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This is where using sets becomes really tedious, as Postgres severely lacks an upsert-like statement.
I don't think there are joins allowed in UPDATE statement, so I will need to use WITH query, right?
Also, I'm not sure how LEFT JOIN will help me isolate and insert "missed" entries...
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Yes, you can do UPDATE with joins (http://www.postgresql.org/docs/9.4/static/sql-update.html) like this:
UPDATE table1 A SET col1 = B.col2
FROM table2 B
WHERE A.col3 = B.col4;
I meant using JOIN operator in the update. But it's still possible, though through WITH query.
