When should I use $wpdb->prepare, if at all?

问题:

I have a hard time figuring out if I should be using $wpdb->prepare on my database queries in WordPress to prevent things such as SQL injection.

The $wpdb Codex shows some examples using the $wpdb->prepare function, and other examples not using it.

Also, in this answer on StackOverflow, someone mentioned that a function such as $wpdb->insert has the same level of safety as using $wpdb->prepare. But what about other $wpdb functions such as $wpdb->get_var or $wpdb->query?

When should I use $wpdb->prepare, if at all?

Some of my (simplified) $wpdb class and function usage looks like this:

---

Example 1: $wpdb->insert

$wpdb->insert(
                'special_posts', 
                    array( 
                        'title' => $title,
                        'selftext_html' => $selftext_html,
                        'selftext' => $selftext,
                    ), 

                    array(
                        '%s',
                        '%s',
                        '%s',
                    )
            );

---

Example 2: $wpdb->get_results

$wpdb->get_results("SELECT * FROM special_posts WHERE selftext_html = '$value'");

---

Example 3: $wpdb->get_var

$wpdb->get_var("SELECT title FROM special_posts ORDER BY id DESC LIMIT 1");

---

Example 4: $wpdb->query

$wpdb->query('TRUNCATE TABLE special_posts');

回答1:

As I understand - the methods those have placeholders for query parameters ($wpdb->insert(), $wpdb->update(), $wpdb->delete()) don't need the $wpdb->prepare() method, and they are already safe.

But the others - those don't have placeholders, need additional sql escaping.