[WIP] test(compliance): add query tests for the Relational Query Interface (for discussion)

compliance/query/README.md

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+
+Query compliance tests
+======================
+
+Test cases for the
+[Relational Query Interface](../../core-spec/relational_semantics.md).
+
+[`relational_semantics.sql`](relational_semantics.sql) loads the SCOTT schema
+and defines an Ossie model over it with dataset-scoped metrics (in comments).
+It then gives 29 test cases: 12 on one relation and 17 on joins. The join
+cases include fan-out and many-to-many joins, and two queries that are run
+with each join type. Each case is an Ossie query that uses `AGG()` (in
+comments), an equivalent DuckDB query, and the expected result (in comments).
+The DuckDB queries for joins follow the reference evaluation strategy of
+Section 9 of the spec.
+
+Run the file with DuckDB. It needs no extensions:
+
+```sh
+duckdb < compliance/query/relational_semantics.sql
+```
+
+These tests are a light version of the `.ost` test format
+([#501](https://github.com/apache/ossie/issues/501)) and the view expander
+([#502](https://github.com/apache/ossie/issues/502)). When those are
+available, each case can become an `.ost` test.