Date Dimension Table in Postgres
One of the first dimension tables required for any Data Warehouse project is the Date Dimension table. See Calendar Date Dimensions @kimballgroup.
We want our date dimension table to look like this:
Column | Type | Description
------------------ | ---- | -----------
date_key | int | A number corresponding to the date. e.g. 20150315.
date | date | SQL date.
y | int | Year in number, e.g. 2015.
m | int | Month in number (1..12).
d | int | Day of month (1..31).
q | int | Quarter (1..4).
h | int | First / second half of year (1 or 2).
dow | int | Day of week (1..7).
doy | int | Day of year (1..366).
yow | int | Year of week.
| | Can be different from y only during first week of the year.
woy | int | Week of year (1..52).
| | Note: first few days of the year may be week 52 of last year.
doe | int | Day of epoch, a serial number.
woe | int | Week of epoch, a serial number.
moe | int | Month of epoch, a serial number.
yoe | int | Year of epoch, a serial number.
is_weekday | bool | True if Mon..Fri.
is_weekend | bool | True if Sat..Sun.
first_date_of_week | date | Date of Monday of this week.
last_date_of_week | date | Date of Sunday of this week.
yw | text | e.g. 2015-W5 for week 5 of 2015.
ym | text | e.g. 2015-12 for December 2015.
yq | text | e.g. 2015-Q4 for Q4 of 2015.
is_holiday | bool | Holiday indicator.
The SQL statement follows. A few things to note:
- You can edit section DR1 to set the starting date and the range of your date dimension. In the code below, we start generating dates from year 2000 through 2030.
- A week starts from Monday (1) and ends on Sunday (7).
- The first week of the year usually belongs to the previous year. For example, on January 1, 2000, the year-of-week is 1999, and the week-of-year is 52.
- You should add columns to this table to mark special dates such as holidays and significant events of the company.
create table datedim as
DR1 as (
-- You can change the range here!
-- epoch starting 2000-01-03, for 30 years
-- must start on a Monday and end on a Sunday to avoid partial week
'2000-01-03'::date as first_date,
'2000-01-03'::date + n as date
from generate_series(0, '2031-01-05'::date - '2000-01-03') n
DR2 as (
extract(dow from first_date) as day_of_first_date,
(date - first_date) + 1 as doe,
extract(year from date)::int - 2000 + 1 as yoe,
extract(week from date)::int as woy,
extract(day from date)::int as dom,
extract(dow from date)::int as dow,
extract(doy from date)::int as doy,
extract(month from date)::int as m,
extract(quarter from date)::int as q,
extract(year from date)::int as y
DR3 as (
(case when m = 1 and woy > 50 then y - 1 else y end) as yow,
(8 - (case when day_of_first_date = 0
then 7 else day_of_first_date end))::int % 7 as dayoffset
DR4 as (
((m - 1) / 6) + 1 as h, -- half of year
(doe - 1 - dayoffset + 7) / 7 + 1 as woe,
(yoe - 1) * 12 + m as moe,
(yoe - 1) * 4 + ((m - 1) / 3) + 1 as qoe,
(1 <= dow and dow <= 5) as is_weekday,
not (1 <= dow and dow <= 5) as is_weekend
MonthGroup as (
select moe, max(date) as last_date_of_month from DR4 group by 1
WeekGroup as (
select woe, min(date) as first_date_of_week,
max(date) as last_date_of_week from DR4 group by 1
to_char(date, 'YYYYMMDD')::int as date_key,
y, -- year (YYYY)
m, -- month (1..12)
dom as d, -- day of month (1..31)
q, -- quarter (1..4)
h, -- half (1..2)
dow, -- day of week (1..7)
doy, -- day of year (1..366)
yow, -- year of week
woy, -- week of year
doe, -- day of epoch
DR4.woe, -- week of epoch
DR4.moe, -- month of epoch
yoe, -- year of epoch
is_weekday, -- true if Mon .. Fri
is_weekend, -- true if Sat or Sun
first_date_of_week, -- date of Mon in week
last_date_of_week, -- date of Sun in week
last_date_of_month, -- date of last day in month
yow||'-W'||LPAD(woy::text, '0', 2) as yw, -- e.g.2015-W05
to_char(date, 'yyyy-mm') as ym, -- e.g. 2015-07
y||'-Q'||q as yq, -- e.g. 2015-Q3
0::bool as is_holiday
join MonthGroup using (moe)
join WeekGroup using (woe)
-- Insert the *special date row* indicating invalid date
-- All columns are NULL except the date_key field.
insert into datedim (date_key) values (99999999);
-- Populate holidays manually.
-- Populate other special dates manually.