src/traits/class-groupby.php
public function groupBy( $columns ) { // @codingStandardsIgnoreLine
		if ( is_string( $columns ) ) {
			$columns = $this->argument_to_array( $columns );
		}

		$this->statements['groups'] = $this->statements['groups'] + $columns;

		return $this;
	}
public function groupBy( $columns ) { // @codingStandardsIgnoreLine
		if ( is_string( $columns ) ) {
			$columns = $this->argument_to_array( $columns );
		}

		$this->statements['groups'] = $this->statements['groups'] + $columns;

		return $this;
	}
Add an group by statement to the current query

->groupBy('created_at')

@param array|string $columns Columns.

@return self The current query builder.