将 sql 查询填充到 vb.net 上的数据网格视图中

Question

I wanna populate sql query on datagridview on vb.net I tried this code that I found on inte.net but I get an error

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        DataGridView2.Visible = False

        Dim Dataconnection As SqlConnection
        Dataconnection.ConnectionString = "server=DESKTOP-514KV5J\SQLEXPRESS;database=A;trusted_connection=True"
        Dim cmd As New SqlCommand

        cmd.Connection = Dataconnection
        cmd.CommandText = "select SA,Product,sum(quantity) quantity from(select SA,Product,quantity from tbpurchase union all select SA,Product,-quantity from tbsold)dt group by SA,Product"
        Dim rdr As SqlDataAdapter = cmd.ExecuteReader
        Dim dt As New DataTable
        dt.Load(rdr)
        rdr.close()

        DataGridView2.DataSource = dt
End Sub

I just want to view the query on datagridview2 whitout any storage on any table. if you have a solution how can I do it. because i have 2 tables. 1 for purchasing. 1 for selling. and the query that i did work on ms sql well and caculates the stock still available thanks in advance

Answer 1

There are two obvious issues there and, for all we know, there could be more. Firstly, you never actually create a connection object. All you do is declare a variable and then try to set the ConnectionString of an object that you never created. That's absolute programming fundamentals and nothing specific to do with data access, so that's not something we should have to explain, especially since you are creating objects later in the code so obviously know how.

Secondly, you aren't even trying to open the connection that you didn't create. If you call Fill on a data adapter then it will open and close the connection automatically but, if you use a data reader, you have to do it yourself.

With those two issues in mind and cleaning up the code somewhat, we arrive at this:

Using connection As New SqlConnection("server=DESKTOP-514KV5J\SQLEXPRESS;database=A;trusted_connection=True"),
      command As New SqlCommand("SELECT SA, Product, SUM(Quantity) Quantity FROM (SELECT SA, Product, Quantity FROM tbpurchase UNION ALL SELECT SA, Product, -Quantity FROM tbsold) dt GROUP BY SA, Product", connection)
    connection.Open()

    Dim table As New DataTable

    Using reader = command.ExecuteReader()
        table.Load(reader)
    End Using

    DataGridView2.DataSource = table
End Using

For objects that support it, you should always create them with a Using block if you are only going to use them then and there, because they will be implicitly disposed at the End Using line. It is also preferable to pass arguments to constructors rather than call parameterless constructors and then set properties immediately after.