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Last modified: December 09, 2019
DATE and TIME values in PostgreSQL have a whole special set of functions and operators for their proper use. So many queries deal with DATE and TIME information that it’s important to get to know the date tools. Below we’ll cover and practice the main functions you’ll likely need. If you want to get detailed you can checkout the full list of PostgreSQL functions here.
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There are 4 main ways to store date values in a PostgreSQL database:
DATA TYPE | DESCRIPTION | EXAMPLE | OUTPUT |
---|---|---|---|
TIMESTAMP | DESCRIPTION date and time | EXAMPLE | OUTPUT 2023-04-10T10:39:37 |
DATE | DESCRIPTION date (no time) | EXAMPLE | OUTPUT 2023-04-10 |
TIME | DESCRIPTION time (no day) | EXAMPLE | OUTPUT 10:39:37 |
INTERVAL | DESCRIPTION interval between two date/times | EXAMPLE | OUTPUT 1 day, 2:00:10 |
We’ll go over more about each of these.
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Dates in a database aren’t stored as strings, but we input and fetch data from it as if it were a string with the following format for the information:
YYYY-MM-DD HH:MM:SS
where the letters stand for Year, Month, Day, Hour, Minutes and Seconds. Let’s say for example that we want to record that we got a new user on April 10, 2023 at exactly 10:39. To represent that exact date and time we would use the format:
2023-04-10 10:39:00
TODO: this format is also supported: January 8 04:05:06 1999 PST
To get some familiarity try creating and SELECTing a few TIMESTAMPS below. I was born on May 1st, 1983 at exactly 4:00am. Can you fetch that timestamp?
SELECT TIMESTAMP '2023-04-10 10:39:37';
We’re just going to jump in here. We need to use a different table as none of the previous ones we’ve been using have had date fields in them. Another table available to us in chinook is employees. Let’s get familiar with what columns are in this table by looking at the first few rows. Note that there are several columns so you may have to scroll right to see all of the data:
SELECT * FROM employees LIMIT 3;
Each employee has two TIMESTAMP columns, one for their birth_date and one for their hire_date. You can use all of the ORDERing, GROUPing and other functions we learned for other columns on DATE columns as well. Try getting a list of the 4 youngest employees in the company.
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Often you don’t want to show the full raw TIMESTAMP, but rather a nicely formatted, potentially truncated version. For example, let’s say we want to get a list of the employees names and the year that they were hired. To do so we’ll need to parse the hired_date to just pull out the year. We can do so with the TO_CHAR function which works as follows
TO_CHAR([date type], [pattern])
where [date type] is a column or value of any of the above listed date/time data types, and [pattern] is a string indicating how to format the output date. The main symbols you’ll want to use to create your format patterns are here
PATTERN | DESCRIPTION | EXAMPLE | OUTPUT |
---|---|---|---|
HH | DESCRIPTION Hour (01-12) | EXAMPLE | OUTPUT 04 |
HH24 | DESCRIPTION Hour (01-24) | EXAMPLE | OUTPUT 16 |
MI | DESCRIPTION Minute | EXAMPLE | OUTPUT 15 |
SS | DESCRIPTION Seconds | EXAMPLE | OUTPUT 23 |
am | DESCRIPTION displays whether time is am or pm | EXAMPLE | OUTPUT am |
YY | DESCRIPTION last 2 digits of the Year | EXAMPLE | OUTPUT 23 |
YYYY | DESCRIPTION 4 digits of the Year | EXAMPLE | OUTPUT 2023 |
MM | DESCRIPTION Month # of the year | EXAMPLE | OUTPUT 04 |
Month | DESCRIPTION written Month of the year capitalized | EXAMPLE | OUTPUT April |
Mon | DESCRIPTION abbreviated of Month of year | EXAMPLE | OUTPUT Apr |
DD | DESCRIPTION Day # of the month | EXAMPLE | OUTPUT 10 |
Day | DESCRIPTION written Day of the week | EXAMPLE | OUTPUT Monday |
Dy | DESCRIPTION abbreviated Day of the week | EXAMPLE | OUTPUT Mon |
WW | DESCRIPTION Week # of the year | EXAMPLE | OUTPUT 15 |
Q | DESCRIPTION Quarter of the year | EXAMPLE | OUTPUT 2 |
TZ | DESCRIPTION TimeZone | EXAMPLE | OUTPUT UTC |
The above patterns can be string together to get the format you eventually want. Some common outputs are:
SELECT TO_CHAR(TIMESTAMP '2023-04-10 10:39:37', 'Day, Month DD YYYY');
and
SELECT TO_CHAR(TIMESTAMP '2023-04-10 10:39:37', 'YYYY-MM-DD HH:MI:SS');
and
SELECT TO_CHAR(TIMESTAMP '2023-04-10 10:39:37', 'MM/DD/YY');
You don’t have to memorize these (it’s hard to!). It’s just good to get familiar with how it works and then reference back to it when you need it in the future.
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There are a couple of extra tools you can use on patterns that output numbers.
FORMATTER | DESCRIPTION | EXAMPLE | OUTPUT |
---|---|---|---|
FM | DESCRIPTION Fill Mode will remove any 0’sat the front of a 2 digit number. | EXAMPLE | OUTPUT 5 |
th | DESCRIPTION adds the ordinal suffixeslike st, nd or th to the end of a number | EXAMPLE | OUTPUT 05th |
And of course you can combine the two to get
SELECT TO_CHAR(DATE '2023-04-03', 'Month FMDDth');
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For string outputs, most of the patterns above support different casing output based on the case you use for the pattern. Some examples using different casings of “Day”:
SELECT
TO_CHAR(DATE '2023-04-10', 'DAY') AS "DAY",
TO_CHAR(DATE '2023-04-10', 'Day') AS "Day",
TO_CHAR(DATE '2023-04-10', 'day') AS "day";
And you can see the following common date format in UPPERCASE, Capitalized and lowercase formats:
SELECT
TO_CHAR(TIMESTAMP '2023-04-10 10:39:37', 'FMHH:MMAM DAY, MONTH DDTH YYYY') AS "UPPERCASED",
TO_CHAR(TIMESTAMP '2023-04-10 10:39:37', 'FMHH:MMam Day, Month DDth YYYY') AS "Capitalized",
TO_CHAR(TIMESTAMP '2023-04-10 10:39:37', 'FMHH:MMam day, month FMDDth YYYY') AS "lowercased";
Note that the case for numeric values doesn’t change. Still use DD for the day # of the month and YYYY for year.
We’re going to move on in the tutorial but if you’d like more details checkout the full list of PostgreSQL date formatting functions.
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PostgreSQL supports a number of special values, or functions to help bet the current DATE, TIMESTAMP or TIME. The most used ones are
CURRENT_DATE
CURRENT_TIME
CURRENT_TIMESTAMP
and they are used by just putting them in the query
SELECT CURRENT_DATE, CURRENT_TIME, CURRENT_TIMESTAMP;
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In analytic queries, it’s very common to group things by dates. For example you may want to see new users by year, month, week or day. To do so, you’ll want to use the TO_CHAR function to convert the dates into a truncated string before you GROUP BY it. You don’t want to simply GROUP BY the raw date as those are accurate down to the millisecond so grouping by the unaltered date would be like making GROUPs for each millisecond.
The following examples are using the hire_date field from the employees table and show a lot of common formats you can use for these groups. These are what we use at Atlassian Analytics for our date group formatting standards.
GROUP PERIOD | EXAMPLE SQL | EXAMPLE OUTPUT |
---|---|---|
SECOND | EXAMPLE SQL | EXAMPLE OUTPUT 2018-03-04T00:00:00 |
Minute | EXAMPLE SQL | EXAMPLE OUTPUT 2018-08-14T00:00 |
Hour | EXAMPLE SQL | EXAMPLE OUTPUT 2018-01-02T00 |
Day | EXAMPLE SQL | EXAMPLE OUTPUT 2003-10-17 |
Week | EXAMPLE SQL | EXAMPLE OUTPUT 2002-W33 |
Month | EXAMPLE SQL | EXAMPLE OUTPUT 2002-05 |
Quarter | EXAMPLE SQL | EXAMPLE OUTPUT 2003-Q2 |
Year | EXAMPLE SQL | EXAMPLE OUTPUT Y2012 |
Hour of Day | EXAMPLE SQL | EXAMPLE OUTPUT 14 |
Day of Week | EXAMPLE SQL | EXAMPLE OUTPUT Thursday |
Day of Month | EXAMPLE SQL | EXAMPLE OUTPUT 17 |
Day of Year | EXAMPLE SQL | EXAMPLE OUTPUT 125 |
Month of Year | EXAMPLE SQL | EXAMPLE OUTPUT October |
Feel free to try out any of the above formats on the query below:
SELECT TO_CHAR(hire_date, '"Y"YYYY') AS "Year Hired",
COUNT(*) FROM employees
GROUP BY "Year Hired";
There are only 8 employees in our database so we’re not dealing with too many groups there. You can get a little more granular with the invoices table and it’s invoice_date column with 250 rows.
SELECT TO_CHAR(invoice_date, '"Y"YYYY') AS "Year Invoiced",
COUNT(*) FROM invoices
GROUP BY "Year Invoiced";
The above query returns the number of invoices created per year. Can you modify it to get a SUM of the total amount invoiced by month?
Written by: Dave Fowler
Reviewed by: Matt David