I have a timestamp column in my database, and i use it for almost every field, but now, i just want to update the hit counter.. and i do not want to update the timestamp column with it. I use the timestamp field to see the last 'content' update. Not for every hit.
Is it possible to let mysql stop updating the timestamp column for just one query?
UPDATE mytable SET counter_field = counter_field + 1,
timestamp_field = timestamp_field
That's why I never use timestamp in my web projects.
As I have full control on my queries, I can always set update time manually, without relying on the unpredictable timestamp mechanism.
Related
I have one field in a table that has a time as a value.
table name: sessions
column: time
For example
time
-----
09:00:00 (this should be the correct time)
12:00:00:00:00
09:00:00:00
11:00:00:00:00:00:00
16:00:00:00:00
This table got messed up and I would like to clean it and keep only the 8 first characters of each row and delete everything that is after that.
Is there a way i can do this with a mysql command?
Thanks
All you need is to update the table:
update sessions
set time = left(time, 8)
Use string functions:
update mytable set mytime = substring(mytime, 1, 8)
I would also recommend changing the datatype of your column to time, so the database will properly enforce data integrity for you in the future. If your data can be implicitely converted to the target format (which should be the case once after executing the above query), you can just do:
alter table mytable modify mytime time;
Note that time is not a wise choice for a column name, since it conflicts with a MySQL datatype. I used mytime instead in the queries.
I am dealing with a legacy application that is using MariaDB to emulate a queue. One of the key things missing is that the original design doesn't insert the time the messages in the queue were inserted meaning that the order the messages are processed is not guaranteed.
So far the messages appear to be processed in order as we're only using a single MariaDB instance but I would like to add a created_on column to ensure this continues.
My question is that I need to backfill the created_on column and i was wondering if MariaDB stored the time a given row was inserted into the database?
I realise that unless it is in the schema it is unlikely but occasionally databases will have non-standard extensions that capture this sort of thing. Oracle for example has similar functionality to this.
MariaDB does not have a hidden timestamp. If the table has an AUTO_INCREMENT, that might suffice since you are asking for order, not specifically time.
My opinion of queuing via MySQL/MariaDB: "Don't queue it, just do it". The effort of queuing and dequeuing can become a burden, especially in end cases.
Yes you can, if you were to create a field make sure when you create the field you have the following:
create table test_created_on_table(
created_on timestamp default now() on update now()
);
If you already have a field just take off the "CURRENT_TIMESTAMP" flag on the created field. Whenever you create a new record in the table, just use "NOW()" for a value.
Or.
On the contrary, remove the 'ON UPDATE CURRENT_TIMESTAMP' flag and send the NOW() for that field. That way actually makes more sense.
This would track when row is inserted or updated.
There's another way of doing it by db trigger:
Adding a ModifiedTime
Adding a modified timestamp to a table is the most straight forward. All your have to do is create the field of type TIMESTAMP, and by default, MySQL will automatically update the field when the row is modified.
There are a couple of things to be aware of:
While you can have multiple TIMESTAMP fields in a row, only one of
these can be automatically updated with the current time on update.
If your UPDATE query contains a value for your ModifiedTime field,
this value will be used.
So, to add your modified timestamp field to an existing table, all you need is:
ALTER TABLE my_table ADD ModifiedTime TIMESTAMP;
Adding a CreatedTime
Adding a CreateTime value is a little more involved.
On the latest versions of MySQL it is apparently possible to create a DateTime field with a default value of CURRENT_TIMESTAMP. This wasn’t an option for me as I was having to support a somewhat older version, besides, even on the newer versions of MySQL it is not possible to have more than one field using CURRENT_TIMESTAMP, which of course we are in order to get ModifiedTime working.
So, in order to get a created timestamp, firstly we must add a DATETIME field to the table.
ALTER TABLE my_table ADD CreatedTime datetime NOT NULL;
Note, that this must be created as NOT NULL in order for the next part to work (this is because setting NOT NULL forces an automatic all zeros default).
Next, we must create a trigger, which will automatically be fired when we insert a value into our table and set the created timestamp.
DELIMITER //
DROP TRIGGER IF EXISTS my_table_insert_trigger//
CREATE TRIGGER my_table_insert_trigger
BEFORE INSERT ON my_table
FOR EACH ROW
BEGIN
IF NEW.CreatedTime = '0000-00-00 00:00:00' THEN
SET NEW.CreatedTime = NOW();
END IF;
END;//
DELIMITER ;
Now, when you insert a value into the table, this trigger will fire and, if you’ve not provided a CreatedTime field in your insert query, it will be set to the current time stamp.
I have a large table with a DATETIME column and for index reasons I want to add a column which just contains a DATE type. It seems that MySQL is not able to use an index by the following expression GROUP BY DATE(datetime) therefore I want to add another column with a second index.
For updating I use this simple statement:
UPDATE table SET datecol = DATE(datetimecol)
Now a strange behavior occurs: the datecol-column contains the correct values. But the datetimecolumn changes as well: to the current timestamp. This is the default value for this column.
I'm working now for many years with databases and MySQL but I cannot explain this behavior.
The current version is MySQL 5.1.66-0.
Do you have any suggestions or explanations for this?
Your datetimecol is not of type DATETIME, but of type TIMESTAMP which (by default) has the ON UPDATE CURRENT_TIMESTAMP attribute that automatically updates to the current time when a record is modified.
To suppress automatic properties for the first TIMESTAMP column, do either of the following:
Define the column with a DEFAULT clause that specifies a constant default value.
Specify the NULL attribute. This also causes the column to permit NULL values, which means that you cannot assign the current timestamp by setting the column to NULL. Assigning NULL sets the column to NULL.
Therefore:
ALTER TABLE my_table
MODIFY datetimecol TIMESTAMP DEFAULT CURRENT_TIMESTAMP;
To update the records this one time without altering the values in datetimecol, you should explicitly set them to their incumbent values:
UPDATE my_table SET
datetimecol = datetimecol,
datecol = DATE(datetimecol);
I have a column update_date in a table and type is timestamp. I set the deault value by using phpmyadmin drop down menu to CURRENT_TIMESTAMP. But later when ever I run sql UPDATE x SET ...
it updates the timestamp column if only there is a changes in any of the columns. What I would like to achieve is that whether there is a change or not set the current time everytime the update sql runs. Is there any way of doing it in the mysql or I need to set the update_date explicitly every time the update is called?
Thank you
You need to explicitly update the column. From the MySQL manual, TIMESTAMP properties:
The auto-update TIMESTAMP column, if there is one, is automatically updated to the current timestamp when the value of any other column in the row is changed from its current value. If all other columns are set to their current values, the TIMESTAMP column does not change. Automatic updating does not apply if the TIMESTAMP column is explicitly assigned a value other than NULL.
Emphasis mine.
I was using phpmyadmin to insert some dummy data into a table, and noticed it structured the insert like this:
INSERT INTO `arc`.`transactions` (
`txn_id` ,
`date_time` )
VALUES (
'50005',
CURRENT_TIMESTAMP );
Normally, I'd just not include the field "date_time" and the value is created by mySQL (as the field is of type timestamp), and auto-updates whenever you insert or update a record.
Is there any reason to structure the query as you see it above, explicitly inserting CURRENT_TIMESTAMP? Or is this a phpmyadmin oddity?
It could also be a matter of compatibility. CURRENT_TIMESTAMP is defined by the SQL standard. Automatically updated timestamp columns are not. An app that wants to be portable and record timestamps is better off explicitly specifying the currrent timestamp. And what better way to do that than to use the standard, built-in CURRENT_TIMESTAMP function?
You can have TIMESTAMP fields that don't auto-update (see the TIMESTAMP properties page for how to specify these), so depending on how you define the column, it might not be the case that an UPDATE query automatically adjusts the TIMESTAMP field.
PHPMyAdmin is probably taking the safe approach here and specifying the value to ensure it's updated, no matter the column definition. PHPMyAdmin can probably detect the default value if it wants to, so another possible explanation would be compatibility between various server versions and modes for any SQL that it generates.
It depends solely on date_time column definition. if it is like
`date_time` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
Then you surely don't need to specify anything when inserting or updating a row, but if it is:
`date_time` TIMESTAMP NOT NULL
Then you have to specify something every time you create or update a row