Why is the result of "count" different from what I expected?

[ryokoCL]

a simple class definition：

class SmallInt extends int {
    SmallInt() {
        this = 1 or this = 2
    }

    SmallInt child() {
        result = this + 1
    } 
}

When I run the following query:

from SmallInt i
select count(i)

the resule is 1.

however, when using the "count" like this :

select count(SmallInt i | | i)

the result is 2. I don't quite understand why.

and further I write :

from int res 
where exists( SmallInt s |   res = count(s.child()) )
select res

the result is 0 and 1.

if I modify the query to:

select count(SmallInt s | | s.child())

I get a result of 1.

Why does a variable behave differently when it's defined in the "from" clause as opposed to directly in the "count"? Could someone explain this for me?

Thank you for your assistance.

[0x73746F70626F74686572696E676D65]

count() determines the possible values of a variable.

from SmallInt i
select count(i)

from creates the variable i which is a value set of [1,2] and count is passed those values independently in the select. so 1 is only one value, and 2 is only one value. this is how the select statements are evaluated.

though select count(SmallInt i | | i) will result in 2 because you are using temporary variables and counting the possible values in the expression part of the aggregate, and then selecting that.

hope this helps. you could also just read: https://codeql.github.com/docs/ql-language-reference/expressions/#aggregations-1

[aibaars]

That's right. The following example illustrates what is going on:

from SmallInt i
select i, count(i)

gives

| i | count |
| 1 | 1 |
| 2 | 1 |

The query

from SmallInt i
select count(i)

is the same as the above except that it drops a column and would result in something like

| count |
| 1 |
| 1 |

However, QL results are real sets and have no duplicates, so the final answer is

| count |
| 1 |

[ryokoCL]

Thank you for your responses and examples, I have understood now.