Feature Request: Add support for multiple UPDATE/DELETE actions in MERGE INTO statements

[onnimonni]

Hey!

I just learned about the MATCHED BY SOURCE directive which is pretty cool and very helpful when one would want to delete missing records.

Following example works perfectly in plain DuckDB:

CREATE TABLE my_table (
    ts TIMESTAMP,
    id BIGINT,
    another BIGINT
);
CREATE TABLE my_table_new AS
    FROM my_table LIMIT 0;

insert into my_table VALUES
    ('2025-09-14', 41, 39),
    ('2025-09-15', 43, 39),
    ('2025-09-16', 44, 40);

insert into my_table_new VALUES
    ('2025-09-14', 41, 39),
    ('2025-09-15', 43, 20),
    ('2025-09-17', 45, 20);

MERGE INTO my_table
USING my_table_new
USING(id)
WHEN MATCHED THEN UPDATE
WHEN NOT MATCHED BY SOURCE THEN DELETE
WHEN NOT MATCHED THEN INSERT;

But when I try to use it inside a Ducklake it crashes with:

libc++abi: terminating due to uncaught exception of type duckdb::NotImplementedException: {"exception_type":"Not implemented","exception_message":"MERGE INTO with DuckLake only supports a single UPDATE/DELETE action currently"}