Monday 8 October 2018

Should I use the datetime or timestamp data type in MySQ

Timestamps in MySQL are generally used to track changes to records, and are often updated every time the record is changed. If you want to store a specific value you should use a datetime field.
If you meant that you want to decide between using a UNIX timestamp or a native MySQL datetime field, go with the native format. You can do calculations within MySQL that way ("SELECT DATE_ADD(my_datetime, INTERVAL 1 DAY)") and it is simple to change the format of the value to a UNIX timestamp ("SELECT UNIX_TIMESTAMP(my_datetime)") when you query the record if you want to operate on it with PHP.


I always use DATETIME fields for anything other than row metadata (date created or modified).
As mentioned in the MySQL documentation:
The DATETIME type is used when you need values that contain both date and time information. MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'.
...
The TIMESTAMP data type has a range of '1970-01-01 00:00:01' UTC to '2038-01-09 03:14:07' UTC. It has varying properties, depending on the MySQL version and the SQL mode the server is running in.
You're quite likely to hit the lower limit on TIMESTAMPs in general use -- e.g. storing birthdate.


The main difference is that DATETIME is constant while TIMESTAMP is affected by the time_zone setting.
So it only matters when you have — or may in the future have — synchronized clusters across time zones.
In simpler words: If I have a database in Australia, and take a dump of that database to synchronize/populate a database in America, then the TIMESTAMP would update to reflect the real time of the event in the new time zone, while DATETIME would still reflect the time of the event in the au time zone.
A great example of DATETIME being used where TIMESTAMP should have been used is in Facebook, where their servers are never quite sure what time stuff happened across time zones. Once I was having a conversation in which the time said I was replying to messages before the message was actually sent. (This, of course, could also have been caused by bad time zone translation in the messaging software if the times were being posted rather than synchronized.)




I recommend using neither a DATETIME or a TIMESTAMP field. If you want to represent a specific day as a whole (like a birthday), then use a DATE type, but if you're being more specific than that, you're probably interested in recording an actual moment as opposed to a unit of time (day,week,month,year). Instead of using a DATETIME or TIMESTAMP, use a BIGINT, and simply store the number of milliseconds since the epoch (System.currentTimeMillis() if you're using Java). This has several advantages:
  1. You avoid vendor lock-in. Pretty much every database supports integers in the relatively similar fashion. Suppose you want to move to another database. Do you want to worry about the differences between MySQL's DATETIME values and how Oracle defines them? Even among different versions of MySQL, TIMESTAMPS have a different level of precision. It was only just recently that MySQL supported milliseconds in the timestamps.
  2. No timezone issues. There's been some insightful comments on here on what happens with timezones with the different data types. But is this common knowledge, and will your co-workers all take the time to learn it? On the other hand, it's pretty hard to mess up changing a BigINT into a java.util.Date. Using a BIGINT causes a lot of issues with timezones to fall by the wayside.
  3. No worries about ranges or precision. You don't have to worry about what being cut short by future date ranges (TIMESTAMP only goes to 2038).
  4. Third-party tool integration. By using an integer, it's trivial for 3rd party tools (e.g. EclipseLink) to interface with the database. Not every third-party tool is going to have the same understanding of a "datetime" as MySQL does. Want to try and figure out in Hibernate whether you should use a java.sql.TimeStamp or java.util.Date object if you're using these custom data types? Using your base data types make's use with 3rd-party tools trivial.
This issue is closely related how you should store a money value (i.e. $1.99) in a database. Should you use a Decimal, or the database's Money type, or worst of all a Double? All 3 of these options are terrible, for many of the same reasons listed above. The solution is to store the value of money in cents using BIGINT, and then convert cents to dollars when you display the value to the user. The database's job is to store data, and NOT to intrepret that data. All these fancy data-types you see in databases(especially Oracle) add little, and start you down the road to vendor lock-in.


A timestamp field is a special case of the datetime field. You can create timestamp columns to have special properties; it can be set to update itself on either create and/or update.
In "bigger" database terms, timestamp has a couple of special-case triggers on it.
What the right one is depends entirely on what you want to do.


2016 +: what I advise is to set your Mysql timezone to UTC and use DATETIME:

Any recent front-end framework (Angular 1/2, react, Vue,...) can easily and automatically convert your UTC datetime to local time.
Additionally:
(Unless you are likely to change the timezone of your servers)
Example with AngularJs
// back-end: format for angular within the sql query
SELECT DATE_FORMAT(my_datetime, "%Y-%m-%dT%TZ")...

// font-end Output the localised time
{{item.my_datetime | date :'medium' }}
All localised time format available here: https://docs.angularjs.org/api/ng/filter/date


I would always use a Unix timestamp when working with MySQL and PHP. The main reason for this being the the default date method in PHP uses a timestamp as the parameter, so there would be no parsing needed.
To get the current Unix timestamp in PHP, just do time();
and in MySQL do SELECT UNIX_TIMESTAMP();.


From my experiences, if you want a date field in which insertion happens only once and you don't want to have any update or any other action on that particular field, go with date time.
For example, consider a user table with a REGISTRATION DATE field. In that user table, if you want to know the last logged in time of a particular user, go with a field of timestamp type so that the field gets updated.
If you are creating the table from phpMyAdmin the default setting will update the timestamp field when a row update happens. If your timestamp filed is not updating with row update, you can use the following query to make a timestamp field get auto updated.
ALTER TABLE your_table
      MODIFY COLUMN ts_activity TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP;


I always use a Unix timestamp, simply to maintain sanity when dealing with a lot of datetime information, especially when performing adjustments for timezones, adding/subtracting dates, and the like. When comparing timestamps, this excludes the complicating factors of timezone and allows you to spare resources in your server side processing (Whether it be application code or database queries) in that you make use of light weight arithmetic rather then heavier date-time add/subtract functions.
Another thing worth considering:
If you're building an application, you never know how your data might have to be used down the line. If you wind up having to, say, compare a bunch of records in your data set, with, say, a bunch of items from a third-party API, and say, put them in chronological order, you'll be happy to have Unix timestamps for your rows. Even if you decide to use MySQL timestamps, store a Unix timestamp as insurance.




I found unsurpassed usefulness in TIMESTAMP's ability to auto update itself based on the current time without the use of unnecessary triggers. That's just me though, although TIMESTAMP is UTC like it was said.
It can keep track across different timezones, so if you need to display a relative time for instance, UTC time is what you would want.


I prefer using timestamp so to keep everything in one common raw format and format the data in PHP code or in your SQL query. There are instances where it comes in handy in your code to keep everything in plain seconds.


A TIMESTAMP requires 4 bytes, whereas a DATETIME requires 8 bytes.


A lot of answers here suggest to store as timestamp in the case you have to represent well defined points in time. But you can also have points in time with datetime if you store them all in UTC by convention.

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