How to separate data columns by year using basic SQL (bigquery) - sql

Question

I am trying to create a visualization using bigquery and chartio. I want to display traffic volumes by day for each year to compare on one viz, to help identify seasonality.
I can break down the traffic by having a single column for traffic and another column for month and one for year, but this data structure doesn't work when I try to build the viz is chartio.
So what I am trying to do is to set a column for each year, where I have the traffic numbers set out by month. I am not sure of the way to do this, I know I probably need a union or a join here.
The code below combines the values, but doesn't get what I want.
Thanks in advance for the help!
SELECT
EXTRACT(MONTH FROM date) AS month,
EXTRACT(YEAR FROM date) AS year,
SUM(CAST(traffic AS INT64)) AS traffic
FROM
data.source
GROUP BY month, year
This is the output I get:
month year traffic
1 2017 11991865
3 2019 3482067
8 2017 21345567
6 2016 85207567
3 2018 22010756
What I want is:
month traffic_2016 traffic_2017
1 233391865 11991865
2 1123465 3482067
3 11996545 21345567
4 119916655 85207567
5 34571865 22010756

Answer 1

By using IF-ELSE / CASE WHEN statement with GROUP BY
SELECT
EXTRACT(MONTH FROM date) AS month,
SUM(IF(EXTRACT(YEAR FROM date) = 2016, CAST(traffic AS INT64), 0) AS traffic_2016,
SUM(IF(EXTRACT(YEAR FROM date) = 2017, CAST(traffic AS INT64), 0) AS traffic_2017,
FROM
data.source
GROUP BY month
Simply with Join
SELECT
*
FROM
(SELECT
EXTRACT(MONTH FROM date) AS month,
SUM(CAST(traffic AS INT64)) AS traffic_2016
FROM
data.source
WHERE
EXTRACT(MONTH FROM date) = 2016
GROUP BY month)
JOIN
(SELECT
EXTRACT(MONTH FROM date) AS month,
SUM(CAST(traffic AS INT64)) AS traffic_2017
FROM
data.source
WHERE
EXTRACT(MONTH FROM date) = 2017
GROUP BY month)
USING(month)

Answer 2

Below is for BigQuery Standard SQL and provides less verbose and easier to read and maintain and extend with more columns version
#standardSQL
SELECT month,
SUM(IF(year = 2016, value, 0)) traffic_2016,
SUM(IF(year = 2017, value, 0)) traffic_2017,
SUM(IF(year = 2018, value, 0)) traffic_2018,
SUM(IF(year = 2019, value, 0)) traffic_2019
FROM `project.data.source`,
UNNEST([STRUCT(
EXTRACT(MONTH FROM `date`) AS month,
EXTRACT(YEAR FROM `date`) AS year,
CAST(traffic AS INT64) AS value
)])
GROUP BY month