Need help on merging Partition

Question

I have a scenario where two tables (main and archive table) are using the same partition number and file group.
Row count is different on each table, though partition range value is same. The way it happened was that we created archived partition process to archive partitions in which data is older than 4 years. After running archiving process , business users inserted
data which were older than 4 years and fell into that archiving range in main table.
Now we have rows in both main table and archiving table. I want to merge those record together in archive table.
I was thinking to switch partition but I believe for that I need to empty archive table partition. Any guidance would be appreciated..
ZK

Answer 1

Use $PARTITION function to get partition number for the value. Look at BOI for more details. 
example from BOI
CREATE PARTITION FUNCTION RangePF1 ( int )
AS RANGE FOR VALUES (10, 100, 1000) ;
GO
SELECT $PARTITION.RangePF1 (100) ;
Then check rows values in sys.partitions table for that partition number and object_id 
select rows from sys.partitions where object_id = object_id (<archive table>) and partition_number = @partition_number
if rows are 0 then that partition is empty you can switch partition else do delete and insert.