在复合类型数组中，如何为数组中的所有条目选择复合类型的第一个元素？

Question

I have a composite type:

create type p as (a int, b int);

Then I created an table:

create table f(pv p[]);

Fill in value with it:

insert into f values(array[(10,20),(30,40)]::p[]);
insert into f values(array[(1,20)]::p[]);

Now what I want is a "select" statement that display element "a" for each entry in the array, the expected may be like:

|pv     |
|{10,30}|
|{1}    |

I've tired lots of statement combination but it doesn't provide the answer.

Can anyone help?

Thank you!

Shore

Answer 1

You should consider adding an id column to your table, so that the grouping can be done on that id after unnest ing .

SELECT array_agg(a)
FROM f
    ,unnest(pv)
GROUP BY id;

Otherwise, you could group by the whole array

SELECT array_agg(a)
FROM f
    ,unnest(pv)
GROUP BY pv;

Demo