How to correctly calculate the amounts for different tables?

Question

SQLite

correct amount

SELECT DISTINCT
  SUM("20.09".total)
FROM
  "20.09.2021" AS "20.09"

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if you make two queries on two tables (for today and yesterday) - the amount will be different

SELECT DISTINCT
  SUM("20.09".total), 
  SUM("21.09".total)
FROM
  "20.09.2021" AS "20.09",
  "21.09.2021" AS "21.09"

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it is necessary that it works like this, as for each record:

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why is that?

Answer 1

This is the so-called join multiplying. To get the correct result, you must first aggregate, and only then link.
PS. DISTINCT in all queries is superfluous, because each of the queries is guaranteed to return only one record - well, what else is there to distinct?