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I have a "bill_date" field that I want to be blank (NULL) until it's been billed, at which point the date will be entered.

I see that MySQL does not like NULL values in datetime fields. Do any of you have a simple way to handle this, or am I forced to use the min date as a "NULL equivalent" and then check for that date?

Thanks.

EDITED TO ADD:

Ok I do see that MySQL will accept the NULL value, but it won't accept it as a database update if I'm updating the record using PHP.

The variable name is $bill_date but it won't leave the variable as NULL if I update a record without sending a value to $bill_date -- I get this error:

Database query failed: Incorrect datetime value: '' for column 'bill_date' at row 1

I assume I need to actually send the word NULL, or leave it out of the update query altogether, to avoid this error? Am I right? Thanks!!!

Answers

MySQL does allow NULL values for datetime fields. I just tested it:

mysql> create table datetimetest (testcolumn datetime null default null);
Query OK, 0 rows affected (0.10 sec)

mysql> insert into datetimetest (testcolumn) values (null);
Query OK, 1 row affected (0.00 sec)

mysql> select * from datetimetest;
+------------+
| testcolumn |
+------------+
| NULL       | 
+------------+
1 row in set (0.00 sec)

I'm using this version:

mysql> select version();
+-----------+
| version() |
+-----------+
| 5.0.45    | 
+-----------+
1 row in set (0.03 sec)

Speaking of inserting NULL values through PHP code, given you are using prepared statements (as you definitely should), there is no problem with inserting NULL values. Just bind your variable the usual way, and all PHP variables that contain null values will be stored as NULL in MySQL.

Just make sure that your PHP variable indeed contains NULL and not an empty string.