Aggregate and concatenate from two tables based on comparison between columns - sql

Question

I have two tables like this:
Table1
A T
a1 t1
a2 t2
a3 t3
a4 t4
a5 t5
...
...
Table2
E T
e1 t1
e2 t2
e3 t3
e4 t4
e5 t5
...
...
what I wanted to achieve is this:
Table 3
E A'
e1 a1,a2,a3
e2 a4,a5,a6
...
...
The aggregation A' is done like this: In table 2 for each e there is a value in column T : t and with that t you look for the last 3 values in Table 1 that are less than the t in question. So a1, a2, a3 are values of A whose t values are less than t1 in Table 2 whose E is e1.
I know that I could write two queries for this like this:
ResultSet (rt) -> select t from e
and then iterate ResultSet and do something like this :
select A from Table1 where t < rt[i] limit 3 - not sure how to concatenate here :)
but I m pretty sure this is utterly inefficient. There should be a better way to do this.
I m working with Postgresql.
If it had been a dataframe from a file I would use python's pandas. Also I know that python has read_sql but the tables are very huge I don't want to load the whole table in memory which I think it won't but not sure either - anyway its a separate story.
How do we solve this in SQL? Any ideas please.

Answer 1

In table 2 for each e there is a value in column T : t and with that t you look for the last 3 values in Table 1 that are less than the t in question.
I don't understand the results follow this logic. But based on your description, you can use a lateral join:
select t2.*, t1.the_as
from t2 left join lateral
(select array_agg(t1.a) as the_as
from (select t1.*
from t1
where t1.T <= t2.T
order by t1.T desc
limit 3
) t1
) t1
on 1=1;
Note that this uses arrays rather than strings because I think arrays are a better data structure for storing multiple values. That said, you can just use string_agg() instead, if you really want a string. The syntax would be string_agg(t1.a, ',').