code: public function groupBy( $columns ) { // @codingStandardsIgnoreLine
  		if ( is_string( $columns ) ) {
  			$columns = $this->argument_to_array( $columns );
  		}
  
  		$this->statements['groups'] = $this->statements['groups'] + $columns;
  
  		return $this;
  	}
docs: Add an group by statement to the current query
  
  ->groupBy('created_at')
  
  @param array|string $columns Columns.
  
  @return self The current query builder.