row objects keyed by the value of each row's
	 *                           first column's value.
	 *
	 * @return array|object|null Database query results.
	 */
	public function get_results( $cols = array(), $where_cols = array(), $limit = 25, $offset = null, $output = OBJECT ) {

		// Bail if no columns have been passed
		if ( empty( $cols ) ) {
			return null;
		}

		// Fetch all the columns for the table being queried
		$column_names = $this->get_column_names();

		// Ensure valid column names have been passed for the `SELECT` clause
		foreach ( $cols as $index => $column ) {
			if ( ! array_key_exists( $column, $column_names ) ) {
				unset( $cols[ $index ] );
			}
		}

		// Columns to retrieve
		$columns = implode( ',', $cols );

		// Get the table name
		$table = $this->get_table_name();

		// Setup base query
		$query = implode( ' ', array(
			"SELECT",
			$columns,
			"FROM {$table} {$this->table_alias}",
			"WHERE 1=1"
		) );

		// Ensure valid columns have been passed for the `WHERE` clause
		if ( ! empty( $where_cols ) ) {

			// Get keys from where columns
			$columns = array_keys( $where_cols );

			// Loop through columns and unset any invalid names
			foreach ( $columns as $index => $column ) {
				if ( ! array_key_exists( $column, $column_names ) ) {
					unset( $where_cols[ $index ] );
				}
			}

			// Parse WHERE clauses
			foreach ( $where_cols as $column => $compare ) {

				// Basic WHERE clause
				if ( ! is_array( $compare ) ) {
					$pattern   = $this->get_column_field( array( 'name' => $column ), 'pattern', '%s' );
					$statement = " AND {$this->table_alias}.{$column} = {$pattern} ";
					$query    .= $this->get_db()->prepare( $statement, $compare );

				// More complex WHERE clause
				} else {
					$value = isset( $compare['value'] )
						? $compare['value']
						: false;

					// Skip if a value was not provided
					if ( false === $value ) {
						continue;
					}

					// Default compare clause to equals
					$compare_clause = isset( $compare['compare_query'] )
						? trim( strtoupper( $compare['compare_query'] ) )
						: '=';

					// Array (unprepared)
					if ( is_array( $compare['value'] ) ) {

						// Default to IN if clause not specified
						if ( ! in_array( $compare_clause, array( 'IN', 'NOT IN', 'BETWEEN' ), true ) ) {
							$compare_clause = 'IN';
						}

						// Parse & escape for IN and NOT IN
						if ( 'IN' === $compare_clause || 'NOT IN' === $compare_clause ) {
							$value = "('" . implode( "','", $this->get_db()->_escape( $compare['value'] ) ) . "')";

						// Parse & escape for BETWEEN
						} elseif ( is_array( $value ) && 2 === count( $value ) && 'BETWEEN' === $compare_clause ) {
							$_this = $this->get_db()->_escape( $value[0] );
							$_that = $this->get_db()->_escape( $value[1] );
							$value = " {$_this} AND {$_that} ";
						}
					}

					// Add WHERE clause
					$query .= " AND {$this->table_alias}.{$column} {$compare_clause} {$value} ";
				}
			}
		}

		// Maybe set an offset
		if ( ! empty( $offset ) ) {
			$values = explode( ',', $offset );
			$values = array_filter( $values, 'intval' );
			$offset = implode( ',', $values );
			$query .= " OFFSET {$offset} ";
		}

		// Maybe set a limit
		if ( ! empty( $limit ) && ( $limit > 0 ) ) {
			$limit  = intval( $limit );
			$query .= " LIMIT {$limit} ";
		}

		// Execute query
		$results = $this->get_db()->get_results( $query, $output );

		// Return results
		return $results;
	}
}