Postgresql Days Between Two Dates
Postgresql Days Between Two Dates – The ability to store date values in your database allows you to add a time element to your queries and analysis of your data. It’s important to know how to work with date types in the relevant database so that you can be accurate in your reports, whether it’s information about orders, ages of people, or any other use case.
PostgreSQL uses 4 bytes to store a date value. The range of date values in PostgreSQL is 4713 BC. to 5874897 AD.
Postgresql Days Between Two Dates
In PostgreSQL, you can set the current date as the default value. This can be done by creating a table using
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Write PostgreSQL, then you can use functions that work with the information you store. We will consider some general functions built on the basis of the table presented in the previous section.
If the time is not of interest to you, you can also specify that the date be returned with just a colon
Depending on your system requirements, you may need the date in a specific format. This is a scenario where the ability to specify output data in PostgreSQL is useful.
Operator. Using this operator allows you to calculate things like how long an employee has been at work or the time since a book was published.
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In our example, we want to find how many days have passed since Joyce’s Ulysses was published by subtracting the current date from
We can continue with the same example to calculate the age on the current date in years, months and days using
Function, then it will automatically use the current date for subtraction and calculation. You can also pass two dates to a function to calculate the age, for example:
A function in PostgreSQL that allows you to separate date components such as year, quarter, month, and day.
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This is a useful feature to consider when you only need part of a date value, for example, to calculate your data.
Data type in PostgreSQL. It’s important to know how date data works in your database. Finding out how you can access and work with it allows you to do age calculations, perform extractions on your queries, and adjust your output if necessary to meet the requirements of another system.
If you are using the Client with PostgreSQL, here is some information on how the Client translates the PostgreSQL date type.
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Function. This function can parse the year, month, or day from the date type, as well as specify the quarter.
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I am trying to calculate the difference between two dates in postgres and found that in several cases my tests fail, while debugging I found something interesting: when I subtract one date from another it seems to be missing an hour, here is the script (the table has only one timestamp field):
This seemed strange to me, so I decided to check it with another service and got the expected result (23 hours instead of 22):
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Can someone explain these results? Am I doing something wrong or am I missing something obvious? I’m using Postgres 9.6 on macOS.
Many countries switch to daylight saving time from March 2 to April 1. Because the hands of the clocks are ahead, there is one hour less between March 2, 2019 and April 1, 2019.
Note that Postgres has its own time zone, which may not match the user’s time zone, especially for web applications. To work around this, set the app to the user’s time zone and the database to UTC. Convert all dates to UTC before sending them to the database.
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Writing SQL is frustrating in part because there are hundreds of different options. MySQL syntax is slightly different from PostgreSQL (for example) and some dialects have features that others don’t (for example
In Snizhinka). When you work with dates, there are prototypes for function types: although the exact syntax may differ in different dialects, the idea is the same. We will consider 5 broad categories:
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For each function prototype, we provide the correct syntax and documentation for the 5 most popular SQL dialects:
Computers interpret dates in a variety of formats, from Unix time to strings and timestamps, and they are generally not friendly to each other. Here are the function prototypes:
Unix time (also called epoch time) is staggering: it corresponds to the number of seconds that have passed since January 1, 1970. A typical Unix timestamp might look like this:
There’s a whole “language” of string format arguments that developers have to frantically google every time they use them. Almost all dialects conform to the C strftime standard, with the exception of Postgres. They are usually the same
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Rounding dates allows you to reduce the precision of your date; this is useful for aggregations and viewing trends over time. Here is the prototype function.
, which reduces the date to a lower level of specificity, such as month or year. This is how it looks in different dialects:
For a more concrete example of when you use the date drop function, imagine we have an orders table where each row represents an order and each order has a date. If we want to see the growth of orders month by month, we will use the shrink function in Postgres:
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If you have a timestamp, the date part functions will select some part of that timestamp; This is useful if you want to show what day of the week a user logged in, what time someone placed an order, or aggregate event data by month to see which months of the year your website gets the most traffic. Our prototype function:
Each dialect of SQL has different approaches to how to specify the data to be extracted, so be sure to consult the documentation referenced in the table above.
If you need to calculate the duration or the difference between two dates, most SQL dialects have functions for this. DATE_DIFF() is popular here:
If you want to get the difference between two dates in days (that is, how many days are there between date number one and date number two), you would use something like
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. Surprisingly, some languages have time deltas as their own data type (see, for example, Python’s Pandas package). This is how these functions are built:
Intervals allow you to add and subtract time from dates in SQL. This function is useful for calculating constant widows, such as filtering your data for each record in the last 7 days or last year.
The SQL statement has unusual syntax: you usually need to use a keyword, followed by a number in single quotes, and then a date range. In Presto, for example, you can add a day to your appointment with
. Note that the quotes in the string only contain the count (1), not the selected interval (day).
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Time zones are one of the biggest nightmares in date processing, and you’re probably not in UTC right now. Fortunately, most SQL dialects have a bunch of functions to handle the TZ conversion. Our feature prototypes:
Time zones also rear their ugly head when it comes to string parsing and date formatting. This is how these cross-dialect functions are constructed:
) to add a time zone to a date and to convert a date that already has one time zone to another. MySQL also has a special function (
The last major category of time-related functions is getting the current time. This type of functionality is useful in similar cases as intervals, such as the rolling construction.
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