A basic example for querying data from custom tables created using ACF Custom Database Tables. For more info see https://hookturn.io/2019/09/how-to-use-acf-custom-database-tables-data-with-wp_query-objects/

0-readme.md

How to use your custom table data with WP_Object queries

This example illustrates how to query an array of post IDs from a custom DB table then use the array in a WP_Query.

This can be much faster than using meta queries on a WP_Query object, particularly if you are matching multiple fields.

how-to-query-custom-database-tables.php

<?php

// Declare the global wpdb variable
global $wpdb;

// Write our custom query. In this query, we're only selecting the post_id field of each row that matches our set of
// conditions. Note the %s placeholders – these are dynamic and indicate that we'll be injecting strings in their place.
$SQL = "SELECT `post_id` FROM `wp_custom_db_table`
        WHERE `date_start` >= %s
        AND `location` = %s
        ORDER BY `date_start` ASC;";

// Use $wpdb's prepare() method to replace the placeholders with our actual data. Doing it this way protects against
// injection hacks as the prepare() method santizes the data accordingly. The output is a prepared, sanitized SQL
// statement ready to be executed.
$SQL = $wpdb->prepare( $SQL, current_time( 'Ymd' ), 'some location' );

// Query the database with our prepared SQL statement, fetching the first column of the matched rows. In our case, we
// only queried the post_id field of each row so we know that the post_id fields will be the first column. The result
// here is an array of post_ids (provided we have a match)
$post_ids = $wpdb->get_col( $SQL );

if ( $post_ids ) {

	// When calling WP_Query, we no longer need to worry about specifying a post type because we know exactly which post
	// IDs we're after. We've also already ensured the correct order of our post IDs through the ORDER BY xxx ASC
	// portion of our SQL query. So, we just tell WP_Query to return the WP_Post objects in the order of the post IDs we
	// pass to it.
	$query = new WP_Query( [
		'post__in' => $post_ids,
		'post_type' => 'any',
		'posts_per_page' => 4,
		'orderby' => 'post__in',
	] );

	// The rest is exactly as you normally would handle a WP_Query object. 
	if ( $query->have_posts() ) {
		while ( $query->have_posts() ) {
			$query->the_post();

			// Do yo thang...
		}

		wp_reset_postdata();
	}

} else {
	// …
}