SQL select & group by a period of time (timestamp)

Question

I want to select the count number and group by a period of time (month, week, day, hour , ...) . So for example I want to select the number of rows and group them by 24h.
My table is created as follow. The date is timestamp.

CREATE TABLE MSG
(
   MSG_ID decimal(22) PRIMARY KEY NOT NULL,
   MSG_DATE timestamp,
   FILE_REF varchar2(32),
   FILE_NAME varchar2(64),
   MSG_TYPE varchar2(2),
);
CREATE UNIQUE INDEX PK_FEI_MSG ON MSG(MSG_ID);

I tried with this query. With length depending on the time period. But how can I group from now .

SELECT substr(MSG_DATE, 1,length),COUNT(*) as total FROM MSG GROUP BY substr(MSG_DATE, 1, length)

But how can I group the date from now + time_period ?

Solution

You can group by the TO_CHAR function.

select to_char(msg_date, 'X')
      ,count(*)
  from msg
 group 
    by to_char(msg_date, 'X')

...where X is the format specification:

• YYYY = 4-digit Year
• MM = Month of year
• DD = Day in month
• WW = Week
• HH24 = Hour of day

You can combine them pretty much however you like