Example using WP caching.

cache-example.php

<?php
// This example assumes an object cache is available. If its not, transients WITHOUT EXPIRATION will be used.

$cache_key = 'unique_prefix_' . md5( $variables );
$results = wp_cache_get( $cache_key );
if ( false === $results ) {
  $results = $wpdb->get_var( $wpdb->prepare("SQL STATEMENT HERE", $variables['here'], $variables['etc'] ));
    wp_cache_set( $cache_key, $results );
}

cache-with-transients-example.php

<?php
// Define a unique group for your plugin/feature.
$cache_group = 'my-unique-cache-group';
// A hash of the variables makes for a good key.
$cache_key = md5( wp_json_encode( $variables ) );

// Attempt the get the results from object cache if available; or from a transient.
$results = wp_using_ext_object_cache() ? wp_cache_get( $cache_key, $cache_group ) : get_transient( $cache_group . '-' . $cache_key );

// If no results are retrieved.
if ( false === $results ) {
	
	// Get the results as intended. This is an example. This code will not execute!
	$results = $wpdb->get_var( $wpdb->prepare("SQL STATEMENT HERE", $variables['here'], $variables['etc'] ));
	
	// If object cache is available...
	if ( wp_using_ext_object_cache() ) {
		// Cache without expiration as this is the job of the cache provider.
		wp_cache_set( $cache_key, $results, $cache_group );
	} else {
		// Set a transient that expires. In this case, a month.
		set_transient( $cache_group . '-' . $cache_key, $results, MONTH_IN_SECONDS );
	}
}

// Proceed as normal...