mysql update multiple columns with same now()

问题:

I need to update 2 datetime columns, and I need them to be exactly the same, using mysql version 4.1.20. I'm using this query:

mysql> update table set last_update=now(), last_monitor=now() where id=1;

It is safe or there is a chance that the columns are update with different time, because of the 2 visible calls to now()?
I don't think that it can be update with different values (I think internally mysql calls now() just once per row or something similar), but I'm not an expert, what do you think?

Second question why this query doesn't update the columns:

mysql> update table set last_update=last_monitor=now() where id=1;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 0  Warnings: 0

回答1:

Found a solution:

mysql> UPDATE table SET last_update=now(), last_monitor=last_update WHERE id=1;

I found this in MySQL Docs and after a few tests it works:

the following statement sets col2 to the current (updated) col1 value, not the original col1 value. The result is that col1 and col2 have the same value. This behavior differs from standard SQL.

UPDATE t1 SET col1 = col1 + 1, col2 = col1;

On the second question, don't have any answers, did find nothing on mysql docs about this.

回答2:

Mysql isn't very clever. When you want to use the same timestamp in multiple update or insert queries, you need to declare a variable.

When you use the now() function, the system will call the current timestamp every time you call it in another query.

回答3:

You can store the value of a now() in a variable before running the update query and then use that variable to update both the fields last_update and last_monitor.

This will ensure the now() is executed only once and same value is updated on both columns you need.

回答4:

You can put the following code on the default value of the timestamp column: CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, so on update the two columns take the same value.

回答5:

MySQL evaluates now() once per statement when the statement commences execution. So it is safe to have multiple visible now() calls per statement.

select now(); select now(), sleep(10), now(); select now();
+---------------------+
| now()               |
+---------------------+
| 2018-11-05 16:54:00 |
+---------------------+
1 row in set (0.00 sec)

+---------------------+-----------+---------------------+
| now()               | sleep(10) | now()               |
+---------------------+-----------+---------------------+
| 2018-11-05 16:54:00 |         0 | 2018-11-05 16:54:00 |
+---------------------+-----------+---------------------+
1 row in set (10.00 sec)

+---------------------+
| now()               |
+---------------------+
| 2018-11-05 16:54:10 |
+---------------------+
1 row in set (0.00 sec)

回答6:

If you really need to be sure that now() has the same value you can run two queries (that will answer to your second question too, in that case you are asking to update last_monitor = to last_update but last_update hasn't been updated yet)

you could do something like:

mysql> update table set last_update=now() where id=1;
mysql> update table set last_monitor = last_update where id=1;

anyway I think that mysql is clever enough to ask for now() only once per query.

回答7:

There are 2 ways to this;

First, I would advice you declare now() as a variable before injecting it into the sql statement. Lets say;

var x = now();
mysql> UPDATE table SET last_update=$x, last_monitor=$x WHERE id=1;

Logically if you want a different input for last_monitor then you will add another variable like;

var y = time();
mysql> UPDATE table SET last_update=$x, last_monitor=$y WHERE id=1;

This way you can use the variables as many times as you can, not only in mysql statements but also in the server-side scripting-language(like PHP) you are using in your project. Remember these same variables can be inserted as inputs in a form on the front-end of the application. That makes the project dynamic and not static.

Secondly if now() indicates time of update then using mysql you can decalre the property of the row as a timestamp. Every time a row is inserted or updated time is updated too.