Question

I have a customer table:

id   name
1    customer1
2    customer2
3    customer3

and a transaction table:

id   customer   amount   type
1    1          10       type1
2    1          15       type1
3    1          15       type2
4    2          60       type2
5    3          23       type1

What I want my query to return is the following table

name        type1    type2
customer1   2        1
customer2   0        1
customer3   1        0

Which shows that customer1 has made two transactions of type1 and 1 transaction of type2 and so forth.

Is there a query which I can use to obtain this result or do I have to use procedural code.

Answer 1

You could try

select c.id as customer_id
   , c.name as customer_name
   , sum(case when t.`type` = 'type1' then 1 else 0 end) as count_of_type1
   , sum(case when t.`type` = 'type2' then 1 else 0 end) as count_of_type2
from customer c
   left join `transaction` t
   on c.id = t.customer
group by c.id, c.name

This query needs to iterate only once over the join.