SQL - summarize results from multiple tables - sql

Question

I have the following simple SQL query that I need to run on 3 tables:
SELECT
A.date,
SUM(A.number)
FROM A
GROUP BY
A.date
But I have two other tables (B and C) on which I'd like to run the same query. And combine the results into one table as output.
I am expecting the output to look something like:
date
A.number
B.number
C.number
2022
12322.1
9999999
888888

Answer 1

We can try the following union approach:
SELECT
date,
SUM(CASE WHEN src = 'A' THEN number ELSE 0 END) AS A_sum,
SUM(CASE WHEN src = 'B' THEN number ELSE 0 END) AS B_sum,
SUM(CASE WHEN src = 'C' THEN number ELSE 0 END) AS C_sum
FROM
(
SELECT date, number, 'A' AS src FROM A
UNION ALL
SELECT date, number, 'B' FROM B
UNION ALL
SELECT date, number, 'C' FROM C
) t
GROUP BY date
ORDER BY date;

Answer 2

Here is my approach:
Create Table TableA
(
Dates Date,
Number Int
)
GO
Create Table TableB
(
Dates Date,
Number Int
)
GO
Create Table TableC
(
Dates Date,
Number Int
)
GO
Insert Into TableA
Values ('2023-01-01', 100000),
('2023-01-02',30000)
GO
Insert Into TableB
Values ('2023-01-01', 200000),
('2023-01-02',10000)
GO
Insert Into TableC
Values ('2023-01-01', 400000),
('2023-01-02',20000)
GO
SELECT * from
(
Select *,'A' Det from TableA
UNION ALL
Select *,'B' from TableB
UNION ALL
Select *,'C' from TableC
)ABC
PIVOT
(SUM(ABC.Number) FOR Det IN (A,B,C))
XYZ
DROP TABLE TableA
DROP Table TableB
DROP table TableC

Answer 3

IMO best option is to create calendar first and then left join created calendar with different tables:
Calendar is important to gather data from join.
And i'ts quite odd if you have column date as year.
In my oppinion it's to small level of complexity.
But ok. Let's say that you have only year.
Create table with years.
Create Table Years
(
years int
)
next:
INSERT INTO Years(years )
VALUES
(2022),(2021),(2020),(2019),(2018),(2017),(2016),(2015)
eg.
SELECT
y.*,
sum(a.number) as SumA,
sum(b.number) as SumB,
sum(c.number) as SumC
FROM Years as y
left join
table_a a
on
y.years=a.date
left join
table_b b
on
y.years=b.date
left join
table_c c
on
y.years=c.date
GROUP BY
y.years
Hopefully this helps!
Please let me know if it works as you wanted.