How to build a WP custom db table

custom-table.php

<?php
/**
 * Install placement table
 * upon plugin activation, create custom table to store user placements
 *
 * @since  4.0.0
 * @author acarter
 *
 * @return void
 */
function install_placement_table() {
	// Get the wpdb object
	global $wpdb;
	// Name the table
	$table_prefix = $wpdb->base_prefix;
	$table_name   = $table_prefix . 'peta_placements';

	$wpdb->peta_placements = $table_name;
	
	// Get the charset
	$charset_collate = $wpdb->get_charset_collate();

	// Build the table
	// If you want you can add NOT NULL so the field has to be set.
	// Example, the id for the thing you want to keep track of so it can be called easy from the
	// the db would require and ID. 
	// Be sure to set that ID as the primary key for the table with PRIMARY KEY (name_of_your_id)
	$sql = "CREATE TABLE $table_name (
		placement_id mediumint(9) NOT NULL,
		pos_order_number text NOT NULL,
		placement_order text,
		placement_name text,
		post_types text,
		categories text,
		taxonomies text,
		last_update text,
		placement_url varchar(100) DEFAULT '' NOT NULL,
		PRIMARY KEY (placement_id)
	) $charset_collate;";
		require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
		dbDelta( $sql );
}

// Run the function somewhere, functions.php etc...

/**
 * Check to see if the site already has placement table installed
 * Nice way to create a bool check for a table's existence. 
 *
 * @since. 4.0.0
 * @author adamc
 *
 * @return bool
 */
function has_placement_table() {
	global $wpdb;

	$table_name  = $wpdb->prefix . 'your_table_name';
	$table_check = $wpdb->get_var( "SHOW TABLES LIKE '$table_name'" );

	if ( isset( $table_check ) ) {
		return true;
	} else {
		return false;
	}
}

// See here how I implement a custom table for plugin-placement
// https://github.com/PETAF/plugin-placement/blob/placement-version-4.0/inc/class-custom-tables.php