Question

I have an arraylist that holds a subset of names found in my database. I need to write a query to get a count of the people in the arraylist for certain sections i.e.
There is a field “City” in my database from the people in the arraylist of names I want to know how many of them live in Chicago, how many live in New York etc.

Can someone help me how I might set up an sql statement to handle this. I think somehow I have to pass the subset of names to sql somehow.

Here is a sample how I am writing my sql in my code

Public Shared Function GetCAData(ByVal employeeName As String) As DataTable
        Dim strQuery As String = "SELECT EMPLID, EMPLNME, DISP_TYPE, BEGIN_DTE FROM Corr WHERE (EMPLNME = @name)"
        Dim cmd As New SqlCommand(strQuery)
        cmd.Parameters.Add("@name", SqlDbType.VarChar)
        cmd.Parameters("@name").Value = employeeName
        Dim dt As DataTable = GenericDataAccess.GetData(cmd)
        Return dt
End Function

I need a way to create a function using your sql statement to return a datatable object of names and the parameters would be city, and the List of names.

The above example isnt the sql I am looking for its just a skeleton of what the function would look like that I want to create.

So then you would use the function by iterating through all the cities passing in the same set of names each time in the fron end.

Answer 1

The best approach would depend on how big the subset of names is.

The very simplest, and probably worst approach would be to just create some dynamic sql like this..

"select city,count(*) from table where user in ("name1","name2","name3") GROUP BY city"

Where the in section is generated from the arraylist, perhaps like the following

private string CreateSQL(List<string> names)
{
    var sb = new StringBuilder();
    sb.Append("select city,count(*) from table where user in ('");
    foreach ( var name in name)
    {
       sb.Append("'");
       sb.Append(name);
       sb.Append("',");
    }
    sb.Remove(sb.Length-1,1); //remove trailing ,
    sb.Append(") GROUP BY city");

    return sb.ToString();
}

The where in clause has a 100 item limit I believe. If your subset is anything like that length, then you really need a better approach, like putting the users in another table and doing a join. In fact if the subset of users is taken from the same database by another query, then post that and we’ll right a single query.

edit: I actually don’t know how to do where user in type queries as parameterised commands, be wary of sql injection !