Yii2 merge queries like cdbcriteria in yii1 - mysql

Question

I have a problem regarding merge of multiple queries.
In Yii 1.x you could merge a CDbCriteria with
$criteria->merge($otherCriteria)
How can I achieve the same nested conditions etc with queries in Yii2?
Edit:
Let's say I want separate queries to form subqueries. And after all subqueries are done I want to merge them together to the one big query.

Answer 1

There is no CDbCriteria concept in Yii2 anymore. Instead you can refer to the following classes:
http://www.yiiframework.com/doc-2.0/yii-db-query.html (yii\db\Query)
http://www.yiiframework.com/doc-2.0/yii-db-activequery.html (yii\db\ActiveQuery)
All you did before with CDbCriteria now you can do with above classes. So, there will be no need to merge two criteria with each other.
update
Yii2 also supports sub-queries like below (as Yii2's official guide):
$subQuery = (new Query)->select('COUNT(*)')->from('user');
$query = (new Query)->select(['id', 'count' => $subQuery])->from('post');
Which results in:
SELECT `id`, (SELECT COUNT(*) FROM `user`) AS `count` FROM `post`
http://www.yiiframework.com/doc-2.0/guide-db-query-builder.html#building-query

Answer 2

I also recently ran into this issue. Complete fusion of queries (select, join etc.) does not exist (as I understand it). But you can manually merge conditions, for example:
$query1 = \app\models\User::find()
->where(['column1' => 'value1', 'column2' => 'value2']);
$query2 = \app\models\User::find()
->where(['and', '[[column3]] = :column3', '[[column4]] = :column4'])
->addParams([
':column3' => 'value3',
':column4' => 'value4'
]);
// merge conditions
$query1->orWhere($query2->where);
$query1->addParams($query2->params);
// build SQL
echo $query1->createCommand()->rawSql;
Built SQL:
SELECT * FROM `yii2_user` WHERE
((`column1`='value1') AND (`column2`='value2')) OR
((`column3` = 'value3') AND (`column4` = 'value4'))

Answer 3

In addition to both the excellent answers above, should you need to merge two conditions to send on to a method which takes a 'condition' parameter in the same format as where(), then you can either build your own array:
$condition1 = ['column1' => 'value1', 'column2' => 'value2'];
$condition2 = ['column3' => 'value3', 'column4' => 'value4'];
$condition = [
'or',
$condition1,
$condition2,
];
$model->updateAll([ 'column5' => 'value5' ], $condition);
Or, if it feels more logical, you can use a temporary query object to build the conditions up and merge, etc. Then pass the generated where condition from it, e.g.:
$query1 = new \yii\db\Query;
$query1->andWhere(['column1' => 'value1', 'column2' => 'value2']);
$query2 = new \yii\db\Query;
$query2->andWhere(['column3' => 'value3', 'column4' => 'value4']);
$query = new \yii\db\Query;
$query->where($query1->where)
->orWhere($query2->where);
$model->updateAll([ 'column5' => 'value5' ], $query->where);
Obviously, this makes more sense when the queries or conditions are built up elsewhere in the code and separately passed the the place where the model method is executed.