Why am I seeing multiple months in the results when I am joining with dim_date - sql

Question

Here is my simple Postgresql Query
SELECT dd.year_actual as yr, sum("Ordered_Amount") from channel_sales cs
JOIN dim_date dd ON cs."date" = dd.date_actual
GROUP BY
dd.year_actual,
cs."Ordered_Amount"
Here is the result below. What I was expecting was a single line result with the year and total amount, but instead it is breaking it down into multiple rows of 2018. I am not sure what I am doing wrong here.
2018 2226
2018 357
2018 616
2018 1074
2018 1422
2018 3080
2018 2106
2018 924
2018 176
2018 580
2018 1587
2018 14350
2018 306
2018 2516
2018 1482
2018 2880
2018 8400
2018 5200
2018 16758
2018 781
2018 135
2018 4056
2018 150
2018 500
2018 2338
2018 3850
2018 1432
2018 1396
2018 1230
2018 274
2018 1494
2018 1068
2018 878
2018 1441
2018 1832
2018 3042
2018 4180
2018 2327
2018 206
2018 426
2018 2090
2018 1003
2018 62499
2018 900
2018 2274
2018 399
2018 1980
2018 278
2018 736
2018 24070
2018 561
2018 648
2018 1256
2018 120
2018 21912
2018 1639
2018 4452
2018 1008
2018 96577
2018 3240
2018 1386
2018 388
2018 260
2018 1080
2018 5525
2018 2672
2018 24674
2018 4392
2018 948
2018 801
2018 658
2018 1908
2018 692
2018 498
2018 630
2018 8999
2018 4056
2018 2990
2018 1745
2018 1280
2018 126
2018 988
2018 422
2018 936
Is it how I am making the join, or is it because I am using group by clause in the wrong way. I cannot figure out for the life of it.

Answer 1

Because you are not grouping by only year. You are also grouping by ordered_amount which you also sum(). Thus you are effectively summing by year and common ordered_amounts. If say in 2018, there are 4 ordered_amounts of 100 that would show as 2018, 400. And this would be repeated per ordered_amount. ie:
2018,100
2018,100
2018,100
2018,100
2018,200
2018,300
2018,300
would be:
2018,400
2018,200
2018,600
Write it as:
SELECT dd.year_actual as yr, sum("Ordered_Amount")
from channel_sales cs
JOIN dim_date dd ON cs."date" = dd.date_actual
GROUP BY
dd.year_actual
Also note that if this is not a 1-to-many or 1-to-1 relation, then sum results would be wrong. To prevent that, you may first do the sum and then join. Depending on table structures and which data is coming from where, a join may not even be needed.