Add support for MERGE DDL

[munro]

Is your feature request related to a problem? Please describe.
Yes, programmatically generating MERGE statements.

Describe the solution you'd like
Adding support MERGE DDL operation in BigQuery.

https://cloud.google.com/bigquery/docs/reference/standard-sql/dml-syntax#merge_statement

Describe alternatives you've considered
Directly writing SQL code

Additional context
None

[jlynchMicron]

Since the BiqQuery SQLalchemy dialect is based on the PostgreSQL dialect, would this implementation most likely follow the UPSERT syntax? https://docs.sqlalchemy.org/en/20/dialects/postgresql.html#insert-on-conflict-upsert

[jlynchMicron]

NVM, I now see the failed merge request that would just use more native BQ MERGE syntax.

[jlynchMicron]

For anyone else waiting on this support that does not mind using the failed merge code above, I just copied that code to a folder and added it to my sqlalchemy_bigquery module by importing the folder (module) with the following init.py code:

import sqlalchemy_bigquery as sa_bq
from .merge import Merge

sa_bq.merge = Merge #Adding merge to sq_bq namespace.
sa_bq.__all__.append('MERGE')

[munro]

yea it's just for BQ— don't quote me but if BQ & PG's MERGE syntax are that similar, it would probably work for 90% of the use cases if ya wanna fork it. I spent a bit of time making the SQL it spits out actually look nice 🪞

I am using the code in production, but ya I hadn't signed the CLA so it got dropped (my bad)