I have this SQL query to a PostgreSQL database. Can it be shortened? I am thinking about the where part.
SELECT *
FROM reservations
WHERE (starts_at BETWEEN ? AND ?) OR (ends_at BETWEEN ? AND ?)
The values for the question-marks is:
The beginning of the current date in datetime format
The end of the the current date in datetime format
Same as point one
Same as point two
The code is meant to return all the reservations that begins or ends on a certain date. And works as it is supposed to. But I have to supply the same information multiple times into the query.
I so not actually use this exactly SQL, so there might be an obvious error somewhere, but please focus on the where part
I'm not a huge fan of BETWEEN in this context, because timestampor datetime can be fractional. In particular, specifying the last possible value on a given date is much more complicated than specifying the first possible value (midnight) because you have to specify the time as 23:59:59.999... out to whatever precision your RDBMS uses. PostgreSQL's timestamp is supposed to be accurate to the microsecond (1e-6 seconds), for example, so it's easy to specify a range that either includes times you don't want, or misses times that you do.
On the other hand, if you use BETWEEN with midnight of the following day so you don't have to know the precision of the time, you're including a time that doesn't exist in the date you're interested in. If your application is only precise to the second, or the minute, or to 5 minutes, then you may mis-categorize data or, worse, count it twice since it suddenly counts as being in two dates.
I would prefer:
WHERE (starts_at >= ? AND starts_at < ?)
OR (ends_at >= ? AND ends_at < ?)
Where the ? map to:
Midnight of the target date.
Midnight of the date after the target date.
Midnight of the target date.
Midnight of the date after the target date.
It's not as short, but it's decidedly safer unless you really want to specify your intervals that precisely.
However, you should not do the following, even though it's shorter:
WHERE DATE(starts_at) = ?
OR DATE(ends_at) = ?
You don't want to do that because it's not SARGEable.
This is also an example of why shortness or brevity is a poor measure of code quality. Generally, I'd order my preference like so:
Accuracy.
Performance.
Readability/maintainability.
Brevity.
No, you can't improve upon this.
WHERE (starts_at BETWEEN ? AND ?) OR (ends_at BETWEEN ? AND ?)
Related
My table has a category 'timestamp' where the timestamps are formatted 2015-06-22 18:59:59
However, using DBVisualizer Free 9.2.8 and Vertica, when I try to pull up rows by timestamp with a
SELECT * FROM table WHERE timestamp = '2015-06-22 18:59:59';
(directly copy-pasting the stamp), nothing comes up. Why is this happening and is there a way around it?
FYI, saying "the timestamps are formatted 2015-06-22 18:59:59" is incorrect if you are indeed using a TIMESTAMP type. Such types have their own internal representation of a date-time value, almost always a count since epoch. In your case with Vertica, 8 bytes are used for such storage. The formatting of the date-time value happens when a string representation is generated. Never confuse the string representation with the date-time value. Conflating the two may well be related to your problem/confusion.
A few different thoughts about possible problems…
String Literals
Are you sure Vertica takes strings as timestamp literals? That format you used is common SQL format. But given that Vertica seems to be a specialized database, I would double-check that.
If strings are not allowed, you may need to call some kind of function to transform the string into a date-time values.
Fractional Second
As the comment by Martin Smith points out, the doc for Timestamp-related data types in Vertica 7.1 says those types can have a fractional second to resolution of microseconds. That means up to 6 decimal places of a fraction.
So if you are searching for "2015-06-22 18:59:59" but the stored value is "2015-06-22 18:59:59.012345", no match on the query.
Half-Open
The fractional seconds issue described above is often the cause of problems people have when handling a span of time. If you naïvely try to pinpoint the ending time, you are likely to have problems. Seeing the "59:59" in your example string makes me think this applies to you.
The better approach to spans of time is "Half-Open" (or Half-Closed, whatever) where the beginning is inclusive while the ending is exclusive. Common notation for this is [). In comparison logic this means: value >= start AND value < stop. Notice the lack of EQUALS SIGN in the stop comparison. In English we would say "look for an hour's worth of invoices starting at 2:00 PM and going up to, but not including, 3:00 PM".
Half-Open for a week means Monday-Monday, for a month the first of one month to the first of the next month, and for a year the January 1 of one year to January 1 of the following year.
Half-Open means not using BETWEEN in SQL. SQL's BETWEEN has often be criticized. Instead do something like the following to look for an hour's worth of invoices. Notice the Z on the end of string literal which means "UTC time zone" ("Z" for "Zulu"). (But verify, as my SQL syntax may need fixing.)
SELECT *
FROM some_table_
WHERE invoice_received_ >= '2015-06-22 18:00:00Z'
AND invoice_received_ < '2015-06-22 19:00:00Z'
;
This query will catch any values such as '2015-06-22 18:59:59.654321" which seems to be eluding you.
Reserved Word
I hope you have not really named your table 'table' and your column 'timestamp'. Such use of keywords and reserved words can cause explicit errors or more subtle weird problems.
Tip: The easy way to avoid any of the over a thousand reserved words in various databases is to append a trailing underscore. The SQL standard explicitly promises to never using a trailing underscore in its reserved words. So use "timestamp_" rather than "timestamp". Another example: "invoice_" table and "received_" column. I recommend doing that as a habit on everything your name in SQL: columns, tables, constraints, indexes, and so on.
Time Zone
You are using the TIMESTAMP which is short for TIMESTAMP WITHOUT TIME ZONE. Or so I presume; the Vertica doc is vague but that is the common usage as seen in the Postgres doc, and may even be standard SQL.
Anyways, TIMESTAMP WITHOUT TIME ZONE is usually the wrong type for most business purposes. The WITH time zone is misnamed and often misunderstood as a consequence: It means "with respect for time zone" where data inputs that include an offset or other time zone information from UTC are adjusted to UTC during the INSERT/UPDATE operations. The WITHOUT type simply ignores any such offset or time zone information.
The WITHOUT type should only be used for the concept of a date-time generally without being tied to any one locality. For example, saying "Christmas this year starts at beginning of December 25, 2015". That means in any time zone rather than a specific time zone. Obviously Christmas starts earlier in Paris, for example, than in Montréal.
If you are timestamping legal documents such as invoices, or booking appointments with people across time zones, or scheduling shipments in various localities, you should be using WITH time zone type.
So back to your possible problem: Test how Vertica or your client app or your database driver is handling your input string. It may be adjusting time zones as part of the parsing of the string using your client machine’s current default time zone. When sent to the database, that value will not match the stored value if during storage no adjustment to UTC was made.
Tip: Generally best practice is to do all your storage and business logic in UTC, adjusting to local time zones only where expected by user.
We are using a Vertica database with table columns of type timestamptz, all data is inserted according to the UTC timezone.
We are using spring-jdbc's NamedParameterJdbcTemplate
All queries are based on full calendar days, e.g. start date 2013/08/01 and end date 2013/08/31, which brings everything between '2013/08/01 00:00:00.0000' and '2013/08/31 23:59:59.9999'
We are trying to modify our queries to consider timezones, i.e. I can for my local timezone I can ask for '2013/08/01 00:00:00.0000 Asia/Jerusalem' till '2013/08/31 23:59:59.9999 Asia/Jerusalem', which is obviously different then '2013/08/01 00:00:00.0000 UTC' till '2013/08/31 23:59:59.9999 UTC'.
So far, I cannot find a way to do so, I tried setting the timezone in the session:
set timezone to 'Asia/Jerusalem';
This doesn't even work in my database client.
Calculating the difference in our Java code will not work for us as we also have queries returning date groupings (this will get completely messed up).
Any ideas or recommendations?
I am not familiar with Veritca, but some general advice:
It is usually best to use half-open intervals for date range queries. The start date should be inclusive, while the end date should be exclusive. In other words:
start <= date < end
or
start <= date && end > date
Your end date wouldn't be '2013/08/31 23:59:59.9999', it would instead be the start of the next day, or '2013/09/01 00:00:00.0000'. This avoids problems relating to precision of decimals.
That example is for finding a single date. Since you are querying a range of dates, then you have two inputs. So it would be:
startFieldInDatabase >= yourStartParameter
AND
endFieldInDatabase < yourEndParameter
Again, you would first increment the end parameter value to the start of the next day.
It sounds like perhaps Vertica is TZ aware, given that you talked about timestamptz types in your answer. Assuming they are similar to Oracle's TIMESTAMPTZ type, then it sounds like your solution will work just fine.
But usually, if you are storing times in UTC in your database, then you would simply convert the query input time(s) in advance. So rather than querying between '2013/08/01 00:00:00.0000' and '2013/09/01 00:00:00.0000', you would convert that ahead of time and query between '2013/07/31 21:00:00.0000' and '2013/08/31 21:00:00.0000'. There are numerous posts already on how to do that conversion in Java either natively or with Joda Time, so I won't repeat that here.
As a side note, you should make sure that whatever TZDB implementation you are using (Vertica's, Java's, or JodaTime's) has the latest 2013d update, since that includes the change for Israel's daylight saving time rule that goes into effect this year.
Okay, so apparently:
set time zone to 'Asia/Jerusalem';
worked and I just didn't realize it, but for the sake of helping others I'm going to add something else that works:
select fiels at time zone 'Asia/Jerusalem' from my_table;
will work for timestamptz fields
If I want to store date and time, is it better to store them in a separate date and time or use a single datetime?
When should we use date and time instead of a single datetime?
I want to filter my queries either using date or time.
When you are talking about a moment in time, whether a universal moment, or a specific date and time on someone's local calendar, you use a datetime. If you want to be sure that you are talking about an exact moment in time, regardless of the observer, then you use a datetimeoffset.
If you are storing just a date then you mean a date without a time component, meaning "any time on this date".
If you are storing just a time then you mean a time without a date component, meaning "this time on any date", or "this time on a date determined by some other means".
There is no practical purpouse to having both a date and a time that are about the same thing, sitting on the same row. Just use a datetime for that.
In SQL Server 2008 you have date and time data types so this becomes a non issue.
If it is good choice it really depends by your business and how you will query you data.
If for example you want to know all the orders places between 1 and 2 PM for any day using a separated Date and Time column will make it quicker
If you intentionally do not care about the time, it's more efficient to store this data as a date datatype. Think a customer birthday column, there's not too many cases I can think of that would use this time. If there happens to be a time attached to it (often a bug), this needs to be removed via a convert statement in order to do a compare. It also consumes additional space if you don't need these values (3 bytes compared to 8).
I think it's similar to having a status code table with the id as a bigint instead of a tinyint or the like (depending on how many status codes you would plan to have).
It's just a matter of what you're using the data for, if you think there's a good chance you'll ever need the that data, then use datetime, otherwise use date.
Nothing brilliant about separating date and time,
Better you save date and time in Same column,
Here they have discussed the same issue check it : are-there-any-good-reasons-for-keeping-date-and-time-in-separate-columns
you can also get date and time separately by query
SELECT
CONVERT(VARCHAR(10),GETDATE(),111) as DatePart,
convert(varchar(15), getdate(), 108) TimePart
When allowing a user to select a date range, let's say:
Show me entries from [August 1] to [September 1]
As a user, I would generally expect this to include the results for September 1. Especially when you consider that when I select the same date for both ends, I obviously mean "from start of day to end of day":
Show me entries from [September 1] to [September 1]
As a programmer, I think of date boundaries as "zero-hour", i.e. "start-of-day"; logically, the entries for September 1 are actually after "2010-09-01 00:00:00" (hence outside of the range).
For example, in SQL the following condition would exclude everything:
SELECT * FROM entries
WHERE created_at >= DATE('2010-09-01') AND created_at <= DATE('2010-09-01')
Obviously, an adjustment needs to be made from the user input to the SQL to advance the end date by 24 hours.
However, that only applies to timestamp or datetime columns. When the column is a date then a direct comparison works and this adjustment should not be added.
In an MVC framework such as Rails, where do you handle the logic for this input mismatch before sending a query? If it's in the controller, it seems that relies too much on knowing the internal fields of the model (date vs. datetime), and if it's in the model, would a "find_in_date_range" method be understood as inclusive, or does that just invite off-by-day errors?
Finally, is my assumption correct for having the user interface represent inclusive ranges? Is this always the case or are there situations where a strict (exclusive) date boundary is more appropriate? For example, in my rake scripts I use the parameter END_DATE=2010-09-01 to capture up to this date, which is inconsistent with the UI, but makes sense to me: where do you draw this line?
My Personal Bias:
Program it however you think the users will like it, but ALWAYS make the inclusive vs exclusive choice explicit in the GUI with labeling. Rather than saying
Between _____________ and ______________
I always label as
On or after _______________ but before _______________
or
On or after ______________ through ______________
(If you are writing software that a few users use every day and you can carefully train them, then they aren't going to be reading the GUI labels anyway, so don't bother.)
What is the best way to count the time between two datetime values fetched from MySQL when I need to count only the time between hours 08:00:00-16:00:00.
For example if I have values 2008-10-13 18:00:00 and 2008-10-14 10:00:00 the time difference should be 02:00:00.
Can I do it with SQL or what is the best way to do it? I'm building a website and using PHP.
Thank you for your answers.
EDIT: The exact thing is that I'm trying to count the time a "ticket" has been in a specific state during working hours. The time could be like a couple weeks.
EDIT2: I have no problems counting the actual time difference, but substracting that off-time, 00:00:00-08:00:00 and 16:00:00-00:00:00 per day.
-Samuli
The TIMEDIFF function
TIMEDIFF() returns expr1 – expr2
expressed as a time value. expr1 and
expr2 are time or date-and-time
expressions, but both must be of the
same type.
mysql> SELECT TIMEDIFF('2000:01:01 00:00:00',
-> '2000:01:01 00:00:00.000001');
-> '-00:00:00.000001'
mysql> SELECT TIMEDIFF('2008-12-31 23:59:59.000001',
-> '2008-12-30 01:01:01.000002');
-> '46:58:57.999999'
I think you should calculate the difference in your own code instead of using a more-complex SQL sentence because:
That calculation seems to be part of your business logic. It seems easier to maintain if you integrate it with the rest of your business code.
The database is often the bottleneck, so don't load it more than needed.
You may want to try it in PHP, but I'm thinking it'll be faster in DB
code to return difference in an array