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If each post only belongs to one category, and each category has different post template, just modify single.php, we can use the following code:

if ( in_category('fruit') ) {
    include 'single-fruit.php';
} elseif ( in_category('vegetables') ) {
    include 'single-vegetables.php';
} else {
    // Continue with normal Loop
    if ( have_posts() ) : while ( have_posts() ) : the_post();
    // ...
}

We also can use the following:

$post = $wp_query->post;
if ( in_category('9') ) {
include(TEMPLATEPATH . '/single9.php');
elseif ( in_category('12') ) {
include(TEMPLATEPATH . '/single12.php');
elseif ( in_category('42') {
include(TEMPLATEPATH . '/single42.php');
} else {
include(TEMPLATEPATH . '/single1.php');
} }

If each post may bebelongs to more than one category, and the post only use 1 main template, say a post belongs to category 2, category 4, category 5, and we only choose category 2 as main category, we can not use the above method, we can use the following code:

// get category id of the current post
$cats=get_the_category();   

$cats=get_the_category();   
foreach($cats as $cat) {
	$newcatsarr [] = $cat->cat_ID; 
}

if (in_array("2", $newcatsarr))
{
    include 'single-book.php';
}
 
else
{
    include 'single-video.php';
}

Here, category book’s id is 2, and category video’s id is 1.