Major Description
Now the MERGE INTO syntax is,
MERGE INTO [db_name.]target_table [AS target_alias] USING [db_name.]source_table [<time_travel_version>] [AS source_alias] ON <merge_condition> [ WHEN MATCHED [ AND <condition> ] THEN <matched_action> ] [ WHEN MATCHED [ AND <condition> ] THEN <matched_action> ] [ WHEN NOT MATCHED [ AND <condition> ] THEN <not_matched_action> ]
It would be nice if we support unlimited MATCHED and NOT MATCHED clauses in MERGE INTO statement, because users may want to deal with different "AND <condition>"s, the result of which just like a series of "CASE WHEN"s. The expected syntax looks like
MERGE INTO [db_name.]target_table [AS target_alias] USING [db_name.]source_table [<time_travel_version>] [AS source_alias] ON <merge_condition> [when_clause [, ...]]
where when_clause is
WHEN MATCHED [ AND <condition> ] THEN <matched_action>
or
WHEN NOT MATCHED [ AND <condition> ] THEN <not_matched_action>
