How to scale creating extra columns in an SQL statement instead of creating extra rows using GROUP BY? - sql

Question

The schema of the database is mentioned here: http://sqlzoo.net/wiki/Guest_House
The request is for each day of the week beginning 2016-11-14 show how many guests are checking out that day by floor number.
The final table should be like this one (the naming of the columns is of little importance of course):
+------------+-----+-----+-----+
| i | 1st | 2nd | 3rd |
+------------+-----+-----+-----+
| 2016-11-14 | 5 | 3 | 4 |
| 2016-11-15 | 6 | 4 | 1 |
| 2016-11-16 | 2 | 2 | 4 |
| 2016-11-17 | 5 | 3 | 6 |
| 2016-11-18 | 2 | 3 | 2 |
| 2016-11-19 | 5 | 5 | 1 |
| 2016-11-20 | 2 | 2 | 2 |
+------------+-----+-----+-----+
In the attempt to implement it using SQL for the middle column (2nd) resulted in this script:
select ADDDATE(booking_date, nights) as checkout, count(distinct guest_id) as '2nd'
from booking
where CAST(room_no as char) like '2%'
and ADDDATE(booking_date, nights) >= '2016-11-14'
group by checkout
order by checkout
LIMIT 7
The issue is that this script produces only one column at a time
A scalable version is this, but this is only per row:
select ADDDATE(booking_date, nights) as checkout,
SUBSTR(CAST(room_no as char), 1, 1) as floor, count(distinct guest_id) as 'guest count'
from booking
where ADDDATE(booking_date, nights) >= '2016-11-14'
group by checkout, floor
order by checkout, floor
LIMIT 21
and the formatted output of this approach is not ideal to be presented:
checkout floor guest count
2016-11-14 1 5
2016-11-14 2 3
2016-11-14 3 4
2016-11-15 1 6
2016-11-15 2 4
2016-11-15 3 1
2016-11-16 1 2
2016-11-16 2 2
2016-11-16 3 4
2016-11-17 1 5
2016-11-17 2 3
2016-11-17 3 6
2016-11-18 1 2
2016-11-18 2 3
2016-11-18 3 2
2016-11-19 1 5
2016-11-19 2 5
2016-11-19 3 1
2016-11-20 1 2
2016-11-20 2 2
2016-11-20 3 2

Answer 1

Twice as many columns versus twice as many rows == not much difference on scaling. And there is a much smaller limit for the number of columns.
Go for more rows, then use pivoting for displaying as columns. (See the [pivot-table] tag)