting it here[^]
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Yea I am using SQL Server.
At the moment the query is designed to open file which the user requests (file_name). This works really well if the input matches a record in the database.
The problem is when it doesn't match a stored record. Will using the query as above with ISNULL work? And if so, am I right in assuming that the query first executes the where part of the query, then the IFNULL portion and, if still valid after that, retrieves the correct record?
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Joe Stansfield wrote: The problem is when it doesn't match a stored record. Will using the query as above with ISNULL work?
Dunno - it will not return an empty record, but it'll return '1' when the value is empty. Would that work with the rest of your code?
Joe Stansfield wrote: And if so, am I right in assuming that the query first executes the where part of the query, then the IFNULL portion and, if still valid after that, retrieves the correct record?
Yes.
Bastard Programmer from Hell
if you can't read my code, try converting it here[^]
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Yea that will fix up the error from no value being entered.
But I'm struggling to understand and make it work if, say, an input of 100 is entered. If 100 isn't a valid ID of a record, it won't return anything. I'd like it to return 1 for ease of avoiding an error and possible loop error (which could also be put down to poor programming by me on the C# side of things).
With that said, if 99 is entered and that is a valid record, then 99 would need to be returned by the query.
Sorry for not understanding your explanation.
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Joe Stansfield wrote: With that said, if 99 is entered and that is a valid record, then 99 would need to be returned by the query.
That's not how things are done usually; all records in the database "should" be valid records with a link. If not, you got a modeling-error that could bite you later on.
We're using "empty" values (nulls) if we can not link to another record. The database-server is built around that concept; you can get all records from two (or more tables) "joined" into a single table, explicitly asking for a collection with/without the linked records.
Joe Stansfield wrote: I'd like it to return 1 for ease of avoiding an error and possible loop error
What loop? Do you have an example?
FWIW, you don't need to loop to load all records; you fetch 'em in a single haul.
Bastard Programmer from Hell
if you can't read my code, try converting it here[^]
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Yea, from reading what you have said, I think my error may be in allowing a user to input a value into the query that is not a valid record (ie one that doesn't exist).
So would the best approach be to run a query to test if input value is a record, then if true, proceed to the query which returns the record?
The loop error is because of my attempting to sort a datagrid with linked data in it - if a null value is returned (ie no record), it gets caught in a loop and error because it can't sort a null record (along those lines).
Thanks for all your help, even though I haven't fixed the problem I am picking up some valuable tips!
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You shouldn't need to run 2 queries. You should just need to check the resulting dataset to see if you get any results. (ie. rowcount > 0)
I wasn't, now I am, then I won't be anymore.
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Thanks for all the help guys.
After reading what you have all posted and doing some more research I managed to solve the problem I was having.
The basic query I was running was sufficient, it turned out the problem was in an automatically created linked query. By fixing that I have got ride of the errors I was getting.
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Joe Stansfield wrote: So would the best approach be to run a query to test if input value is a record, then if true, proceed to the query which returns the record?
The best approach is one without loops. Are you displaying the "linked record" in a separate (child) gridview?
Usually, we combine the tables in Sql, like this;
SELECT a.Field1
,a.Field2
,b.Field1 AS Field3
FROM Employee AS a
JOIN Department AS b ON a.DepartmentId = b.Id
This will generate a single resultset, and depending on the type of join, the database will fetch the employees with, those without a linked record, or both 
Bastard Programmer from Hell
if you can't read my code, try converting it here[^]
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Yea I have a linked record showing in a child view.
So from what I can gather the first query executes to fill the main record table, and then a second query populates the correct record in the grid view (and a few other text boxes on the form).
It is working now - but is it worth the time to write it as it should be? It won't affect any of my datasource bindings by combing it into a single query will it?
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Joe Stansfield wrote: It is working now - but is it worth the time to write it as it should be?
It's already as it should be; if you have a separate grid for the children, you'll need a second query. Also explains why the second grid might get an empty resultset.
Joe Stansfield wrote: It won't affect any of my datasource bindings by combing it into a single query will it?
It would! The net effect is that you only pull the data over the network-line once. You're using a different concept here - if you'd fetch all tables, you'd probably have fetched a lot of child-records that the user isn't going to use. That means that your current approach might be the more efficient one, depending on what the user does
Bastard Programmer from Hell
if you can't read my code, try converting it here[^]
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ITS BETTER U YO USE THE IMMEDIATE CONSTRAINT DURING THE CODING
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Please don't shout (capital letters are considered shouting!).
I would also refrain from using text speak as it upsets a lot of people on these forums.
Lobster Thermidor aux crevettes with a Mornay sauce, served in a Provençale manner with shallots and aubergines, garnished with truffle pate, brandy and a fried egg on top and Spam - Monty Python Spam Sketch
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Hi Joe,
You can use following query:
SELECT ISNULL( (SELECT File_Name
FROM Files
WHERE File_Name = @FileName1),1)
This would solve your problem, in case there is no result, it will return you 1 otherwise the matching result.
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There are also a number of other columns for each row, I assume that I just populate the query with those separated by columns?
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Yup
SELECT Col1
,ISNULL(Col2, '1')
,Col3
,Col4
FROM SomeTable
WHERE SomeCondition
..or even nest another select, like this;
SELECT Col1
,ISNULL(Col2, '1')
,(SELECT 1) AS Col3
,Col4
FROM SomeTable
WHERE SomeCondition
Bastard Programmer from Hell
if you can't read my code, try converting it here[^]
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