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It'd be great if SQL Server had a MAX_OF function, where you could ask for the value of one column where another column had the maximum value in a group. Sadly, it doesn't.
My workaround looks like this:
SELECT C.ID_customer, C.Name, Cities.Name AS City
FROM
Customers C
INNER JOIN
Addresses A ON C.ID_customer = A.ID_customer
INNER JOIN
(
SELECT ID_customer, MAX(LivesHereSince) AS LastMove
FROM Addresses
GROUP BY ID_customer
) A2
ON A.ID_customer = A2.ID_customer AND A.LivesHereSince = A2.LastMove
INNER JOIN
Cities ON A.City = Cities.ID_cityI join Addresses onto a subquery of itself. The inner subquery tells me, for each customer, the largest (most recent) value of LivesHereSince. To extract the other fields of that row, I join back onto Addresses using that value as a key.
This technique works quite well as long as you have unique values in your search column. If that doesn't hold you'll get duplicate values. You might consider denormalizing your database slightly and keeping a CurrentAddress column in the Customers table. You can use a trigger to keep that column up-to-date.
Stability. What an interesting concept. -- Chris Maunder
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I tried it and it works, but it doesn't return records for those customers that don't have yet an address (in the process of being captured, not that it should happen often.)
Mike Dimmick wrote:
This technique works quite well as long as you have unique values in your search column.
That might be a problem. I need to perform searches based on first or last name, and so I should get several results.
Mike Dimmick wrote:
You might consider denormalizing your database slightly and keeping a CurrentAddress column in the Customers table.
This might not be a bad idea at all. I have a Customer C# class with an AddressCollection and when I call Save on the Customer class, it can easily walk the collection and get the latest address.
Mike Dimmick wrote:
You can use a trigger to keep that column up-to-date.
This may be another solution, but I've never used them before. Any pointers?
-- LuisR
Luis Alonso Ramos
Intelectix - Chihuahua, Mexico
Not much here: My CP Blog!
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Luis Alonso Ramos wrote:
I tried it and it works, but it doesn't return records for those customers that don't have yet an address (in the process of being captured, not that it should happen often.)
Well, you could change the first INNER JOIN to a LEFT JOIN. You'll then get a NULL City back for anything you've not yet added the address to. You'll probably need to turn the whole Address<->Address<->City join into a subquery so that the INNER JOINs there don't eliminate rows from the Customers table. Example:
SELECT C.ID_customer, C.Name, AC.City
FROM
Customers C
LEFT JOIN
(
SELECT
A.ID_customer, Cities.Name AS City
FROM
Addresses A
INNER JOIN
(
SELECT ID_customer, MAX(LivesHereSince) AS LastMove
FROM Addresses
GROUP BY ID_customer
) A2
ON A.ID_customer = A2.ID_customer AND A.LivesHereSince = A2.LastMove
INNER JOIN
Cities ON A.City = Cities.ID_city
) AC ON C.ID_customer = AC.ID_customer
Luis Alonso Ramos wrote:
That might be a problem. I need to perform searches based on first or last name, and so I should get several results.
When I said 'search column', I meant duplicates in the column you're using to find a single row in the historical table (in this case, LivesHereSince in the Addresses table). If you have two rows where ID_customer = 1 and LivesHereSince = 1/1/2002, you'll get two rows from the Addresses<->Addresses join, because they both match LastMoved. I seem to recall finding some disambiguating mechanism last time I needed to do this - I think I added another condition that selected the maximum ID. Unfortunately this ends up adding another join to the query. My problem was trying to extract the most recent event from a transaction log where things actually could happen within the timer period and so were recorded with the same timestamp - however, you couldn't guarantee that the IDs were necessarily in the right order.
As for the trigger, you probably want to add something like:
CREATE TRIGGER trigUpdateCustomersCurrentAddress
ON Addresses
AFTER INSERT AS
UPDATE Customers
SET CurrentAddress = inserted.ID_address
FROM inserted
WHERE Customers.ID_customer = inserted.ID_customerinserted is a virtual table that exists in the trigger. It contains the new versions of rows inserted or updated (you can create a trigger FOR UPDATE too - you can even use the same trigger for both insert and update). The trigger executes in the context of the statement that performs the update. You might ask, why use this form of UPDATE? Remember that you can perform batch updates from client code and that you can use an INSERT/SELECT statement to insert multiple rows in one operation, so you might actually be operating on multiple rows inside the trigger. If you're writing a trigger FOR UPDATE or FOR DELETE, the deleted table contains the old versions of rows that were updated and the rows that were deleted.
That's the simplistic version. If you might need to insert a customer's old addresses at a later time, you'll need to check what's already in the addresses table:
CREATE TRIGGER trigUpdateCustomersCurrentAddress
ON Addresses
AFTER INSERT AS
UPDATE Customers
SET CurrentAddress = inserted.ID_address
FROM inserted
INNER JOIN Addresses
ON inserted.ID_customer = Addresses.ID_Customer AND
inserted.LivesHereSince > Addresses.LivesHereSince
WHERE Customers.ID_customer = inserted.ID_customerIt's surprising how few people actually 'get' the set-based concepts of SQL and its ability to work on multiple rows simultaneously, and fall back on cursors way too early. INSERT/SELECT and UPDATE FROM are extremely powerful tools.
I can't recall how exactly I first found out about the power of triggers. It was probably somewhere in Inside SQL Server 2000 by Kalen Delaney.
Once you've created the trigger, you'll need to update the existing rows:
UPDATE Customers
SET CurrentAddress = A.ID_address
FROM
Addresses A
INNER JOIN
(
SELECT ID_customer, MAX(LivesHereSince) AS LastMove
FROM Addresses
GROUP BY ID_customer
) A2
ON A.ID_customer = A2.ID_customer AND A.LivesHereSince = A2.LastMove
WHERE
Customers.ID_customer = A.ID_customer
Stability. What an interesting concept. -- Chris Maunder
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Wow! that was a long reply, thank you very much!! 
I had already solved my problem with what you suggested in your last post about denormalizing my table. I developed a simple SP that would update the customer record with the ID_address of the latest address after all addresses had been inserted/updated. The only problem was that I had to call that SP at the end of every customer save operation.
I've copied and tested your query now, and after adding some WHERE clauses, I think it works beautifully. So I've just updated my search stored procedure, and removed the ID_lastaddress field from my Customers table. All I can say is THANKS!
That thing about unique search column, I don't think it'll be a problem. Even if somehow the customer could move twice in the same day, the address is stored in a datetime, and using DateTime objects in C# makes the column contain a date and a time, so they'll always be different.
So, I won't be needing triggers for this particular use anymore, but it sure will be useful to know about them for a later need. I've never had any formal SQL training (not even read a book). I only recently learned about stored procedures, and mainly design all my queries (JOINs and all that) in Access. But know I look back to some other projects I developed earlier and I see how much code I could have replaced with a little stored procedure. I guess it happens to everyone, once you learn a new thing you see a thousand places where that could have been useful in the past.
Well, I better keep on working that I still have plenty of things to do today (and it's Saturday noon for me.) Again, thank you very much for your help!
-- LuisR
Luis Alonso Ramos
Intelectix - Chihuahua, Mexico
Not much here: My CP Blog!
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I'm tring to add a row at my database .mdb but i get the follow error: "Sintax error in the INSERT INTO instruction" why? what do i wrong?
this is the schema of my database:
Nome: text 15
Domanda: text 20
Risposta: text 15
Email: text 20
this is the code:
System.Data.OleDb.OleDbConnection conn = new System.Data.OleDb.OleDbConnection();
conn.ConnectionString = @"Provider=Microsoft.Jet.OLEDB.4.0;" +
@"Data source= myDatabase.mdb;";
try
{
conn.Open();
OleDbDataAdapter myAdapter = new OleDbDataAdapter("SELECT * FROM myTable", conn);
OleDbCommandBuilder myCmd = new OleDbCommandBuilder(myAdapter);
DataSet myDataSet = new DataSet("myTable");
myAdapter.MissingSchemaAction = MissingSchemaAction.AddWithKey;
myAdapter.Fill(myDataSet, "myTable");
DataRow myRow = myDataSet.Tables["myTable"].NewRow();
myRow["Nome"] = mNuovoUtente.mName;
myRow["Domanda"] = mNuovoUtente.mQuestion;
myRow["Risposta"] = mNuovoUtente.mAnswer;
myRow["Email"] = mNuovoUtente.mEmail;
myDataSet.Tables["TabellaUtente"].Rows.Add(myRow);
myAdapter.Update(myDataSet, "myTable");
catch(Exception ex)
{
MessageBox.Show(ex.Message);
}
finally
{
conn.Close();
}
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What is the syntax for your myAdapter.UpdateCommand and myAdapter.InsertCommand?
Torin Blair
'In the immortal words of Socrates - "I drank what?".'
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Sorry i don't understand what you mean, please can you explain me?
Thanks.
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In order for you to be able to update or insert from your dataadapter, you need to specify an insert SQL statement in the OleDbDataAdapter.InsertCommand, and an update SQL statement in the OleDbDataAdapter.UpdateCommand. If you used the wizard to create your adapter, you probably already have these statements.
In debug, or by writing out to a label on the screen, you should output both of these properties and see if their SQL Syntax is correct. This will allow you see the sql commands you are using with the database.
These strings are probably also listed in the WinForms or WebForms generated code if you are using Visual Studio .NET
Torin Blair
'In the immortal words of Socrates - "I drank what?".'
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I'm a newbie can you tell me how run the wizard to create the adapter?
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If you drag a data adapter to a form, then it will open a wizard for you.
No worries you can do this the manual way.
<br />
OleDbCommand cmdUpdate = new OleDbCommand(SQLCOMMAND);<br />
OleDbCommand cmdInsert = new OleDbCommand(SQLCOMMAND);<br />
<br />
myAdapter.UpdateCommand = cmdUpdate;<br />
myAdapter.InsertCommand = cmdInsert;<br />
<br />
And that's how it's done.
For a more detailed explanation you can look at the .NET sdk documentation or this example from csharpcorner.
http://www.csharpfriends.com/Articles/getArticle.aspx?articleID=103[^]
Torin Blair
'In the immortal words of Socrates - "I drank what?".'
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Hi, tojamismis.
Thanks for the answer...
Another user has wrote me this:
the first problem is that your table doesn't show a primary key. Without it, .Update() won't work. Second, you, apparently, don't have an Update command associated with the DataAdapter. Third, if the Primary Key information is missing, you set the .MissingSchemaAction, but never called .FillSchema before you called .Fill
In your opinion what other things have i miss?
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Can add you directly the missing rows of code, i'm not success. 
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I don't know the structure of your tables so I can't write any of the sql statements.
Torin Blair
'In the immortal words of Socrates - "I drank what?".'
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It is compose just by a table "myTable" with:
Name: text 20 primarykey
Surname: text 20
Image: OLEOBJ Byte[]
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I can't think of anything else. It is true that you need these additional things if you aren't using a strongly typed dataset.
But that should be it.
Torin Blair
'In the immortal words of Socrates - "I drank what?".'
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That is correct, without strongly typed datasets you should add this line right before you call myAdapter.Fill.
<br />
myAdapter.FillSchema(ds, SchemaType.Source, "MyTable");<br />
Torin Blair
'In the immortal words of Socrates - "I drank what?".'
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How come when I want to reset a date field to NULL my Enterprise manger locks up and I have to kill it?
This is after I have right clicked on a table and choose return all rows. Then I click on the date/time field and delete what was in there. Is there another way to do this besides running a SQL statement?
Thanks
Tom Wright
tawright915@yahoo.com
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I've never seen it do that before. Is it a big table? Does it have the relevant indexes on it?
Database FAQ
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small table. with only three records. We use it as a heartbeat table so that other apps know that my app is still working.
Davey wrote:
Does it have the relevant indexes on it?
Yeah.
It's strange. I might see if there is a SP for it.
Tom Wright
tawright915@yahoo.com
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Have you tried Running a Query from enterprise manager on the table? You can get only the row or rows you need and then update from the results pane in the Query builder.
Torin Blair
'In the immortal words of Socrates - "I drank what?".'
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yeah. If I do an update on that field, passing NULLS, it works fine. I just cannot click in that field, delete what's in there and click out of it. if I do it freezes up.
Tom Wright
tawright915@yahoo.com
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Pressing Delete in Enterprise Manager is not the same as resetting to NULL. Delete will update the field with '' (Empty string)
If you want to update the field to NULL in Enterprise Manager press Ctrl-0
Wout Louwers
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If I create and XP, and I want to connect to my
XP. How do I go about doing that? Will i do an execute from my app?
Right now I do all my selects and updates using a connection pointer like this:
m_pConnection->Execute(_bstr_t(mySelect),NULL, NULL);
Will I also connect to my XP this way? I'm making an XP that will alert me to a trigger in a table that I have set. So that any new record placed in there will alert me so that I do not have to loop every 3 seconds to see if there's new data.
Thanks
Tom Wright
tawright915@yahoo.com
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As far as TSQL is concerned, an XP is the same as a normal SP. If you can execute SPs using this method (I don't see why you can't), then you should be able to execute XPs also.
Database FAQ
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cool I'll try it. Just gotta write my XP and find out how to return a value. I'm going to assume that, that is the same as SP's too?
Thanks
Tom Wright
tawright915@yahoo.com
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