Postgresql difference between dates of two columns?

Question

Username DateStart DateFinish James 2017-07-01 2017-09-14 Luigi 2017-08-02 2017-09-18 Francesco 2017-09-03 2017-10-25 

How is the sql difference calculated between two date columns in days?

Answer 1

You can just subtract them like

 select "DateFinish"::date - "DateStart"::date; 

And if the date column has a data date, then you can simply do:

 select "DateFinish" - "DateStart" 

Answer 2

If you want to see the difference in quantity (10 instead of the date value that has 10 days in it), you can get it with:

 select extract(day from "DateFinish" - "DateStart") 

Answer 3

Try it. It extracts the number of days between two dates and creates a column called "days":

 select extract(day from DateFinish - DateStart) as days from MyTable;