Query, yii\db\Query

　　The columns to be selected.Columns can be specified in either a string (e.g. "id, name") or an array (e.g. ['id', 'name']).Columns can be prefixed with table names (e.g. "user.id") and/or contain column aliases (e.g. "user.id AS user_id").The method will automatically quote the column names unless a column contains some parenthesis(which means the column contains a DB expression). A DB expression may also be passed in form ofan yii\db\ExpressionInterface object.
　　Note that if you are selecting an expression like CONCAT(first_name, ' ', last_name), you shoulduse an array to specify the columns. Otherwise, the expression may be incorrectly split into several parts.
　　When the columns are specified as an array, you may also use array keys as the column aliases (if a columndoes not need alias, do not use a string key).
　　Starting from version 2.0.1, you may also select sub-queries as columns by specifying each such columnas a Query instance representing the sub-query.

---