我总是听到使用准备好的陈述，但是在wordpress中呢？

Question 1. I know I don't need to do them for wbdb->insert or wbpdb->delete, but people say do it anyway even though the class will do it for you.

// I've got a select count query function and should use it here.

global $wpdb;
$entries = $wpdb->prefix . 'simpledir_entries';
$count = $wpdb->get_var("SELECT COUNT(*) FROM $entries WHERE rel = $id;");

if($count <= 0){
    return false;
}else{
    return true;
}   

Question 2. I'm not sure how to formulate this into the proper prepared statement which is completely secured. Does $entries and $id both need it?

Thanks

Answering to your first question: Prepare statement in WordPress prepares a SQL query for safe execution. It uses sprintf()-like syntax.

For your second question: In your case of get_var query the statement should be something like this:

global $wpdb;
$entries = $wpdb->prefix . 'simpledir_entries';
$count = $wpdb->get_var($wpdb->prepare("SELECT COUNT(*) FROM %s WHERE rel = %d;", $entries, $id));

if($count <= 0){
    return false;
}else{
    return true;
}

Good Luck!!