The difference between dao and ar in yii2

Active Record (AR) is a popular object-relational mapping (ORM) technology. Each AR class represents a data table (or view), the columns of the data table (or view) are reflected in the AR class as attributes of the class, and an AR instance represents a row in the table. Common CRUD operations are implemented as AR methods.

Therefore, we can access the data in a more object-oriented way. $c = new CDbCriteria(); is a way of writing ActiveRecord, which makes ActiveRecord more flexible, while assembling sql is the most common. Let’s look at two examples below. (Recommended learning: yii tutorial)

Note: AR is not intended to solve all database-related tasks. Its best applications are modeling data tables into PHP structures and executing queries that do not contain complex SQL statements. For complex query scenarios, Yii DAO should be used.

$criteria = new CDbCriteria;       
//函数方式 
$criteria->addCondition("id=1"); //查询条件，即where id = 1   
$criteria->addInCondition('id', array(1,2,3,4,5)); //代表where id IN (1,23,,4,5,);   
$criteria->addNotInCondition('id', array(1,2,3,4,5));//与上面正好相法，是NOT IN   
$criteria->addCondition('id=1','OR');//这是OR条件，多个条件的时候，该条件是OR而非AND   
$criteria->addSearchCondition('name', '分类');//搜索条件，其实代表了。。where name like '%分类%'   
$criteria->addBetweenCondition('id', 1, 4);//between 1 and 4    
     
$criteria->compare('id', 1);    //这个方法比较特殊，他会根据你的参数自动处理成addCondition或者addInCondition，   
                                //即如果第二个参数是数组就会调用addInCondition   
$criteria->addCondition("id = :id");   
$criteria->params[':id']=1;   
      
//属性方式   
$criteria->select = 'id,parentid,name'; //代表了要查询的字段，默认select='*';   
$criteria->join = 'xxx'; //连接表   
$criteria->with = 'xxx'; //调用relations    
$criteria->limit = 10;    //取1条数据，如果小于0，则不作处理   
$criteria->offset = 1;   //两条合并起来，则表示 limit 10 offset 1,或者代表了。limit 1,10   
$criteria->order = 'xxx DESC,XXX ASC' ;//排序条件   
$criteria->group = 'group 条件';   
$criteria->having = 'having 条件 ';   
$criteria->distinct = FALSE; //是否唯一查询

Yii Data Access Object (DAO) is built on the PHP Data Object (PDO) extension, enabling access to different database management systems (DBMS) in a single unified interface . Applications developed using Yii's DAO can easily switch to use different database management systems without modifying the data access code. The following is an example of DAO:

Sample code

/**
     * 关于DAO的例子
     * 获取所有用户名
     */
    public function getUsernames ()
    {
        $sqlStatement = ' SELECT `username` FROM `testdrive`.`tbl_user` ';
        $this->command = $this->connection->createCommand($sqlStatement);
        return $this->command->queryAll();
    }
    /**
     *
     * 添加一个用户
     */
    public function addUser ()
    {
        $sqlStatement = " INSERT INTO `tbl_user` (`username`, `password`, `email`) VALUES ('test', 'test', 'test@test.com') ";
        $this->command = $this->connection->createCommand($sqlStatement);
        return $this->command->execute();
    }

There are a few points to note:

1 , the difference from the DAO method is that each element in the DAO method array is still an array. With the CDbCriteria method, the elements in the array are objects.

2, even if this function is very powerful, there are still some requirements that cannot be met, and sql statements are still needed at this time. For example, select avg(num) amount from ****.

3, print the results after running, you can learn more about how Yii implements this

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