What is difference between datetime and timestamp datatype in Sql Server?.
One is a date and time, the other is a column type that is updated every time a row is updated.
[Note timestamp is being deprecated; use rowversion instead]
Timestamp (deprecated synonym for rowversion) :
Is a data type that exposes automatically generated, unique binary numbers within a database. rowversion is generally used as a mechanism for version-stamping table rows. The storage size is 8 bytes. The rowversion data type is just an incrementing number and does not preserve a date or a time. To record a date or time, use a datetime2 data type.
http://msdn.microsoft.com/en-us/library/ms182776.aspx
Normally time-stamp used when ever you inserted new record into database automatically system would take default date time
ex : transaction like bank deposit or with draw
data-time datatype used at the movement of inserting the user defined date into the record
ex : date of birth
timestamp is the synonym for the rowversion data type and is subject to the behavior of data type synonyms. In DDL statements, use rowversion instead of timestamp wherever possible. In simple way to tell, It means the updating time of row. datetime means the time of creation of row.
DateTime is constant and other is changeable as the real time and local time.
Related
I have a large table shipments with the column time which has the data type timestamp(0) with time zone, i.e. all microseconds are rounded by PostgreSQL to seconds.
I'd like to convert time's data type to allow microseconds, i.e. timestamp with time zone, but the table is large, so I'd like to alter the type:
ALTER TABLE shipments
ALTER COLUMN time
TYPE timestamptz;
...but do not convert data stored in this column, taking into account timestamp(0) and timestamp are perfectly compatible - because in my case data conversion will take a lot of time. How can I accomplish this?
Thanks in advance!
ALTER TABLE shipments ALTER COLUMN time TYPE timestamptz
won't convert the data which are already stored in your table.
Only the new inserted data and updated data will be stored with the microseconds.
See the demo in dbfiddle.
Widening the value is not a problem. But changing from 'without timezone' to 'with timezone' probably is. It will not rewrite the table if your setting of "timezone" at the time you run the command is 'UTC'.
If the setting is not UTC, then it will need to rewrite the table so it can apply the offset. (I don't know that I agree this is what it should do, I haven't thought it through before. But nonetheless that is what it does do.)
I have a PostgreSQL table with a column of type timestamp. I had included this column a while ago just in case I wanted to use it for something in the future. I am now looking to convert it into an int8 and use it as an epoch time column. All rows of the table have this column set to null at the moment. When I try to alter the row using:
ALTER TABLE public.new_ambient_data
ALTER COLUMN sensor_date TYPE int8 USING sensor_date::int8;
I get the error:
ERROR: cannot cast type timestamp without time zone to bigint
Is it better to just drop the column and make a new one of the data type that I want, or is here a better SQL script to convert the empty timestamp column to an int8.
Note: The table in question has over a million rows in it.
First of all, the objective is undefined without clearing up what that int8 is going to represent. Seconds since the epoch? Milliseconds? Microseconds? (Won't matter in your particular case with all NULL values, but the next reader might be misguided.)
Next, in Postgres there is no cast defined for timestamp --> bigint (basically for the same reason). You need a valid expression for the USING clause.
Assuming you want microseconds because that's preserving the original microsecond resolution of Postgres timestamps, this will do the job:
ALTER TABLE public.new_ambient_data
ALTER COLUMN sensor_date TYPE int8 USING (extract(epoch FROM sensor_date)*1000000)::int8;
Notably, the Postgres epoch for timestamps starts at 2000-01-01 00:00:00 UTC, unlike the UNIX epoch starting at 1970-01-01 00:00:00 UTC. But extract() returns the UNIX epoch (which can be converted back to timestamptz with to_timestamp()). So just converting the internal value wouldn't do.
For your particular case (all values NULL), it's simpler to use text as stepping stone. Every type can be cast from and to text (as long as the value is compatible).
ALTER TABLE public.new_ambient_data
ALTER COLUMN sensor_date TYPE int8 USING sensor_date::text::int8;
And yes, it's probably cheaper to convert the column in place, than to drop and recreate it. While the column is all NULL, the operation is very cheap either way, as there is no actual tuple data, only a bit in the NULL bitmap. Neither way will trigger a table rewrite.
A newly added column always goes to the end of the columns list, while the converted one stays in place. Depends on what you want.
Finally, don't do it at all. The data type timestamp (or timestamptz) is typically superior to storing temporal information as generic bigint in multiple ways. See details in Laurenz' answer!
See:
Ignoring time zones altogether in Rails and PostgreSQL
How to get the date and time from timestamp in PostgreSQL select query?
How to round off milliseconds value from timestamp(0) in PostgreSQL?
First, I want to dissuade you from doing that. It is much better to use a timestamp column than to use a numeric column:
the values can more easily be understood by a human
you can make use of date arithmetic, so you don't lose anything:
timestamp - timestamp → interval
timestamp + interval → timestamp
interval / double precision → interval
This makes your queries more readable
if you use timestamp with time zone, you can have PostgreSQL handle time zone conversions for you
useful functions like date_part and date_trunc (and date_bin from v14 on!) to truncate the values and extract individual components
Internally, a timestamp is stored as a number anyway, so you don't lose performance.
The actual conversion is given in Erwin's answer, so I won't repeat that here.
please tell me how to show the value of timestamp field of a table in datetime format (human understandable). and also how to use it in comparison while querying it.
Thanks
This can't be done. The TIMESTAMP datatype is misnamed - the name has been deprecated in favour of ROWVERSION in SQL 2008, which is a much clearer name.
TIMESTAMP stores an incrementing value which changes every time a row is updated.
If you want last updated dates stored against each row, you will need to add a DATETIME or SMALLDATETIME to your table and update it with the current date on each update.
You mean timestamp data type I think - http://msdn.microsoft.com/en-us/library/ms182776%28v=SQL.90%29.aspx
You can't convert it to datetime. According to the documentation in the link above :
> The timestamp data type is just an incrementing number and does not preserve a date or a time. To record a date or time, use a datetime data type.
This is a follow up to this question: Unique responses rails gem
I'm going to create an index based on the user id, url and a date type.
I want date type (not datetime type) because I want the day, the 24 hour day to be part of the index to avoid duplication of page views counts on the same day.
In other words: A view only counts once in a day by a visitor.
I also want the default value of that column (viewdate) to be the function GETDATE().
This is what I have in my migration:
execute "ALTER TABLEpage_viewsADD COLUMN viewdate datetime DEFAULTGETDATE()`"
But the value viewdate is always empty. What am I missing?
(as an aside, any other suggestions for accomplishing this goal?)
You're declaring the column as datetime type, not as date. Also I'm not sure MySQL supports default value seeding when altering the table.
Try this:
execute "ALTER TABLE page_views ADD COLUMN viewdate DATE"
PageView.update_all('viewdate=CURDATE()')
Anyone know how to extract the date from a datetime stamp as part of the where clause?
eg.
select *
from tableA
where date between '01/08/2009' and '31/08/2009'
(Date is a timestamp!)
Many thanks,
Fiona
If this is sql server, it's not possible. The timestamp data type's name is misleading, as it does not store any date information of any kind. All it holds is a sequential value that allows you to establish record order (eg, item A was created before item B), and therefore you don't have enough information in that column alone to know on what day the row was created.
Since the link I provided is Sql Server 2000 specific, also check this link for information on SQL Server 2008:
http://msdn.microsoft.com/en-us/library/ms182776.aspx
timestamp is the synonym for the rowversion data type and is subject to the behavior of data type synonyms. In DDL statements, use rowversion instead of timestamp wherever possible.
To build a real timestamp column in Sql Server, use a DateTime (or DateTime2) data type and set it's default value to getdate() or current_timestamp.
If a real datetime value, not TIMESTAMP/ROWVERSION which is binary(8)...
SELECT DATEADD(DAY, DATEDIFF(DAY, 0, #MyValue), 0)