I am attempting to retrieve one field of one row in an access database. The current setup i have takes around 100 ms. Is there a faster way for a single field?
Public Function GetEntityConfig(ByVal pintID As Integer) As aadXML.XMLDoc
Dim objRtn As New aadXML.XMLDoc, myreader As OleDbDataReader
Dim Query As New OleDbCommand("Select e_config from entity where e_id = " & pintID, CurrentActiveConnectionToDB)
If CurrentActiveConnectionToDB.State = False Then
CurrentActiveConnectionToDB.Open()
End If
myreader = Query.ExecuteReader()
myreader.Read()
objRtn.LoadXML(myreader.Item("e_config"))
myreader.Close()
GetEntityConfig = objRtn
CurrentActiveConnectionToDB.Close()
End Function
First, you should learn to use parameters, rather than munging a query string with input values.
Your query is:
Select e_config
from entity
where e_id = ?
For this query, an index will speed it up:
create index idx_entity_e_d_e_config on entity(e_id, e_config);
You don't have to include e_config in the index; it is helpful but only marginal.
Related
I have a Public Sub to move a collection of records from one table to another in the same SQLite database. First it reads a record from strFromTable, then writes it to strToTable, then deletes the record from strFromTable. To speed things up, I've loaded the entire collection of records into a transaction. When the list involves moving a lot of image blobs, the db gets backed up, and throws the exception "The Database is Locked". I think what is happening is that it's not finished writing one record before it starts trying to write the next record. Since SQLite only allows one write at a time, it thows the "Locked" exception.
Here is the code that triggers the error when moving a lot of image blobs:
Using SQLconnect = New SQLiteConnection(strDbConnectionString)
SQLconnect.Open()
Using tr = SQLconnect.BeginTransaction()
Using SQLcommand = SQLconnect.CreateCommand
For Each itm As ListViewItem In lvcollection
SQLcommand.CommandText = $"INSERT INTO {strToTable} SELECT * FROM {strFromTable} WHERE id = {itm.Tag}; DELETE FROM {strFromTable} WHERE ID = {itm.Tag};"
SQLcommand.ExecuteNonQuery()
Next
End Using
tr.Commit()
End Using
End Using
When I get rid of the transaction, it executes without error:
Using SQLconnect = New SQLiteConnection(strDbConnectionString)
SQLconnect.Open()
Using SQLcommand = SQLconnect.CreateCommand
For Each itm As ListViewItem In lvcollection
SQLcommand.CommandText = $"INSERT INTO {strToTable} SELECT * FROM {strFromTable} WHERE id = {itm.Tag}; DELETE FROM {strFromTable} WHERE ID = {itm.Tag};"
SQLcommand.ExecuteNonQuery()
Next
End Using
End Using
I'm not very good with DB operations, so I'm sure there is something that needs improvement. Is there a way to make SQLite completely finish the previous INSERT before executing the next INSERT? How can I change my code to allow using a transaction?
Thank you for your help.
.
Ok ... here is the solution that I decided to go with. I hope this helps someone finding this in a search:
Dim arrIds(lvcollection.Count - 1) As String
Dim i as Integer = 0
' Load the array with all the Tags in the listViewCollection
For i = 0 to lvcollection.Count - 1
arrIds(i) = lvcollection(i).Tag 'item.Tag holds the Primary Key "id" field in the DB
Next
'build a comma-space separated string of all ids from the array of ids.
Dim strIds as String = String.Join(", ", arrIds)
Using SQLconnect = New SQLiteConnection(strDbConnectionString)
SQLconnect.Open()
Using tr = SQLconnect.BeginTransaction()
Using SQLcommand = SQLconnect.CreateCommand
SQLcommand.CommandText = $"INSERT INTO {strToTable} SELECT * FROM {strFromTable} WHERE id IN ({strIds});"
SQLcommand.ExecuteNonQuery()
SQLcommand.CommandText = $"DELETE FROM {strFromTable} WHERE ID IN ({strIds});"
SQLcommand.ExecuteNonQuery()
End Using
tr.Commit()
End Using
End Using
The IN statement allows me to pass all of the "id" values to be deleted as a batch. This solution is faster and more secure than doing them one by one with no transaction.
Thanks for the comments, and best wishes to everyone in their coding.
I am trying to find the max product ID and store the value in a local variable "MaxID" and return this value. I am trying to convert the result of the query into an Integer type but I am not able to do it. Below is the code:
Public Function GetMaxID(ByVal TableName As String, ByVal ID As String) As Integer
Dim MaxID As Integer
Dim sqlquery As SqlCommand
Dim field_name As String = ID
Dim con As SqlConnection
con = New SqlConnection()
con.ConnectionString = "Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename='D:\Docs Dump\Work\Srinath\SrinathDB.mdf';Integrated Security=True;Connect Timeout=30"
con.Open()
Try
sqlquery = New SqlCommand("SELECT MAX( #field ) FROM #table ", con)
sqlquery.Parameters.AddWithValue("#field", field_name)
sqlquery.Parameters.AddWithValue("#table", TableName)
MaxID = CInt(sqlquery.ToString)
con.Close()
Return MaxID
Catch ex As Exception
Return 0
Exit Function
con.Close()
End Try
End Function
End Class
MaxID = CInt(sqlquery.ExecuteScalar())
You also should know about SqlCommand.ExecuteReader(), SqlCommand.ExecuteNonQuery() (for inserts/updates/deletes), and SqlDataAdapter.Fill().
Where you'll still have a problem is you can't use a parameter value for the table name or column name. The Sql Server engine has a "compile" step, where it has to be able to work out an execution plan, including permissions/security, at the beginning of the query, but variable names like #table and #field aren't resolved until later. It's not what actually happens, but think of it as if you had string literals in those places; imagine trying to run this:
SELECT MAX('ID') FROM 'MyTable'
MAX('ID') will always return the string value ID, and not anything from an ID column in any rows. But the MyTable part is not the correct place for a string literal, and such a query wouldn't even compile.
I also see people here from time to time try to create functions like GetMaxId(), and it's almost always misguided in the first place. If the intended use for this function is the same as what I usually see, you're setting up a major race condition issue in your application (one that probably won't show up in any testing, too). Sql Server gives you features like identity columns, sequences, and the scope_identity() function. You should be using those in such a way that new IDs are resolved on the server as they are created, and only (and immediately) then returned to your application code.
But that issue aside, here's a better way to structure this function:
Public Class DB
Private conString As String = "Data Source=(LocalDB)\MSSQLLocalDB;AttachDbFilename='D:\Docs Dump\Work\Srinath\SrinathDB.mdf';Integrated Security=True;Connect Timeout=30"
'You want a separate method per-table that already knows the table and column names
Public Function GetMyTableMaxID() As Integer
Dim sql As String = "SELECT MAX(ID) FROM MyTable"
Using con As New SqlConnection(conString), _
sqlQuery As New SqlCommand(sql, con)
'Parameters would go here.
'Do NOT use AddWithValue()! It creates performance issues.
' Instead, use an Add() overload where you provide specific type information.
'No exception handling at this level. The UI or business layers are more equipped to deal with them
con.Open()
Return CInt(sqlQuery.ExecuteScalar())
End Using
'No need to call con.Close()
'It was completely missed in the old code, but handled by the Using block here
End Function
End Class
net and would to have the Header Text of columns in a datagridview be named after results from the database, e.g the query in my code returns four dates,30/08/2017,04/09/2017,21/09/2017 and 03/02/2018. My aim is to have the column headers in the data grid named after those dates. Your help will highly be appreciated.
sql = "SELECT COUNT (ServiceDate) As NoOfServiceDates FROM (SELECT DISTINCT ServiceDate FROM tblattendance)"
Using command = New OleDbCommand(sql, connection)
Using reader = command.ExecuteReader
reader.Read()
ColumnNo = CInt(reader("NoOfServiceDates")).ToString
End Using
End Using
DataGridView1.ColumnCount = ColumnNo
For i = 0 To DataGridView1.Columns.Count - 1
sql = "SELECT DISTINCT ServiceDate FROM tblattendance"
Using command = New OleDbCommand(sql, connection)
Using reader = command.ExecuteReader
While reader.Read
DataGridView1.Columns(i).HeaderText = reader("ServiceDate").ToString
End While
End Using
End Using
Next
The current code re-runs the query each time through the column count loop, meaning it will set the column header for that column to all of the date values in sequence, so the last value in the query shows in the all the columns. You only need to run the query once:
Dim i As Integer = 0
sql = "SELECT DISTINCT ServiceDate FROM tblattendance"
Using command As New OleDbCommand(sql, connection), _
reader As OleDbDatareader = command.ExecuteReader()
While reader.Read
DataGridView1.Columns(i).HeaderText = reader("ServiceDate").ToString
i+= 1
End While
End Using
Additionally, this still results in two separate trips to the database, where you go once to get the count and again to get the values. Not only is this very bad for performance, it leaves you open to a bug where another user changes your data from one query to the next.
There are several ways you can get this down to one trip to the database: loading the results into memory via a List or DataTable, changing the SQL to include the count and the values together, or adding a new column each time through the list. Here's an example using the last option:
DataGridView1.Columns.Clear()
Dim sql As String = "SELECT DISTINCT ServiceDate FROM tblattendance"
Using connection As New OleDbConnection("string here"), _
command As New OleDbCommand(sql, connection)
connection.Open()
Using reader As OleDbDataReader = command.ExecuteReader()
While reader.Read
Dim column As String = reader("ServiceDate").ToString()
DataGridView1.Columns.Add(column, column)
End While
End Using
End Using
Even better if you can use something like Sql Server's PIVOT keyword in combination with the DataGridView's AutoGenerateColumns feature for DataBinding, where you will write ONE SQL statement that has both column info and data, and simply bind the result set to the grid.
The For Next is incorrect. You execute your command for every column, when you only need to execute it once. The last result from the DataReader will be the header for every column as currently written.
You should iterate through your DataReader and increment the cursor variable there:
Dim i As Integer = 0
Using command = New OleDbCommand(sql, connection)
Using reader = command.ExecuteReader
While reader.Read
DataGridView1.Columns(i).HeaderText = reader("ServiceDate").ToString
i += 1
End While
End Using
End Using
I have a parameterized query GET_CUSTOMER:
SELECT * FROM Customer WHERE id = [customer_id]
I want to call this query from another query and pass it a parameter:
SELECT * FROM GET_CUSTOMER(123)
Note the above code is not valid, it is here to give you an idea of what I'm trying to do. Is it possible to do this in MS Access?
UPDATE 1:
The queries I posted are for example. The actual queries are much more complex. I know I can use table joins, but in my specific case it would be much easier if I could run parameterized queries inside other queries (that are parameterized as well). I can't use access forms because I'm using access with my .NET application.
This is how I end up solving this with help of https://stackoverflow.com/a/24677391/303463 . It turned out that Access shares parameters among all queries so there is no need to specifically pass parameters from one query to another.
Query1:
SELECT * FROM Customer WHERE ID > [param1] AND ID < [param2]
Query2:
SELECT * FROM Query1
VB.NET code:
Dim ConnString As String = "Provider=Microsoft.Jet.OleDb.4.0;Data Source=Database.mdb"
Dim SqlString As String = "Query2"
Using Conn As New OleDbConnection(ConnString)
Using Cmd As New OleDbCommand(SqlString, Conn)
Cmd.CommandType = CommandType.StoredProcedure
Cmd.Parameters.AddWithValue("param1", "1")
Cmd.Parameters.AddWithValue("param2", "3")
Conn.Open()
Using reader As OleDbDataReader = Cmd.ExecuteReader()
While reader.Read()
Console.WriteLine(reader("ID"))
End While
End Using
End Using
End Using
You can build the SQL on the fly.
MyID = prompt or get from user some ID
strSQl = "Select * from tblCustomer where ID in " & _
"(select * from tblTestCustomers where id = " & MyID
So you can nest, or use the source of one query to feed a list of ID to the second query.
I need to read row by row in a column in a table then I need to store this then call procedure to insert data to a different column using vb.net.
I have already create the DB connection and I know how to call the procedure
but I'm not sure of how to read in the loop and then to assign it to a variable to call it in the store procedure.
Dim drDocs As SqlClient.SqlDataReader
Dim cmdDocs As SqlClient.SqlCommand
Dim Doc As Long
Dim l As Long
Using conn As New SqlConnection(DBpath)
cmdDocs = New SqlClient.SqlCommand("Select (RecordID) from DocID", conn)
drDocs = cmdDocs.ExecuteReader
Do While drDocs.Read
'need it read each row in that field and hold value'
Loop
drDocs.Close()
cmdDocs.Dispose()
If Doc Then
cmdDocs = New SqlClient.SqlCommand("Insert_Doc", conn)
cmdDocs.CommandType = CommandType.StoredProcedure
cmdDocs.Parameters.Add("path", SqlDbType.NVarChar).Value =need to put hold value from reading that cloumn row by row
End If
End If
The code you've provided actually works now. It is as Juergen D says, sql functions like Max(), min() and using Limit will only return 1/certain number of rows based on their conditions.
if I may, just use this SQL command
"select `RecordID` from DocID asc;"
If you want it in descending format, use desc instead
...now reading further, I realize that what you want to do is to store the results, then loop again through it so that you can do an sql command with it, correct? what you can do then is to pass the SQL results to a container (I use datagridviews) then loop through the container.