How to find count of transaction of type1 and type2 for each customer in mysql

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

SELECT  name, 
        (
        SELECT  COUNT(*)
        FROM    transaction t
        WHERE   t.customer = c.id
                AND t.type = 'type1'
        ) AS type1,
        (
        SELECT  COUNT(*)
        FROM    transaction t
        WHERE   t.customer = c.id
                AND t.type = 'type2'
        ) AS type2
FROM    customer c

To apply WHERE conditions to these columns use this:

SELECT  name
FROM    (
        SELECT  name, 
                (
                SELECT  COUNT(*)
                FROM    transaction t
                WHERE   t.customer = c.id
                        AND t.type = 'type1'
                ) AS type1,
                (
                SELECT  COUNT(*)
                FROM    transaction t
                WHERE   t.customer = c.id
                        AND t.type = 'type2'
                ) AS type2
        FROM    customer c
        ) q
WHERE   type1 > 3

Answer 2

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.

Answer 3

dirk beat me to it ;)

Similar but will work in mysql 4.1 too.

Select c.name,
sum(if(type == 1,1,0)) as `type_1_total`,
sum(if(type == 2,1,0)) as `type_2_total`,
from 
customer c
join transaction t on (c.id = t.customer)
;