In-Memory Relational Tables and Queries

Question

Implement a small in-memory database supporting table creation, row insertion, filtering, projection, and ordering.

```python
class Database:
    def create_table(self, name: str, columns: list[str]) -> None: ...
    def insert(self, table: str, row: dict) -> None: ...
    def query(
        self, table: str, columns=None, where=None, order_by=None
    ) -> list[dict]: ...
```

## Requirements

- Reject duplicate tables and rows containing unknown columns; define defaults for missing columns.
- `where`, when supplied, is a predicate over a row.
- Projection returns only requested existing columns and must not expose mutable internal rows.
- `order_by=(columns, ascending)` sorts lexicographically by listed columns with deterministic ties.
- Define null and mixed-type comparison behavior.
- Preserve insertion order when no ordering is requested.

## Follow-ups

1. Add joins and grouped aggregations.
2. Build an index for repeated equality predicates.
3. Add transactions and snapshot reads.
4. Parse a constrained SQL string into the query representation.