Snowflake subquery

Question

I have two tables. Transaction(ID, TERMINALID) and Terminal(ID, TERMINALID, EXPORT_DATE). The goal is to obtain for each row from Transaction table newest recored from Terminal table. Snowflake is used as a backend.

I have this SQL query:

SELECT tr.ID,
       (SELECT te.ID
        FROM "Terminal" te
        WHERE te.TERMINALID = tr.TERMINALID
        ORDER BY te.EXPORT_DATE DESC
        LIMIT 1)
FROM "Transaction" tr;

But I get this error:

SQL compilation error: Unsupported subquery type cannot be evaluated

Error disappears if I replace tr.TERMINALID with a specific value. So I can't reference parent table from nested SELECT. Why this is not possible? Query works in MySQL.

Answer 1

I'm afraid Snowflake doesn't support correlated subqueries of this kind.

You can achieve what you want by using FIRST_VALUE to compute best per-terminalid id :

-- First compute per-terminalid best id
with sub1 as (
  select 
    terminalid, 
    first_value(id) over (partition by terminalid order by d desc) id
  from terminal 
),
-- Now, make sure there's only one per terminalid id
sub2 as (
  select 
    terminalid, 
    any_value(id) id
  from sub1
  group by terminalid
)
-- Now use that result
select tr.ID, sub2.id
FROM "Transaction" tr
JOIN sub2 ON tr.terminalid = sub2.terminalid

You can run subqueries first to see what they do.

We're working on making our support for subqueries better, and possibly there's a simpler rewrite, but I hope it helps.