How do you get the sum of values by day, but day in columns SQL

Question

I have a table that looks like below where day, client_name and order_value are stored/

select day, client_name, order_value
from sample_table

day	client_name	order_value
2021-01-01	A	100
2021-01-01	A	100
2021-01-02	A	200
2021-01-03	A	100
2021-01-01	B	300
2021-01-01	B	400
2021-01-01	C	500
2021-01-02	C	500
2021-01-02	C	500

and I want to get the sum of order_value per client by day, but days in columns. Basically, I want my result to come out something like this.

client_name	2021-01-01	2021-01-02	2021-01-03
A	200	200	100
B	700	Null	Null
C	500	1000	Null

Answer 1

If you know what the days are, you can use conditional aggregation:

select client_name,
       sum(case when date = '2021-01-01' then order_value end) as date_20210101,
       sum(case when date = '2021-01-02' then order_value end) as date_20210102,
       sum(case when date = '2021-01-03' then order_value end) as date_20210103
from t
group by client_name ;

If you don't know the specific dates (i.e., you want them based on the data or a variable number), then you need to use dynamic SQL. That means that you construct the SQL statement as a string and then execute it.