Which three are true about the MERGE statement？

B: True. The Oracle Database MERGE statement can use a subquery that involves views to generate the source data for the merge operation. This allows for greater flexibility in specifying the data to be merged.

C: True. MERGE can conditionally combine rows from one or more source tables (or views, or the results of a subquery) to insert or update rows in a single target table based on whether or not a match exists.

D: True. MERGE can indeed use subqueries to produce source rows. The source data for a merge operation can be a table, view, or the result of a subquery.

The MERGE statement is a powerful SQL operation in Oracle that allows for conditional INSERT or UPDATE operations in one statement, often referred to as an "upsert" operation. It can handle complex scenarios where the decision to insert or update is based on whether the data matches between the target and source datasets.

References:Oracle SQL documentation on the MERGE statement provides information on how it can use different data sources, including subqueries and views, and perform conditional operations on the target table.