SQL UNION算子如何识别重复

Question

Executing the following SQL (on an PostgreSQL data base) results in 9 rows, even tough the data sets from both tables are obviously not completely identical.

removed

Result:

removed

Why does it not result in 13 rows?

Using UNION ALL does the trick, but I am wondering how SQL UNION operator identifies duplicates?

Answer 1

UNION removes duplicates from the result set . It guarantees that the result has no duplicates at all. So, it removes duplicates both within tables and between tables.

You seem to have total duplicates within the tables. They are removed.