Wordpress Show all posts in each category

wp_all_posts_in_category

<?php

$category_IDs = get_all_category_ids();

// Loop through categories
foreach ( $category_IDs as $category_ID ) :
	$query = new WP_Query( array(
		'cat'            => $category_ID,
		'posts_per_page' => - 1,
	) );

	if ( $query->have_posts() ) :
		// Show the category name first
		$category_link = get_category_link( $category_ID );
		$category_name = get_the_category_by_ID( $category_ID );
		?>
		<h2><a href="<?php echo $category_link; ?>"><?php echo $category_name; ?></a></h2>

		<ul>
		<?php	while ( $query->have_posts() ) : $query->the_post(); ?>
			<li>
				<a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
			</li>

<?php	endwhile; ?>
		</ul>
<?php	endif;
endforeach; wp_reset_postdata();