How to update column with null value

In the above answers, many ways and repetitions have been suggested for the same. I kept looking for an answer as mentioned is the question but couldn't find here.

Another way to put the above question "update a column with a null value" could be "UPDATE ALL THE ROWS IN THE COLUMN TO NULL"

In such a situation following works

update table_name
set field_name = NULL
where field_name is not NULL;

is as well is not works in mysql

Another possible reason for the empty string, rather than a true null is that the field is an index or is part of an index. This happened to me: using phpMyAdmin, I edited the structure of a field in one of my tables to allow NULLs by checking the "Null" checkbox then hitting the "Save" button. "Table pricing has been altered successfully" was displayed so I assumed that the change happened -- it didn't. After doing an UPDATE to set all of those fields to NULL, they were, instead, set to empty strings, so I took a look at the table structure again and saw that the "Null" column for that field was set to 'no'. That's when I realized that the field was part of the Primary key!

if you set NULL for all records try this:

UPDATE `table_name` SET `column_you_want_set_null`= NULL

OR just set NULL for special records use WHERE

UPDATE `table_name` SET `column_you_want_set_null`= NULL WHERE `column_name` = 'column_value' 

On insert we can use

$arrEntity=$entity->toArray();        
    foreach ($arrEntity as $key => $value) {    
        if (trim($entity->$key) == '' && !is_null($entity->$key) && !is_bool($entity->$key)){
        unset($entity->$key);
        }
    }

On update we can use

$fields=array();
foreach ($fields as $key => $value) {
        if (trim($value) == '' && !is_null($value) && !is_bool($value)){
            $fields[$key] = null;
        }
    }