Allow filtering on discriminator column in dataset

[spdr870]

What is the best way to allow filtering on a discriminator column in a dataset? In the example below, I filter rows in the table Person on the PersonType.

datasets:
- name: Employee
  source: |
    SELECT *
    FROM dba.Person
    WHERE PersonType = 1

Because the source is a now a complete SQL query, it makes it harder to apply the filter. I could use CTE for this is SQL Server, but I would prefer a cleaner way. Like the example below. Is there another option available, or a proposal? Or should I use the source sql query?

datasets:
- name: Employee
  source: dba.Person
  constraints
    expression:
      dialects:
        - dialect: ANSI_SQL
          expression: PersonType = 1