单个表的平均出生日期（Oracle SQL）

Question

Average date of birth from a single table where the column's date format is '01-JAN-2001'. Looking to return total count of users, and the average DOB YEAR (oracle sql)

mock table:

|User_ID|birth_date|
|123|01-JAN-2001|
|123|01-JAN-2001|
|123|01-JAN-2001|

Answer 1

Assuming your intention is to just average the year, you can parse it to a date, extract the year and average it as a number:

SELECT COUNT(*), AVG(EXTRACT(YEAR FROM TO_DATE(birth_date, 'dd-mon-yyyy'))
FROM   users

Answer 2

(Oracle 12c:) Test table and data:

create table dt (
  userid_ number generated always as identity ( start with 123 )
, d_ date
);

begin
  for i in 1 .. 15
  loop
    insert into dt ( d_ ) values (
      to_date ( trunc( dbms_random.value( 2446067, 2458100 ) ), 'J' )
    ) ;
   end loop;
end;
/

SQL> select userid_, to_char( d_, 'DD-MON-YYYY' ) dob from dt;

   USERID_ DOB                 
---------- --------------------
       123 19-JUN-2000         
       124 06-OCT-2005         
       125 27-JAN-2012         
       126 09-JUL-2003         
       127 23-JUL-2010         
       128 07-FEB-1992         
       129 20-DEC-2002         
       130 19-MAY-2002         
       131 23-FEB-1990         
       132 26-DEC-1990         
       133 19-JUN-1999         
       134 16-DEC-1994         
       135 13-APR-2017         
       136 31-MAR-2000         
       137 23-MAY-1987

Query and result:

select 
  count(*) user_count
, trunc( avg( extract( year from d_ ) ) ) avg_dob_year
from dt ;

USER_COUNT AVG_DOB_YEAR
---------- ------------
        15         2000

(Not taking "days" into account, just "years").

When using the method suggested by @mathguy

"... It can be calculated by subtracting a fixed date, like 2000/01/01, from all dates, taking the average of the differences, and adding it to the fixed date."

... this may be a query we could start with (I'm sure it can be refined):

select
  count(*) usercount
, to_date( '2000/01/01', 'YYYY/MM/DD' ) fixeddate
, avg( differences ) difftofixed
, to_date('2000/01/01', 'YYYY/MM/DD' ) + avg( differences ) fixedplusdiff
, extract( year from ( to_date('2000/01/01', 'YYYY/MM/DD' ) + avg( differences ) ) ) dob_year
from (
  select 
    d_ - to_date( '2000/01/01', 'YYYY/MM/DD' ) differences
  from dt 
);

-- result
USERCOUNT  FIXEDDATE  DIFFTOFIXED  FIXEDPLUSDIFF  DOB_YEAR  
15         01-JAN-00  250.6        07-SEP-00      2000 

Answer 3

I don't know why you're trying to get the average date but I think it would go something like this :

“SELECT COUNT(user_id), AVG(EXTRACT(YEAR FROM TO_DATE(birth_date, 'dd-MM-yyyy'))
FROM users”