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Hi all, I have a question about avg function.
How do I query from 6 different tables and avegrage them.
All of the tables have same number and type of coloumn.
thanks in advance.
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I'm not sure if these work, but they should worth a try:
Select (t1.col1, t2.col1,t3.col1, t4.col1, t5.col1, t6.col1)/6
From table1 as t1, table2 as t2, table3 as t3, table4 as t4, table5 as t5, table6 as t6
or
use nested Select statement (use the sql AVG function on a grand total, and in the nested select statement you can define that the grand total is sum of individual columns).
or
use User-Defined-Function (you can create a function that takes in 6 parameters and return an average).
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Hi James Shao, Thanks for your answer, I tryied the first one it seems like not wroking with sqlite.
I am not sure how to implement the second statement since I am (sorry to say) not good at sql yet.
this is the first time I am working with sql. reading books and getting help form online this is how
I am getting my work done. May be after this app I will have some knowledge about sql.
if second statement does not wrok then third option is the one I will have to do.
thanks very much for your feedback and help.
modified on Wednesday, January 13, 2010 11:19 AM
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You may want to use a sub-query to solve this problem. Try something like this:
Suppose you have six tables t1, t2, ..., t6, with a column called c1 you want to average.
select avg(c1) from
(
select c1 from t1 where ...<put your condition here>
union all
select c1 from t2 where ...<put your condition here>
union all
select c1 from t3 where ...<put your condition here>
union all
select c1 from t4 where ...<put your condition here>
union all
select c1 from t5 where ...<put your condition here>
union all
select c1 from t6 where ...<put your condition here>
)
That will return the average of the column from all six tables.
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Try this
Assuming the tables have the following structure with data
Tbl1
ID1 NUM1
1 10
2 20
Tbl2
ID2 NUM2
1 11
2 22
Tbl3
ID3 NUM3
1 1
2 2
3 3
Tbl4
ID4 NUM4
1 41
Tbl5
ID5 NUM5
1 109
Tbl6
ID6 NUM6
1 61
2 62
Query:
SELECT AvgNum = AVG(N)FROM(
SELECT N=NUM1 FROM Tbl1 UNION ALL
SELECT NUM2 FROM Tbl2 UNION ALL
SELECT NUM3 FROM Tbl3 UNION ALL
SELECT NUM4 FROM Tbl4 UNION ALL
SELECT NUM5 FROM Tbl5 UNION ALL
SELECT NUM6 FROM Tbl6
)X
Output:
AvgNum
31
Niladri Biswas
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I have written a small Scalar UDF that excepts a VarChar value and check to see if the value is NULL.
If the value is null an empty string should be returned else the value passed to the UDF should be returned.
If the value is NULL an empty string is returned, so that bit works fine, but is I do something like this:-
SELECT dbo.NullToString('Hello world')
The value returned by the UDF is 'H', where I would expect 'Hello world'
Here's the UDF code, hopefully someone can enlighten me to my mistake.
CREATE FUNCTION NullToString
(
@value VarChar(255)
)
RETURNS VarChar
AS
BEGIN
DECLARE @ReturnValue VarChar(255)
IF(@value Is NULL)
BEGIN
SET @ReturnValue = ''
END
ELSE
BEGIN
SET @ReturnValue = @value
END
RETURN @ReturnValue
END
GO
Thanks
Steve Jowett
-------------------------
Real programmers don't comment their code. If it was hard to write, it should be hard to read.
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Sounds like a ANSI->UTF problem.
Try to use NVarChar(255) istead of VarChar(255).
Greetings
Covean
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Im guessing that the default length of a VarChar is 1
RETURNS VarChar
should possibly be
RETURNS VarChar(255)
BTW, you know there is a built-in function for this:
ISNULL(@someVariable,'')
IsNull Function (T-SQL)[^]
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Thanks for the quick reply, and no I did not know there was a build-in function, so thanks for pointing that out to me.
You learn something new everyday
Thanks again
Steve Jowett
-------------------------
Real programmers don't comment their code. If it was hard to write, it should be hard to read.
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There is also the coalesce function, which returns the first non-null value from a list
select coalesce(col1val,col2val,'')
from table1
I cannot remember how many values you can have in the list, but its quite a few. I find it useful instead of nesting isnull's
Bob
Ashfield Consultants Ltd
Proud to be a Code Project MVP
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Ashfield wrote: There is also the coalesce function, which returns the first non-null value from a list
That's interesting, I wish I'd known about that one 12 months ago.
I shell be making use of that.
Thanks Bob
Steve Jowett
-------------------------
Real programmers don't comment their code. If it was hard to write, it should be hard to read.
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Pleased to have been of help
Bob
Ashfield Consultants Ltd
Proud to be a Code Project MVP
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I have a update query, But need with Order by Clause.
I cannot identify the error....
update sales_master set un_rate = (select qtnmaster.un_rate where qtnmaster.itemcode=sales_master.itemcode order by (0+qtnmaster.Qtnno))
Thanks For The Ideas...
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Why on earth would you need an order by in an update clause (illegal for a start) you are identifying the individual records to update!
Never underestimate the power of human stupidity
RAH
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You can use an ORDER BY clause in an UPDATE statement as long as it is in a subquery.
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Have a look at CROSS APPLY.
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There is no problem with the order by clause. However, your subquery may return multiple values, which makes the assignment not valid.
You need to refine your subquery so that only the desired value is returned. For example, when you use order by clause and use "top 1", then the "un_rate" of the record with the smallest "Qtnno" value is returned. Without a clear knowledge of your original intension, I don't have a clear answer to your question.
modified on Tuesday, January 12, 2010 11:02 AM
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In T-SQL
Update sales_master
set un_rate = qtnmaster.un_rate
from sales_master
inner join qtnmaster
ON sales_master.itemcode = qtnmaster.itemcode
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This will only work if there are no more than 1 record in qtnmaster per itemcode.
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(Posted on c# forum - better suited to DB)
I have an Access db with a number of related tables.
Some tables have a boolean column, eg HTReadOnly, to determine whether it is a system row (true) or not (false).
If HTReadOnly is true the user cannot edit the table row.
In a databound DataGridView, if the user chooses to Edit a row I need to check the underlying table column value, HTReadOnly for the current row and allow editing if false.
My current solution, using static methods, although reusable for different DataGridViews, requires some manual checking of design time values to protect against run time errors. eg. DataRow,column name and column type are valid etc
If I could get the row index of the table row matching the DataGridViewRow I could use...
<pre>if(!MyDataSet.MyTable1[pos].HTReadOnly ) {
else {
The benefit of this method (in my mind) is the IDE and intellisense will provide the table names and check the type etc. So I know they are correct.
I tried to get pos from....
int pos = MyTable1bindingSource.Position;
however this seems to return the position of the row in the DataGridView not the table, as it doesn't match the table row when DGV sorted.
I can't seem to find a method or property for either, DataGridView, bindingSource, or TableAdapter that will return the record row index matching the currently selected DataGridViewRow.
Any Ideas?
Should I just be content getting the value of the DataGridView current row cell for my boolean value?
I read somewhere that it is better to get the actual data rather than the data loaded into the DGV, not sure why?
My current method utilizes various statics methods with
DataGridViewRow dgvr = dgv.CurrentRow;
DataRowView drv = (DataRowView)dgvr.DataBoundItem;
to be able to get the column value
eg retVal = (bool)drv.Row[colName] after various tests.
Any help would be appreciated.
Thanks...
modified on Wednesday, January 13, 2010 1:54 AM
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ok, I'm game to take a shot at this, IMHO your design of allowing the user to edit into the DGV is wrong, in all but the most trivial cases I force the user to double click the list control item (DGV, listview etc) and pop a dialog that allows the crud work to be done on the single row. Trap the results from the dialog and reload the list control if the data has been modified/deleted.
The double click event will give you the id from the datarow.
Having said that have you tried the cellformat event to deal with each row as it is displayed. DGV has a myriad of event and you need to get the one that will allow you to identify the row and make it read only before the user gets at it. Search for alternate row colouring and you will see the events that can be adopted for this.
I assume your Boolean flag is in the DGV, probably a hidden column!
Never underestimate the power of human stupidity
RAH
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Thanks for your reply!
To expand a bit further...
I have set up the DGV so that no edits , updates etc can be done in the DGV.
So the whole DGV is ReadOnly.
I am providing the edit/add fields ( as databound textboxes etc )separately elsewhere on the form.
These normally display a bit more info than the DGV itself, and are all ReadOnly unless an Edit button (or row double click - yet to be done) is clicked.
If the Boolean flag (as you said,in a hidden column) allows edits, I change all the appropriate TextBox.ReadOnly values to allow changes there.
(There are some rows that I don't want the user ever to edit or delete)
(1)So, if it is considered to be best practice to get the column value from the table itself rather than the DGV , I need to get the row index of the table that matches the current row in the DGV.
(2) Also being able to use MyDataSet.MyTable1[pos].HTReadOnly ) syntax allow for error checking in the IDE in case of wrong column name or wrong type etc
Otherwise I can get the DGV row/column value from ( off the top of my head) with
DataGridViewRow dgvr = table1DataGridView.CurrentRow;
DataRowView drv = (DataRowView )dgvr.DataBoundItem;
bool isReadOnly = drv.Row["htreadonly"];
By using drv.Row["htreadonly"]; am I in fact looking directly at the data?
If so it leaves only point (2) which I think is significant instead of coding (as at present) to catch runtime errors.
Hope this serves to clarify, not confuse....
LJL
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I am not sure I agree with (1) that you should be getting the data from the underlying datatable, I don't distinguish between the 2 as a datasource for editing. Then again I never edit into the underlying datasource.
AussieLew wrote: I am providing the edit/add fields ( as databound textboxes etc )separately elsewhere on the form
When you get he data for this do you use the DGV row or go back to the database get a row to bind to the textboxes.
Never underestimate the power of human stupidity
RAH
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Mycroft Holmes wrote: I am not sure I agree with (1) that you should be getting the data from the underlying datatable,
You are probably right, unless there is a reason they may not always match?
Something I read somewhere that made sense at the time!
I guess also that this would become more unwieldy in the case of a DGV based on multi table queries?
(That one is coming up soon..)
Mycroft Holmes wrote: When you get he data for this do you use the DGV row or go back to the database get a row to bind to the textboxes.
The textboxes etc are bound to the same bindingSource as the DGV's DataSource and change as the DGV row is selected etc.
I worked out how I can do what I (feel I) want to do. I needed to go one more level and get the DataRow.
DataGridViewRow dgvr = MyTable1DataGridView.CurrentRow;
DataRowView drv = (DataRowView)dgvr.DataBoundItem;
DataRow dr = (DataRow)drv.Row;
int pos = dr.Table.Rows.IndexOf(dr);
bool boolColumnValue = MyDataSet.MyTable1[pos].HTReadOnly;
if (boolColumnValue)
{
}
else
{
}
Ah...It's been an interesting exercise, learnt a lot..There's nothing like a real life project for giving incentive to learn.
Now all I need to determine is the "best" way to do the updates and deletes etc. There seems to be many classes where this can done.
Thanks for your help, it's been fun!
LJL
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Your challenge now is to get the bool check to one line/statement. Breaking the steps up is the correct way to go when understanding the process of achieving a goal, well done. Then finding and understanding the correct syntax that achieves the 5 lines of code in 1 statement is the real bonus  !
Never underestimate the power of human stupidity
RAH
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