Question

I’m trying to get difference in days, casting result to decimal:

SELECT
CAST( TO_DATE('2999-01-01','yyyy-mm-dd') - TO_DATE('2909-01-01','yyyy-mm-dd') AS DECIMAL )
;

Now if I add 1 month to the 2nd date:

SELECT
CAST( TO_DATE('2999-01-01','yyyy-mm-dd') - (TO_DATE('2909-01-01','yyyy-mm-dd') + INTERVAL '1 MONTH' * (1) ) AS DECIMAL )
;

I’m getting an error:
ERROR: cannot cast type interval to numeric

OK, I can cast to char to get result:

SELECT
CAST( TO_CHAR( TO_DATE('2909-02-10','yyyy-mm-dd') - (TO_DATE('2909-01-01','yyyy-mm-dd') + INTERVAL '1 MONTH' * (1) ), 'DD') AS DECIMAL )
;

But in this case the 1st query modified with TO_CHAR casting stop working:

SELECT
CAST( TO_CHAR(TO_DATE('2999-01-01','yyyy-mm-dd') - TO_DATE('2909-01-01','yyyy-mm-dd'), 'DD') AS DECIMAL )
;

I’m getting ERROR: multiple decimal points.

So, my question is, how can I get days using the same sql statement? For both sql queries.

Answer 1

Look at your first two examples again. If you remove the outer CAST … AS DECIMAL you get

 ?column? 
----------
    32872

  ?column?  
------------
 32841 days

Clearly the difference is in the “days”. The second is an interval value rather than a simple number. You only want the number (because you always just want days) so you need to extract that part. Then you can cast to whatever precision you like:

SELECT extract(days FROM '32841 days'::interval)::numeric(9,2);
 date_part 
-----------
  32841.00

---

Edit responding to Alexandr’s follow-up:

Your first example fails with a fairly specific error:

SELECT extract(days FROM (TO_DATE('2999-01-01','yyyy-mm-dd') - TO_DATE('2909-01-01','yyyy-mm-dd'))::interval)::numeric(9,2);

ERROR:  cannot cast type integer to interval
LINE 1: ...yyyy-mm-dd') - TO_DATE('2909-01-01','yyyy-mm-dd'))::interval...

Here you’ve got an integer (which is what you originally wanted) and try to cast it to an interval (for reasons I don’t understand). It’s complaining it doesn’t know what units you want. You want 32872 what in your interval – seconds, hours, weeks, centuries?

The second example is complaining because you are trying to extract the “day” part from a simple integer, and of course there’s no extract() function in the system to do that.

I think you probably need to take a step back and just take the time to understand the values your various expressions return.

Subtracting one date from another gives the number of days separating them – as an integer. There is no other sensible measure, really.

Adding (or subtracting) an interval to a date gives you a timestamp (without time zone) since the interval may contain whole days, days and hours, seconds etc.

Subtracting a timestamp from a date will give you an interval since the result may contain days, hours, seconds etc.

If you have an interval and you just want the days part then you use extract() on it and you will get an integer number of days back.

You will need an integer (or floating-point) number of days if you want to cast to numeric, not an interval because casting an interval to an scalar number makes no sense without units.

So – either stick to dates and date arithmetic (easy), or realise you are using timestamps (flexible) but understand which it is.

To get an illustration of what’s happening you can do something like this (in psql):

CREATE TEMP TABLE tt AS SELECT
  ('2909-01-02'::date - '2909-01-01'::date) AS a,
  ('2909-01-02'::date - '2909-01-02 00:00:00'::timestamp) AS b;
\x
SELECT * FROM tt;
\d tt

That will show you the values and types you are dealing with. Repeat for as many columns as you find useful.

HTH