From: khaines@...
Date: 2006-12-06T20:14:55+09:00
Subject: Re: Lisp comprehensions => SQL

On Wed, 6 Dec 2006, Victor "Zverok" Shepelev wrote:

>> Think about the syntax of it, though.
>>
>> From a ruby perspective, you call select(), then sort_by(), then []
>>
>> Three separate method calls, and there is no way any of them can know
>> which is the last one.  So there's no way to assemble the SQL call you
>> would like to see, where a single query is assembled to implement what is
>> implied by those three separate method calls.
>>
>
> It's a matter of philosophy.

No, my point is that philosophy doesn't matter there, because without some 
additional information, it can't work quite like you have described it.

If you requires that a query start with a call to select(), but not be 
invoked until a second method call, then it would be possible;

employees.select{|e| (e.name == 'John') & (e.salary > 50)}.sort_by{|e| 
e.age}[2..10].run

You could assemble all of the information that described the query with 
the chained method calls; the sentinel that indicates the end of query 
construction is the call to run()

> To the contrary, trying to read this
>
> employees.select{|e| (e.name == 'John') & (e.salary > 50)}.sort_by{|e|
> e.age}[2..10]
>
> is very familiar for any rubyist; but also cause a bunch of questions from
> SQL perspective (would it be performed in one request or several? How many
> data would be selected? What's the overhead?)

That syntax seems very straight forward from a Ruby perspective and a 
database interaction perspective.

1) select all employees from the database where the name is 'John' and the 
salary is greater than 50.  Return that result set.  That's a discrete 
method call that invokes a discrete database interaction.

2) Sort the result set by the age field on the records.

3) Return a slice from the result set.

I think your original wish -- that all of those operations magically be 
combined into a single query -- would be counterintuitive if it were 
possible without some sort of a final sentinel method call like I 
illustrated above.


Kirk Haines