[naasking]

I agree the term is often abused, but I think the wikipedia page actually does a decent job of making the notion somewhat precise, along the lines I've been arguing here:

https://en.wikipedia.org/wiki/First-class_citizen

If you want to see what queries as first-class values looks like, LINQ in .NET is pretty close. I can actually write a series of queries that build on and compose with each other, like this:

    IQueryable<Person> RunQuery(int userSelection)
    {
        var first = from x in People
                    select x;
        var second = userSelection == 1
            ? from x in first where x.Birthday > '2000-01-01' select x
            : from x in first where x.Name.Contains("Jane") select x;
        return DumbJoin(first, second);
    }

    IQueryable<Person> DumbJoin(IQueryable<Person> first, IQueryable<second>)
    {
        return from x in second
               join y in first on y.Role equals x.Role into g
               select g;
    }

This query is nonsense, but it just shows you what composition really looks like when queries are first-class values. I wish raw SQL were like this!