Who can explain the "weird" behavior in MySQL and help fix the error?

Question

MySQL

The table has a field of type DATETIME and it can be NULL.
When the user does not fill in the corresponding field in the HTML form, naturally its value will be an empty string. The following query is made like:

UPDATE `tbl` SET ...bla bla bla... `d` = DATE(:date)

it does not pass, an error occurs Error Code: 1292. Incorrect datetime value: ''
Although the query returns NULL without error
SELECT DATE('')
as expected .

Answer 1

Replaced with:

UPDATE `tbl` SET ...bla bla bla... `d` = IF(:date, DATE(:date), NULL)

Answer 2

So null and "" are 2 different things.

Answer 3

Check the value before inserting, and if it's empty explicitly set $date=0; `d`='".$date."'

Answer 4

<?php ........`d`='".(($date=="")?"0000-00-00 00:00:00":$date)."'... ... ?>