# AQL teaser 2 for implementers

**Category:** [AQL](https://discourse.openehr.org/c/aql/43)
**Created:** 2020-02-28 15:57 UTC
**Views:** 377
**Replies:** 10
**URL:** https://discourse.openehr.org/t/aql-teaser-2-for-implementers/388

---

## Post #1 by @Seref

Dear implementers:
@bna @matijap @ian.mcnicoll

Given the composition on the left, and the AQL query after that,    what should the result set for the select clause on the right be? Same as before: think on it whenever you have some free time

![image|690x275](upload://qyV4Vk0RsO0E65CUHZRbdWsODKa.png) 

    SELECT
        c,s,i_1,i_2
    FROM
        COMPOSITION C c
            CONTAINS
        SECTION s
            CONTAINS 
        (INSTRUCTION i_1 AND INSTRUCTION i_2)

<c|s|i|i|i>(i) on the leftmost tree is composition|section|instruction(x3)  instances, i.e. a single composition instance.

---

## Post #2 by @matijap

If I argued collapsing results into as few rows as possible (while still having a characteristic of a cartesian product), I will with even more confidence proclaim that is the correct way in case of conjunction. This is exactly like SQL inner join with a condition that i_1 != i_2.

---

## Post #3 by @Seref

sorry @matijap I understand your point but can't understand how you suggest that point could be applied to this example. 
do you mean you'd collapse rows that are permutations of the same set ([i1,i2], [i2,i1]) ?

---

## Post #4 by @matijap

Yes, and I'll edit my previous message comparing this to inner join: there is a subtle difference that the instructions are never the exact same object; there need to be two instructions.

---

## Post #5 by @matijap

Oh, I'm sorry, I didn't read the question clearly enough; I see you are asking about a result on a composition with 3 instructions, and in this case you'll get 6 rows (3x3=9, minus the 3 cases where both variables bind to the same object).

---

## Post #6 by @Seref

[quote="matijap, post:5, topic:388"]
where both variables bind to the same object
[/quote]


My explanation for removing those is "an object cannot be in two columns of a row at the same time", since merely having two variables implies having two different instances of data :) Glad to agree on yet another one.

---

## Post #7 by @matijap

[quote="Seref, post:6, topic:388"]
My explanation for removing those is “an object cannot be in two columns of a row at the same time”, since merely having two variables implies having two different instances of data
[/quote]

We agree on what having two variables implies. You could, however, use the same variable multiple times in the projection in a way that would cause having "an object in two rows". Or indeed you could have different variables between which there is containment, and it's actually what bothers Pablo, but I say you can have the same field in the projection as many times you like. :)

---

## Post #8 by @Seref

[quote="matijap, post:7, topic:388"]
in a way that would cause having “an object in two rows”
[/quote]

absolutely. My statement is based on "a row". Completely agree with the rest of your statement

---

## Post #9 by @matijap

[quote="matijap, post:7, topic:388"]
having “an object in two rows”
[/quote]

I meant columns, of course, but I think you understood that.

---

## Post #10 by @Seref

no , I did not read it carefully it appears :) but I think you're referring to SELECT reusing an alias/variable from alias multiple times, in which case, I'd certainly agree.

---

## Post #11 by @bna

This is the permutation problem - again :-)  
The result-set will have 3 x 3 = 9 rows. 

![image|172x272](upload://bDrc18voL9qNkQNPz2wvq4hKqlj.png)

The executed AQL was: 

    SELECT
       c/name/value as C1,
       s/name/value as S1,
       i_1/name/value as I1,
       i_2/name/value as I2
    FROM
    COMPOSITION c
        CONTAINS SECTION s
            CONTAINS (
            INSTRUCTION i_1 AND INSTRUCTION i_2
            )

---


**Canonical:** https://discourse.openehr.org/t/aql-teaser-2-for-implementers/388
**Original content:** https://discourse.openehr.org/t/aql-teaser-2-for-implementers/388