Snapshot_merge: 'WHEN MATCHED' cannot appear more than once in a 'UPDATE' clause of a MERGE statement

[mikaelene]

Describe the feature

The SQL generated for updating records in merge statements generated by snapshots does not work on SQL Server.

Describe alternatives you've considered

I want it to work on SQL Server as well as all other databases.

Additional context

This applies to SQL Server, but it could be better written anyway to not confuse developers.

Who will this benefit?

SQL Server dbt users

Are you interested in contributing this feature?

I can make a PR solving this

[mikaelene]

https://github.com/fishtown-analytics/dbt/blob/a4280d7457c32dce072d3224190a4779abc77d77/core/dbt/include/global_project/macros/materializations/snapshot/snapshot_merge.sql#L14-L24

could be rewritten as

    when matched
     and DBT_INTERNAL_DEST.dbt_valid_to is null
     and DBT_INTERNAL_SOURCE.dbt_change_type in ('update', 'delete')
        then update
        set dbt_valid_to = DBT_INTERNAL_SOURCE.dbt_valid_to

[jtcohen6]

Agree that the more concise version is preferable, anyway!