按月对事务进行分组的 SQL 查询

Question

----------------------------------------
|transaction_id|customer_id|month|value|
----------------------------------------

I am looking for a SQL query that would return customers ID of the customers that had more transactions in the month of October instead of November or of customers that ONLY had transactions in November (and none in October). The month column is of type string (not a date)

I was thinking an inner join could do it, but past that I do not know how to approach this problem.

This is what I attempted

select customer_id
from tbl_name
where (select count(month is 'October') > count(month is 'November') from tbl_name)

Thank you

Answer 1

You count records. So you aggregate. You want counts per customer_id. So you'd group by customer_id. You forgot this in your query.

Then you'd compare the counts in the HAVING clause, ie after having counted October and November records.

select customer_id
from tbl_name
where month in ('October', 'November')
group by customer_id
having count(case when month = 'October' then 1 end) >
       count(case when month = 'November' then 1 end)
    or count(case when month = 'October' then 1 end) = 0;

You don't need to access the table twice as you see. Just aggregate and see what you have :-)