# Database Design

The SQLite schema is defined in `db/schema.sql`.

The database has one table, `tasks`. There are no inter-table relationships because the application stores only task records.

## Table Diagram

```text
+-----------------------------+
| tasks                       |
+-----------------------------+
| id INTEGER PK AUTOINCREMENT |
| title TEXT NOT NULL         |
| description TEXT NOT NULL   |
| due_date TEXT NOT NULL      |
| topic TEXT NOT NULL         |
| status TEXT NOT NULL        |
| archived_at TEXT NULL       |
| created_at TEXT NOT NULL    |
| updated_at TEXT NOT NULL    |
+-----------------------------+

Indexes:
  idx_tasks_active_status (archived_at, status, due_date)
  idx_tasks_topic         (topic COLLATE NOCASE)
```

## `tasks` Columns

| Column | Type | Constraints and Purpose |
| --- | --- | --- |
| `id` | `INTEGER` | Primary key with `AUTOINCREMENT`; uniquely identifies each task. |
| `title` | `TEXT` | Required; must not be blank after trimming. |
| `description` | `TEXT` | Required by schema with default `''`; stores the task description. |
| `due_date` | `TEXT` | Required; constrained to the `YYYY-MM-DD` shape using a `GLOB` check. |
| `topic` | `TEXT` | Required; must not be blank after trimming. |
| `status` | `TEXT` | Required; defaults to `Todo`; constrained to `Todo`, `In-Progress`, or `Complete`. |
| `archived_at` | `TEXT` | Nullable timestamp set when a task is archived. |
| `created_at` | `TEXT` | Required; defaults to `CURRENT_TIMESTAMP`. |
| `updated_at` | `TEXT` | Required; defaults to `CURRENT_TIMESTAMP` and is updated by application code when a task changes. |

## Constraints

- `title` and `topic` must contain non-blank text.
- `due_date` must match the date string format `YYYY-MM-DD`.
- `status` must be one of the fixed values `Todo`, `In-Progress`, or `Complete`.
- `description`, `created_at`, and `updated_at` have defaults so inserted rows always have values.

## Archiving

Tasks are not deleted by the application. Archiving sets `archived_at` on the existing task row. Active task lists exclude rows where `archived_at` is set, while archived tasks remain viewable when archived tasks are included.

## Status and Overdue Rules

The only stored statuses are `Todo`, `In-Progress`, and `Complete`.

Overdue is never stored as a status or database column. It is derived when tasks are read: a task is overdue when its due date has passed, it is not `Complete`, and it is not archived.

## Indexes

- `idx_tasks_active_status` on `(archived_at, status, due_date)`: supports active-task filtering, status grouping, and due-date ordering.
- `idx_tasks_topic` on `(topic COLLATE NOCASE)`: supports case-insensitive topic sorting.