David Rowley schrieb am 08.08.2019 um 13:03:
>> The following statement tries to find the overlapping values in id_list
>> between the current row and the next row:
>>
>> select id,
>> id_list,
>> lead(id_list) over (order by id) as next_list,
>> array(select unnest(id_list) intersect select
>> unnest(lead(id_list) over (order by id))) as common_ids
>> from sample_data;
>>
>> The above returns:
>>
>> id | id_list | next_list | common_ids
>> ---+---------+-----------+-----------
>> 1 | {1,2,3} | {2,3,4} | {}
>> 2 | {2,3,4} | {4,5,6} | {}
>> 3 | {4,5,6} | | {}
>>
>> The empty array for "common_ids" is obviously incorrect.
>
> I think you're confused with what the SELECT with the empty FROM
> clause does here. In your subquery "id_list" is just a parameter from
> the outer query. LEAD(id_list) OVER (ORDER BY id) is never going to
> return anything since those are both just effectively scalar values,
> to which there is no "next" value.
id_list is a column in the table and as you can see in the output
lead(id_list) most definitely returns the array from the next row.
and "select unnest(some_array)" works just fine as you can see
when "next_list" is taken from the derived table.
Thomas
