**Usage of this file**
File usage note for LLMs: this file contains task (todo list) for this project as outlined by the user.
Pending tasks begin with "TODO:". 
Tasks that are completed are marked with "DONE:".

If you started here and didn't do so earlier, start with reading the README.md file from the root of the project.

**Task List**

DONE: [NEW-1] Added custom SQL query support with new 'select' command. Users can now execute arbitrary SELECT queries against the accounting database. Example: `llm-accounting select --query "SELECT model, COUNT(*) as count FROM accounting_entries GROUP BY model"`

DONE: Update README.md with new 'select' command usage

DONE: Update README.md with recently added limits/quota module. Include a lot of examples of different combinations of limit configurations.

DONE: [CHECK-0] Checked the code for the use of functions like sleep(). Found `time.sleep(0.01)` in `src/llm_accounting/backends/sqlite.py` within the `close` method. This sleep call has been removed as per user's request.

DONE: [CHECK-1] Reviewed functions exposed by API. All public API functions are covered by the CLI. Internal functions like `initialize` and `close` are not exposed directly, which is expected.

DONE: [CHECK-2] Parameter validation logic consolidated. Moved SELECT query validation from CLI to backend's `execute_query` method. Removed redundant `model` validation from CLI and `LLMAccounting.track_usage` as `UsageEntry` handles it.

DONE: [CHECK-3] Reviewed API (BaseBackend in `src/llm_accounting/backends/base.py`). The API is logical and comprehensive, providing all necessary methods (`initialize`, `insert_usage`, `get_period_stats`, `get_model_stats`, `get_model_rankings`, `purge`, `tail`, `close`, `execute_query`) for client code to implement its own database abstraction layer effectively.

DONE: [CODE-1] Created `src/llm_accounting/backends/mock_backend.py` as an example DB backend implementation with all operations mocked. This verifies that client code can effectively implement a DB abstraction with custom code, as per [CHECK-3].

DONE: [DOCS-1] Added information to README.md about how to implement an example custom DB layer, based on [CODE-1].

DONE: [ARCH-1] Resolved Architectural Inconsistency: The project now uses a unified database interaction layer. `QuotaService` and its models (`APIRequest`, `UsageLimit`) have been refactored to use direct database access via the `BaseBackend` interface, aligning with the core `LLMAccounting` and `SQLiteBackend`'s direct `sqlite3` operations. This involved converting `APIRequest` and `UsageLimit` to dataclasses, extending `BaseBackend` with new methods for quota management, implementing these methods in `SQLiteBackend`, and updating `QuotaService` and `LLMAccounting` to use the new backend interface.

DONE: [BUG-1] Fix Inaccurate Month Calculation in QuotaService: The `time_delta` method in `src/llm_accounting/models/limits.py` no longer supports `TimeInterval.MONTH` directly. Instead, it raises a `NotImplementedError`, forcing `QuotaService` to handle month boundaries accurately.

DONE: [CODE-2] Remove `_validate_db_path` from `src/llm_accounting/backends/base.py`.
    *   **Explanation:** This method was unused and its specific implementation details were not appropriate for a base class. It was effectively dead code.
    *   **Proposed Fix:** The method has already been removed in the previous review step. This task is for documentation purposes.

DONE: [CODE-3] Remove unnecessary `__new__` method from `src/llm_accounting/backends/sqlite.py`.
    *   **Explanation:** The `__new__` method was not adding any necessary functionality and could be safely removed.
    *   **Proposed Fix:** The method has already been removed in the previous review step. This task is for documentation purposes.

DONE: [CODE-4] Replace "magic string" default database path with a named constant in `src/llm_accounting/backends/sqlite.py`.
    *   **Explanation:** The default database path `'data/accounting.sqlite'` was a hardcoded string. Using a named constant improves readability and maintainability.
    *   **Proposed Fix:** The magic string has already been replaced with `DEFAULT_DB_PATH` in the previous review step. This task is for documentation purposes.

DONE: [CODE-5] Refactor CLI context management and error handling in `src/llm_accounting/cli.py`.
    *   **Explanation:** The CLI commands had repetitive `try...except...finally` blocks and manual context management for `LLMAccounting`. This led to code duplication and potential issues with resource management.
    *   **Proposed Fix:** A `with_accounting` decorator has been implemented and applied to all CLI commands, centralizing context management and error handling. This significantly improved code readability, reduced repetition, and made error handling more robust. This task is for documentation purposes.

DONE: [CODE-6] Remove redundant `sys.exit(0)` from `main` function in `src/llm_accounting/cli.py`.
    *   **Explanation:** `sys.exit(0)` was called unconditionally, which is redundant as Click handles exit codes automatically.
    *   **Proposed Fix:** The `sys.exit(0)` call has been removed. This task is for documentation purposes.
