[didibus]

This argues that composability is the most important consideration of a domain specific language. But I think, as proved by SQL taking over, the UX is more important. Any programming language must consider the programmer and its humanity and natural way of thinking and reasoning to win in getting the most adoption and mindshare. Usability does matter as the user of any programming language is a human being.

Was QUEL also more usable and natural to people? As well as being more amenable to composition?

[alquemist]

There are a handful of examples on Wikipedia: https://en.wikipedia.org/wiki/QUEL_query_languages. One example:

    retrieve (a=count(y.i by y.d where y.str = "ii*" or y.str = "foo"), b=max(count(y.i by y.d))) 

Not a particularly clear 'jumps at you' obvious semantic:

* Are a and b aggregation functions or window functions? If aggregations, how do they compose if the 'by' scopes are different?

* What does max(count(... by ...)) mean? What is the aggregation (window?) scope of max?

* How would an outer where clause compose? What is the evaluation order?

[sergeykish]

> The following table lists aggregate functions:

> count() Number of entries in column

> max() Maximum value in column

> The by clause causes the function to return a set of results, as opposed to a single result. One result is returned for each grouping specified by the by clause. Think of by as meaning "for each."

I assume it evaluates like retrieving set and scalar.

      a   |   b
    --------------
    set 1 | scalar
    set 2 | scalar

[1] http://docs.huihoo.com/ingres/9.3/QUELRef.pdf

[alquemist]

Assuming we key a and b by y.d and generalizing b to sets, we are reaching the limits of the relational model: there is no good way to represent multiple sets associated with a given key, we need independent tables to do so. Neither cross product nor null padding is a good way to represent the schema {y, a[], b[]}.

      y.d |  a   |  b
    -------------------
      y0  |  a0  |  b0
      y0  |  a0  |  b1
      y0  |  a1  |  b0
      y0  |  a1  |  b1
      y0  |  a2  |  b0
      y0  |  a2  |  b1

    
      y.d |  a   |  b
    -------------------
      y0  |  a0  |  b0
      y0  |  a1  |  b1
      y0  |  a2  |  null

[sergeykish]

    # create table y (i int, d int);
    # insert into y values (1, 1), (1, 2), (2, 1);
    # select *, (select count(*) from (select count(y.i), y.d from y group by y.d) _) as foo
      from (select count(y.i), y.d from y group by y.d) _ ;
    
     count | d | foo 
    -------+---+-----
         1 | 2 |   2
         2 | 1 |   2
    (2 rows)
    

By the way great example how unwieldy SQL is. A bit better with CTE:

    # with bar as (select count(y.i), y.d from y group by y.d)
      select *, (select count(*) from bar) as bar from foo;

[alquemist]

Nice! Should be easy to do max instead of count. Then top instead of max. Then top3 breaks the model.

[sergeykish]

Yes, should be max(count). I've lost you with top3.

    # select * from
      (select count from (select count(y.i), y.d from y group by y.d) _ limit 2) b,
      (select count(y.i), y.d from y group by y.d) a;
     count | count | d 
    -------+-------+---
         1 |     1 | 2
         2 |     1 | 2
         1 |     2 | 1
         2 |     2 | 1
    (4 rows)