Update:

Since WordPress 4.4. this is now supported by the insert , update , replace and delete methods of wpdb and the ticket #15158 has been closed as fixed.

Thanks to @dmsnell for commenting about that update.

On the other hand, the null support in wpdb::prepare() is currently closed as wontfix in ticket #12819.

Previous answer:

NULL not supported:

It looks like you will have to write your own custom SQL to update the value with NULL .

Currently NULL is not supported by $wpdb->prepare() , that takes the input through the vsprintf formatting function.

Check out these open Trac tickets:

These tickets are about 4 years old, so I wouldn't hold my breath until this gets supported by the core ;-)

You should take a look at the source as @s_ha_dum suggested.

A possible workaround:

If you're adventurous you can try the following with the query filter:

// Add a filter to replace the 'NULL' string with NULL add_filter( 'query', 'wpse_143405_query' ); global $wpdb; $wpdb->update( 'table', array( 'status' => 'NULL', ), array( 'id' => 1 ) ); // Remove the filter again: remove_filter( 'query', 'wpse_143405_query' );

where

/** * Replace the 'NULL' string with NULL * * @param string $query * @return string $query */ function wpse_143405_query( $query ) { return str_ireplace( "'NULL'", "NULL", $query ); }