finished quick start about database [skip ci]

apps/basic/config/web.php

 <?php
 
 $params = require(__DIR__ . '/params.php');
-$db = require(__DIR__ . '/db.php');
 
 $config = [
     'id' => 'basic',
@@ -32,7 +31,7 @@ $config = [
                 ],
             ],
         ],
-        'db' => $db,
+        'db' => require(__DIR__ . '/db.php'),
     ],
     'params' => $params,
 ];

docs/guide/start-databases.md

 Working with Databases
 ======================
 
-In this section, we will describe how to create a new page to display data fetched from a database table.
-To achieve this goal, you will configure a database connection, create an [Active Record](db-active-record.md) class,
-and then create an [action](structure-controllers.md) and a [view](structure-views.md).
+In this section, we will describe how to create a new page to display the country data fetched from
+from a database table `country`. To achieve this goal, you will configure a database connection,
+create an [Active Record](db-active-record.md) class, and then create an [action](structure-controllers.md)
+and a [view](structure-views.md).
 
 Through this tutorial, you will learn
 
@@ -16,17 +17,15 @@ Note that in order to finish this section, you should have basic knowledge and e
 In particular, you should know how to create a database and how to execute SQL statements using a DB client tool.
 
 
+<a name="preparing-database"></a>
 Preparing a Database
 --------------------
 
-To begin with, create a database from which you will fetch data in your application. You may create
-a SQLite, MySQL, PostgreSQL, MSSQL or Oracle database. For simplicity, in the following description
-let's assume you have created a MySQL database named `basic`.
+To begin with, create a database named `yii2basic` from which you will fetch data in your application.
+You may create a SQLite, MySQL, PostgreSQL, MSSQL or Oracle database. For simplicity, we will use MySQL
+in the following description.
 
-Make sure you have installed the [PDO](http://www.php.net/manual/en/book.pdo.php) PHP extension and
-the PDO driver for the database you are using (e.g. `pdo_mysql` for MySQL).
-
-Now create a table named `country` and insert some sample data, using the SQL statements as shown below,
+Create a table named `country` in the database and insert some sample data. You may run the following SQL statements.
 
 ```sql
 CREATE TABLE `country` (
@@ -47,27 +46,215 @@ INSERT INTO `Country` VALUES ('RU','Russia',146934000);
 INSERT INTO `Country` VALUES ('US','United States',278357000);
 ```
 
+To this end, you have a database named `yii2basic`, and within this database there is a `country` table
+with ten rows of data.
 
+
+<a name="configuring-db-connection"></a>
 Configuring a DB Connection
 ---------------------------
 
+Make sure you have installed the [PDO](http://www.php.net/manual/en/book.pdo.php) PHP extension and
+the PDO driver for the database you are using (e.g. `pdo_mysql` for MySQL). This is a basic requirement
+if your application uses a relational database.
+
+Open the file `config/db.php` and adjust the content based on your database information. By default,
+the file contains the following content:
+
+```php
+<?php
+
+return [
+    'class' => 'yii\db\Connection',
+    'dsn' => 'mysql:host=localhost;dbname=yii2basic',
+    'username' => 'root',
+    'password' => '',
+    'charset' => 'utf8',
+];
+```
+
+This is a typical file-based [configuration](concept-configurations.md). It specifies the parameters
+needed to create and initialize a [[yii\db\Connection]] instance through which you can make SQL queries
+against the underlying database.
+
+The DB connection configured above can be accessed in the code via the expression `Yii::$app->db`.
+
+> Info: The `config/db.php` file will be included in the main application configuration `config/web.php`
+  which specifies how the [application](structure-applications.md) instance should be initialized.
+  For more information, please refer to the [Configurations](concept-configurations.md) section.
 
 
+<a name="creating-active-record"></a>
 Creating an Active Record
 -------------------------
 
+To represent and fetch the data in the `country` table, create an [Active Record](db-active-record.md)
+class named `Country` and save it in the file `models/Country.php`.
 
+```php
+namespace app\models;
+
+use yii\db\ActiveRecord;
+
+class Country extends ActiveRecord
+{
+}
+```
+
+The `Country` class extends from [[yii\db\ActiveRecord]]. You do not need to write any code inside of it.
+Yii will guess the associated table name from the class name. In case this does not work, you may
+override the [[yii\db\ActiveRecord::tableName()]] method to explicitly specify the associated table name.
+
+Using the `Country` class, you can manipulate the data in the `country` table easily. Below are some
+code snippets showing how you can make use of the `Country` class.
+
+```php
+use app\models\Country;
+
+// get all rows from the country table and order them by "name"
+$countries = Country::find()->orderBy('name')->all();
+
+// get the row whose primary key is "US"
+$country = Country::findOne('US');
+
+// displays "United States"
+echo $country->name;
+
+// modifies the country name to be "U.S.A." and save it to database
+$country->name = 'U.S.A.';
+$country->save();
+```
+
+> Info: Active Record is a powerful way of accessing and manipulating database data in an object-oriented fashion.
+You may find more detailed information in the [Active Record](db-active-record.md). Besides Active Record, you may also
+use a lower-level data accessing method called [Data Access Objects](db-dao.md).
+
+
+<a name="creating-action"></a>
 Creating an Action
 ------------------
 
+To expose the country data to end users, you need to create a new action. Instead of doing this in the `site`
+controller like you did in the previous sections, it makes more sense to create a new controller specifically
+for all actions about manipulating country data. Name this new controller as `CountryController` and create
+an `index` action in it, as shown in the following,
+
+```php
+namespace app\controllers;
+
+use yii\web\Controller;
+use yii\data\Pagination;
+use app\models\Country;
 
+class CountryController extends Controller
+{
+    public function actionIndex()
+    {
+        $query = Country::find();
+
+        $pagination = new Pagination([
+            'defaultPageSize' => 5,
+            'totalCount' => $query->count(),
+        ]);
+
+        $countries = $query->orderBy('name')
+            ->offset($pagination->offset)
+            ->limit($pagination->limit)
+            ->all();
+
+        return $this->render('index', [
+            'countries' => $countries,
+            'pagination' => $pagination,
+        ]);
+    }
+}
+```
+
+Save the above code in the file `controllers/CountryController.php`.
+
+The `index` action calls `Country::find()` to build a DB query and retrieve all data from the `country` table.
+To limit the number of countries returned in each request, the query is paginated with the help of a
+[[yii\data\Pagination]] object. The `Pagination` object serves for two purposes:
+
+* Sets the `offset` and `limit` clauses for the SQL statement represented by the query so that it only
+  returns a single page of data (at most 5 rows in a page).
+* Being used in the view to display a pager consisting of a list of page buttons, as will be explained in
+  the next subsection.
+
+At the end, the `index` action renders a view named `index` and passes the country data as well as the pagination
+information to it.
+
+
+<a name="creating-view"></a>
 Creating a View
 ---------------
 
+Under the `views` directory, first create a sub-directory named `country`. This will used to hold all
+views rendered by the `country` controller. Within the `views/country` directory, create a file named `index.php`
+with the following content:
+
+```php
+<?php
+use yii\helpers\Html;
+use yii\widgets\LinkPager;
+?>
+<ul>
+<?php foreach ($countries as $country): ?>
+    <li>
+        <?= Html::encode("{$country->name} ({$country->code})") ?>:
+        <?= $country->population ?>
+    </li>
+<?php endforeach; ?>
+</ul>
+
+<?= LinkPager::widget(['pagination' => $pagination]) ?>
+```
+
+The view consists of two parts. In the first part, the country data is traversed and rendered as an unordered HTML list.
+In the second part, a [[yii\widgets\LinkPager]] widget is rendered using the pagination information passed from the action.
+The `LinkPager` widget displays a list of page buttons. Clicking on any of them will refresh the country data
+in the corresponding page.
 
+
+<a name="how-it-works"></a>
 How It Works
 ------------
 
+To see how it works, use your browser to access the following URL:
+
+```
+http://hostname/index.php?r=country/index
+```
 
+You will see a page showing five countries. And below the countries, you will see a pager with four buttons.
+If you click on the button "2", you will see that the page displays another five countries in the database.
+Observe more carefully and you will find the URL in the browser changes to
+
+```
+http://hostname/index.php?r=country/index&page=2
+```
+
+Behind the scene, [[yii\data\Pagination|Pagination]] is playing the magic.
+
+* Initially, [[yii\data\Pagination|Pagination]] represents the first page, which sets the country query
+  with the clause `LIMIT 5 OFFSET 0`. As a result, the first five countries will be fetched and displayed.
+* The [[yii\widgets\LinkPager|LinkPager]] widget renders the page buttons using the URLs
+  created by [[yii\data\Pagination::createUrl()|Pagination]]. The URLs will contain the query parameter `page`
+  representing different page numbers.
+* If you click the page button "2", a new request for the route `country/index` will be triggered and handled.
+  [[yii\data\Pagination|Pagination]] reads the `page` query parameter and sets the current page number 2.
+  The new country query will thus have the clause `LIMIT 5 OFFSET 5` and return back the next five countries
+  for display.
+
+
+<a name="summary"></a>
 Summary
 -------
+
+In this section, you have learned how to work with a database. You have also learned how to fetch and display
+data in pages with the help of [[yii\data\Pagination]] and [[yii\widgets\LinkPager]].
+
+In the next section, you will learn how to use the powerful code generation tool, called [Gii](tool-gii.md),
+to help you rapidly implement some commonly required features, such as the Create-Read-Update-Delete (CRUD)
+operations about the data in a DB table. As a matter of fact, the code you have just written can all
+be automatically generated using this tool.