SQL two “group by” in the same query

Question

We have the table to track what product page a user visited.

product_tracking
____________________
id  user_id  product
1   1        A
2   1        B
3   2        A 
4   1        A

I know we can use group by user_id, and group_by product but I need both.

Expected result :

result_table 
____________________________
user_id      A     B    C 
  1          2     1    0
  2          1     0    0 

any idea how to merge the 2 group by ?

Answer 1

You could do this:

SELECT
    product_tracking.user_id,
    SUM(CASE WHEN product_tracking.product='A' THEN 1 ELSE 0 END) AS A,
    SUM(CASE WHEN product_tracking.product='B' THEN 1 ELSE 0 END) AS B,
    SUM(CASE WHEN product_tracking.product='C' THEN 1 ELSE 0 END) AS C
FROM
    product_tracking
GROUP BY
    product_tracking.user_id

Answer 2

使用sum（...）的替代方法是使用count（...）：

SELECT product_tracking.user_id, COUNT(CASE WHEN product_tracking.product='A' THEN 1 END) AS A, COUNT(CASE WHEN product_tracking.product='B' THEN 1 END) AS B, COUNT(CASE WHEN product_tracking.product='C' THEN 1 END) AS C FROM product_tracking GROUP BY product_tracking.user_id