Question

I have the table below:

+-------------+------------------+------+-----+-------------------+----------------+
| Field       | Type             | Null | Key | Default           | Extra          |
+-------------+------------------+------+-----+-------------------+----------------+
| category_id | int(11) unsigned | NO   | PRI | NULL              | auto_increment |
| title       | varchar(64)      | NO   |     | NULL              |                |
| description | text             | NO   |     | NULL              |                |
| created_on  | timestamp        | NO   |     | CURRENT_TIMESTAMP |                |
| updated_on  | timestamp        | YES  |     | NULL              |                |
+-------------+------------------+------+-----+-------------------+----------------+

I needed to delete some records according to the date created and executed a set of queries:

mysql> select created_on from categories limit 10;
+---------------------+
| created_on          |
+---------------------+
| 2013-01-14 09:26:21 |
| 2012-08-15 11:18:38 |
| 2012-09-06 06:44:46 |
| 2012-09-06 23:27:14 |

then

mysql> select date('2013-01-14 09:26:21');
+-----------------------------+
| date('2013-01-14 09:26:21') |
+-----------------------------+
| 2013-01-14                  |
+-----------------------------+

and finally I got:

mysql> select DATE('created_on') from categories limit 10;
+--------------------+
| DATE('created_on') |
+--------------------+
| NULL               |
| NULL               |
| NULL               |
| NULL               |

As you can see the date wasn’t converted correctly. In about 10 minutes is worked ok:

mysql> select date(created_on) from categories limit 10;
+------------------+
| date(created_on) |
+------------------+
| 2013-01-14       |
| 2012-08-15       |
| 2012-09-06       |
| 2012-09-06       |

I’m sure that the data were intact while I was running the queries.

So my question:

Can anybody explain why date function showed different results on the same input?

Answer 1

You have to use::

select DATE(created_on) from categories limit 10;

By using the inverted comma, you are asking the database to treat 'created_on' as parameter, and since it cannot be converted to date type, you are getting null