Printing Calendar -> SQL Solution.
Do you got any alternative Solution ?
Welcome everyone! Today, in this blog, I am going to explore how we can print a calendar using SQL. Printing a calendar is a common exercise while learning any programming language.
Step 1 -> Generate All the Days in the Current Month
We can use the generate_series
function to generate all the days in the current month. It works similarly to a for loop
select x.id from pg_catalog.generate_series(0, 31) x(id);
0
1
2
3
4
5
6
7
8
---
31
Remember, generate_series($1, $2)
generates a series from $1
to $2
.
Step 2 -> Generate Necessary Details Using generate_series
First, let’s see how we can extract values from the current date
select
current_date as cur_date,
to_char(current_date, 'mm') as month_number,
to_char(current_date, 'month') as month_name,
to_char(current_date, 'iw') as iso_week,
to_char(current_date, 'dd') as dm,
to_char(current_date, 'd') as week_date;
Explanation of format patterns:
iw
: ISO week number. ISO weeks start on Monday and end on Sunday. The first week of the year contains the first Thursday.dm
: Day of the month as a number.d
: Day of the week as a number (1-7), where 1 corresponds to Sunday, 2 to Monday, and so on.
cur_date | month_number | month_name | iso_week | dm | week_date
------------+--------------+------------+----------+----+-----------
2024-07-15 | 07 | july | 29 | 15 | 2
Let’s generate this information for all the dates in the current month
select
cast(date_trunc('month', current_date) as date) + x.id,
to_char(cast(date_trunc('month', current_date) as date) + x.id, 'iw') as wk,
to_char(cast(date_trunc('month', current_date) as date) + x.id, 'dd') as dm,
cast(to_char(cast(date_trunc('month', current_date) as date) + x.id, 'd') as integer) as dw,
to_char(cast(date_trunc('month', current_date) as date) + x.id, 'mm') as cur_month
from generate_series(0, 31) x(id);
?column? | wk | dm | dw | cur_month
------------+----+----+----+-----------
2024-07-01 | 27 | 01 | 2 | 07
2024-07-02 | 27 | 02 | 3 | 07
2024-07-03 | 27 | 03 | 4 | 07
...
2024-07-31 | 31 | 31 | 4 | 07
2024-08-01 | 31 | 01 | 5 | 08
We need to remove the unwanted dates beyond the current month.
Final Step -> Group by Week
The calendar format shows that days should be grouped by week. Let’s achieve this incrementally
select
x.wk
from (select
cast(date_trunc('month', current_date) as date) + x.id,
to_char(cast(date_trunc('month', current_date) as date) + x.id, 'iw') as wk,
to_char(cast(date_trunc('month', current_date) as date) + x.id, 'dd') as dm,
cast(to_char(cast(date_trunc('month', current_date) as date) + x.id, 'd') as integer) as dw,
to_char(cast(date_trunc('month', current_date) as date) + x.id, 'mm') as cur_month
from generate_series(0, 31) x(id)
) X
where to_char(current_date, 'mm')= cur_month
group by wk;
wk
----
27
28
29
30
31
But it does not look like calendar huh. To make it a calendar
select
max(CASE dw WHEN 2 THEN dm END) AS Mo,
max(CASE dw WHEN 3 THEN dm END) AS Tu,
max(CASE dw WHEN 4 THEN dm END) AS Wed,
max(CASE dw WHEN 5 THEN dm END) AS Thu,
max(CASE dw WHEN 6 THEN dm END) AS Fri,
max(CASE dw WHEN 7 THEN dm END) AS Sat,
max(CASE dw WHEN 1 THEN dm END) AS Sun
from (select
cast(date_trunc('month', current_date) as date) + x.id,
to_char(cast(date_trunc('month', current_date) as date) + x.id, 'iw') as wk,
to_char(cast(date_trunc('month', current_date) as date) + x.id, 'dd') as dm,
cast(to_char(cast(date_trunc('month', current_date) as date) + x.id, 'd') as integer) as dw,
to_char(cast(date_trunc('month', current_date) as date) + x.id, 'mm') as cur_month
from generate_series(0, 31) x(id)
) X
where to_char(current_date, 'mm')= cur_month
group by wk
order by wk;
mo | tu | wed | thu | fri | sat | sun
----+----+-----+-----+-----+-----+-----
01 | 02 | 03 | 04 | 05 | 06 | 07
08 | 09 | 10 | 11 | 12 | 13 | 14
15 | 16 | 17 | 18 | 19 | 20 | 21
22 | 23 | 24 | 25 | 26 | 27 | 28
29 | 30 | 31 | | | |
select dm
is invalid because we are doinggroup by
soaggregate
function must be there.max(case dw when 2 then dm end) as Mo
set label for weekdays.max
is required to make this expression valid. It is likemax([number]) = number
Conclusion
By grouping and ordering our data correctly, we can format the output to look like a traditional calendar. This approach demonstrates SQL’s capability to handle complex data transformations and presentations, making it a valuable tool for a wide range of data processing tasks.
So, next time you need to generate a calendar or similar output using SQL, remember that with a few creative queries, it’s entirely possible. Happy querying!