Source code for https://formidable-masterminds.com/use-frm_view_order-to-customize-sql-for-views/

frm_view_order_callback.php

<?php

/* directory search form custom where filter */
add_filter('frm_view_order', 'masterminds_directory_search_filter', 10, 2);
/**
 * masterminds_directory_search_filter() callback for frm_view_order filter
 *
 * this function is where you create custom SQL for any view
 *
 * @param array $query contains the SQL query created by Formidable
 * @param array $args contains the view
 *
 * @return $query with the rewritten custom query
 */
function masterminds_directory_search_filter( $query, $args ) {

	/* developers directory search view */
	if ( isset( $args['display']->post_name ) && $args['display']->post_name == "developers-directory-search-results" ) {

		/* initialize variables for the sql */
		global $wpdb;
	    $wpdb_prefix = $wpdb->prefix;
	    $frm_items_table = $wpdb_prefix . 'frm_items';
		$frm_item_metas_table = $wpdb_prefix . 'frm_item_metas';

		/* get the number of parameters passed */
		$parameters = count($_GET);
		$wautop = ( isset( $_GET[ 'wautop' ] ) ) ? 1 : 0;
		$query_string_parameters = $parameters-$wautop;
		$query_string_parameters_hold = $query_string_parameters;

		/* get the directory field ids */
		$devdir_directory_listing = FrmField::get_id_by_key( "devdir_directory_listing" );
		$devdir_registration_type = FrmField::get_id_by_key( "devdir_registration_type" );
		$devdir_first_name = FrmField::get_id_by_key( "devdir_first_name" );
		$devdir_display_name = FrmField::get_id_by_key( "devdir_display_name" );
		$devdir_price_range = FrmField::get_id_by_key( "devdir_price_range" );
		$devdir_country = FrmField::get_id_by_key( "devdir_country" );
		$form_id = $query['where']['it.form_id'];

		/* Start the SQL statement */
		$sql = "SELECT it.id FROM `{$frm_items_table}` it LEFT JOIN `{$frm_item_metas_table}` t2 ON t2.item_id=it.id AND t2.field_id={$devdir_directory_listing} LEFT JOIN `{$frm_item_metas_table}` t4 ON t4.item_id=it.id AND t4.field_id={$devdir_display_name} LEFT JOIN `{$frm_item_metas_table}` fn ON fn.item_id=it.id AND fn.field_id={$devdir_first_name}";
		$where = "WHERE it.is_draft = 0 AND it.form_id = {$form_id} AND t2.meta_value = 'Yes'";
		$order = "ORDER BY (CASE WHEN t4.meta_value IS NULL THEN fn.meta_value ELSE t4.meta_value END) ASC";

		$query['select'] = $sql . " " . $where;
		/* empty $query['where'] to prevent conflicts with the view */
		$query['where'] = '';
		$query['order'] = $order;

		if ( ( isset( $_GET[ 'resource_type' ] ) ||
			isset( $_GET[ 'display_name' ] ) ||
			isset( $_GET[ 'country' ] ) ||
			isset( $_GET[ 'price_range' ] ) ) ) {

			$where .= " AND ";
			$where .= ( $query_string_parameters_hold > 1 ) ? "( " : "";

			/* get the field ids for search fields */
			if ( isset( $_GET[ 'resource_type' ] ) ) {
				$sql .= " LEFT JOIN `{$frm_item_metas_table}` t3 ON t3.item_id=it.id AND t3.field_id={$devdir_registration_type}";
				$where .= "t3.meta_value = '" . $_GET[ 'resource_type' ] . "'";
				if ( $query_string_parameters > 1 ) {
					$where .= " OR ";
					--$query_string_parameters;
				}
			}
			if ( isset( $_GET[ 'display_name' ] ) ) {
				/* the display name LEFT JOIN is included in the main SQL for the ORDER BY clause */
				// $sql .= " LEFT JOIN `{$frm_item_metas_table}` t4 ON t4.item_id=t1.id AND t4.field_id={$devdir_display_name}";
				$where .= "t4.meta_value = '" . $_GET[ 'display_name' ] . "'";
				if ( $query_string_parameters > 1 ) {
					$where .= " OR ";
					--$query_string_parameters;
				}
			}
			if ( isset( $_GET[ 'price_range' ] ) ) {
				$sql .= " LEFT JOIN `{$frm_item_metas_table}` t5 ON t5.item_id=it.id AND t5.field_id={$devdir_price_range}";
				if( $query_string_parameters_hold == 1 ) {
					$where .= "t5.meta_value = '" . $_GET[ 'price_range' ] . "'";
				} else {
					$where .= "t5.meta_value <= '" . $_GET[ 'price_range' ] . "'";
				}
				if ( $query_string_parameters > 1 ) {
					$where .= " OR ";
					--$query_string_parameters;
				}
			}
			if ( isset( $_GET[ 'country' ] ) ) {
				$sql .= " LEFT JOIN `{$frm_item_metas_table}` t6 ON t6.item_id=it.id AND t6.field_id={$devdir_country}";
				$where .= "t6.meta_value = '" . $_GET[ 'country' ] . "'";
			}

			$where .= ( $query_string_parameters_hold > 1 ) ? " )" : "";

			$query['select'] = $sql . " " . $where;
		}
	}
	return $query;
}