why UNION ALL command in mysql doesn't give back any results? - mysql

Question

I am trying to merge two queries into one, but UNION is not working for me.
Here is the code:
SELECT
Customer_A,
Activity,
Customer_P,
Purchase
FROM (
SELECT
buyer_id as Customer_A,
COUNT(buyer_id) As Activity
FROM
customer_info_mxs
GROUP BY buyer_id
UNION ALL
SELECT
buyer_id as Customer_P,
SUM(purchase_amount) As Purchase
FROM
customer_info_mxs
GROUP BY buyer_id
)sub
I expect to have 4 columns as a result, but I get 2 instead (Customer_A) and(Activity).

Answer 1

If the query is supposed to return a list of customers, their number of purchases, and the total amount they’ve spent, then you can use a single query like this:
SELECT mxs.buyer_id as Customer,
COUNT(mxs.purchase_id) As Activity,
SUM(mxs.purchase_amount) As Purchases
FROM customer_info_mxs mxs
GROUP BY mxs.buyer_id;
Otherwise, your first subquery will always be a buyer_id and a value of 1.
Be sure to change purchase_id to whatever the unique id is for each purchase if you wish to see that number.

Answer 2

I think there is some confusion about the union statement. The union statement returns a row set that is the sum of all of the 'unioned' queries; since these queries have only 2 columns, the combined output only has two columns. The fact that the columns have different names is irrelevant. The column names in the output are being applied from the first query of the union.
One option is to just do
select buyer_id, count(buyer_id), sum(purchase_amount) from customer_info_mxs group by buyer_id
From your question, it looks like you are trying to do a pivot, turning some of the rows into additional columns. That could be done with ... some difficulty.

Answer 3

i read your comment,
'main goal is to creat a dataset in which returns 5 columns as: Customer_A, Activity (top 100), customer_P, Purchase(top 100), inner join of activity and purchase'
please try this query
SET #row_number = 0, #row_number2 = 0;
SELECT t1.Customer_A,t1.Activity, t2.Customer_P, t2.Purchase
from (
SELECT (#row_number:=#row_number + 1) AS n, t.Customer_a, t.Activity
from (
select buyer_id as Customer_A,COUNT(buyer_id) As Activity
FROM customer_info_mxs
GROUP BY buyer_id
order by Activity desc
Limit 100
)t
) t1
left join (
SELECT (#row_number2:=#row_number2 + 1) AS n,
FROM (
select buyer_id as Customer_P, SUM(purchase_amount) Purchase
FROM customer_info_mxs
GROUP BY buyer_id
order by Purchase desc
Limit 100
)t
) t2 on t2.n=t1.n
basic idea is, i just create some temporary number 0-99 to table 1 (t1) and join to temporary number on table 2 (t2)