Take the following pseudo C# code:

using System;
using System.Data;
using System.Linq;
using System.Collections.Generic;

public IEnumerable<IDataRecord> GetRecords(string sql)
{
     // DB logic goes here
}

public IEnumerable<IEmployer> Employers()
{
     string sql = "select EmployerID from employer";
     var ids = GetRecords(sql).Select(record => (record["EmployerID"] as int?) ?? 0);
     return ids.Select(employerID => new Employer(employerID) as IEmployer);
}

Would it be faster if the two Select() calls were combined? Is there an extra iteration in the code above? Is the following code faster?

public IEnumerable<IEmployer> Employers()
{
     string sql = "select EmployerID from employer";
     return Query.Records(sql).Select(record => new Employer((record["EmployerID"] as int?) ?? 0) as IEmployer);
}

I think the first example is more readable if there is no difference in performance.