vb.net DAL Specify columns returned

Question

I have a Data Access Layer class that has a method (GetPeople) that will retrieve records from a SQL Server table (people). This table has more than 20 fields, including varbinary type.

Right now, SQL query is something like

SELECT * FROM people

From my BLL class, I will call DAL.GetPeople(), which will return all columns.

What would be the best way to specify which columns to return, so I could improve performance? For example, sometimes I would like to return all fields, other times, just one or two.

UPDATE To explain it better: In DAL I have a method GetPeople() which calls a SQL Server function GetPeople. In BLL I have a method GetPeople() which calls DAL.GetPeople(), after doing some business logic. In my presentation layer, I call BLL.GetPeople().

This is working, but on SQL function, I have "SELECT * FROM people". Sometimes I would like to retrieve only one column (eg. name) from table, but in this case all columns are returned, which I think is affects performance.

So, I would like to have a kind of dynamic SELECT query on this SQL Server function, whose columns returned would depend on how I call the function...

Answer 1

I think you are after something like this where you can pass in a comma-seperated list of column names

Private Function GenerateQuery(ByVal columnNames As String) As String

    '   columnNames in the following format 'column1,column2,column3'
    Dim lstColumnNames As String() = Split(columnNames, ",")
    Dim strSQL As New StringBuilder
    strSQL.Append("SELECT ")
    For intColNumber As Integer = 0 To lstColumnNames.GetUpperBound(0)
        strSQL.Append("[")
        strSQL.Append(lstColumnNames(intColNumber))
        strSQL.Append("]")
        If intColNumber < lstColumnNames.GetUpperBound(0) Then
            strSQL.Append(", ")
        End If
    Next
    strSQL.Append(" FROM People ")
    Return strSQL.ToString

End Function

You can use it like this: SqlCommand.CommandText = GenerateQuery("column1,column2,column3")

The column names are wrapped in [] symbols so you don't have to worry about reserved words causing the database to error.

Answer 2

Change your SQL-query to something like

SELECT column1, column2, column3 FROM people;

EDIT:

What you are going to need to do is create function that will put your SQL string together for you. When i did this before, I had all of the available fields in a checked-list control, and if i wanted them pulled, I checked them. The checked items were then put through the function to assemble the string. It should be pretty simple since there are not any joins going on.