010-sql-date-differences

Data differences in SQL

In PostgreSQL, there is no direct DATEDIFF function as found in some other SQL databases like SQL Server. However, you can achieve the same result using various date and time functions provided by PostgreSQL.

To calculate the difference between two dates, you can simply subtract them, which will return the difference in days. Here are a few examples:

1. Calculating the difference in days:

   SELECT (date1 - date2) AS date_difference
   FROM your_table;

2. Calculating the difference in months:

   SELECT EXTRACT(MONTH FROM age(date1, date2)) AS month_difference
   FROM your_table;

3. Calculating the difference in years:

   SELECT EXTRACT(YEAR FROM age(date1, date2)) AS year_difference
   FROM your_table;

4. Using the AGE function:

   The AGE function can provide a more detailed interval between two dates, including years, months, and days.

   SELECT age(date1, date2) AS age_difference
   FROM your_table;

5. Calculating the difference in hours, minutes, or seconds:

   You can use the EXTRACT function to get specific parts of the interval.

   SELECT EXTRACT(EPOCH FROM (timestamp1 - timestamp2)) / 3600 AS hours_difference,
          EXTRACT(EPOCH FROM (timestamp1 - timestamp2)) / 60 AS minutes_difference,
          EXTRACT(EPOCH FROM (timestamp1 - timestamp2)) AS seconds_difference
   FROM your_table;

6. Using INTERVAL function:

   SELECT (CURRENT_DATE - INTERVAL '29 days') AS current_date_30days
   FROM your_table

   Using INTERVAL '29 days' covers a 30-day period because it includes both the start and end dates. Here’s a detailed breakdown of why 29 days is used instead of 30 days:

   Period Calculation:

   • If you want a period ending on 2019-07-27 and inclusive of this date, you subtract 29 days from 2019-07-27.
   • This gives you a starting date of 2019-06-28, making the period from 2019-06-28 to 2019-07-27, inclusive.

   Let’s see this with an example:

   • End date: 2019-07-27
   • Start date: 2019-06-28

   The period from 2019-06-28 to 2019-07-27 includes exactly 30 days.

   Inclusive Period:

   Here’s the mathematical reasoning:

   • When you subtract 29 days from 2019-07-27, you are counting backward, including the end date.
   • Therefore, 2019-07-27 minus 29 days covers 2019-07-27 itself and the previous 29 days, which totals 30 days.