How to merge 2 tables in Oracle I have 1 big table with 25 milions of rows and i have to merge data of another table with 500,000 rows into the big table. the rows of both tables have the same columns and i have to check if the key columns exist then i update the rest of the columns , otherwise i insert the new row.
i would like to know which is the best way to do the update:
1) update the columns according to the key columns
2) delete the old column and insert the new one instead.
You can fully achieve this with MERGE:

MERGE INTO big_table B
USING (SELECT key_col, col1, col2 FROM small_table) S
ON (B.key_col = S.key_col)
WHEN MATCHED THEN UPDATE SET B.col1=S.col1, B.col2=S.col2
WHEN NOT MATCHED THEN INSERT (B.col1, B.col2)
VALUES (S.col2, S.col2)
/