How to display all posts from all categories in WordPress

Hello I’d like to display all the posts in a category on a page.

Currently I’m doing using manual else if statement, but I don’t think this is the best way to do it, (below is my code)

<?php
/*
Template Name: categories

*/
?>

<?php get_header(); ?>

<?php
//get all categories then display all posts in each term
$taxonomy = 'category';
$param_type = 'category__in';

wp_dropdown_categories('show_option_none=Select category&show_count=1&orderby=name&echo=0'); 

if ( is_category( '4' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('4'),
);

} elseif ( is_category( '5' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('5'),
);

} elseif ( is_category( '6' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('6'),
);

} elseif ( is_category( '7' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('7'),
);

} elseif ( is_category( '8' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('8'),
);

} elseif ( is_category( '9' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('9'),
);

} elseif ( is_category( '11' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('11'),
);

} elseif ( is_category( '12' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('12'),
);

} elseif ( is_category( '16' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('16'),
);

} elseif ( is_category( '17' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('17'),
);

} elseif ( is_category( '18' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('18'),
);

} elseif ( is_category( '19' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('19'),
);

} elseif ( is_category( '20' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('20'),
);

} elseif ( is_category( '21' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('21'),
);

} elseif ( is_category( '22' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('22'),
);

} elseif ( is_category( '23' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('23'),
);

} elseif ( is_category( '24' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('24'),
);

} elseif ( is_category( '25' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('25'),
);

} elseif ( is_category( '26' )) {

  $term_args=array(
  'orderby' => 'name',
  'order' => 'ASC',
  'include'  => array('26'),
);

} else {
	echo('dwayne');
}

$terms = get_terms($taxonomy,$term_args);
if ($terms) {
  foreach( $terms as $term ) {
    $args=array(
      "$param_type" => array($term->term_id),
      'post_type' => 'post',
      'post_status' => 'publish',
      'posts_per_page' => -1,
      'caller_get_posts'=> 1,

      );
    $my_query = null;
    $my_query = new WP_Query($args);
    if( $my_query->have_posts() ) {  ?>
      <div class="category section">
	    <h3><?php echo 'Category '.$term->name;?></h3>

<ul>
	    <?php
      while ($my_query->have_posts()) : $my_query->the_post(); ?>

        <div class="post-excerpt-post">

				<h2><?php echo get_the_title(); ?></h2>

			 	<h3><?php echo get_the_date(); ?> </h3>

			 	<div class="comment-number">
					<a> "><p><?php comments_number( '0', '1', '% ' ); ?></p></a>
				</div>

				<a>" rel="bookmark"> <span class="side-bar-text"><?php the_excerpt('<p>Read more...»</p>'); ?></span>  </a> 

			</div>
       <?php
      endwhile;
      ?>
      </ul>
      </div>
 <?php
    }
  }
}
wp_reset_query();  // Restore global post data stomped by the_post().
?>

<?php get_footer(); ?>

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