How do I get grand total for orders made by credit card only from such a table :
order_id | meta_name | meta_value
___________________________________
1 | type | credit
1 | total | 1030.00
...
2 | type | check
2 | total | 930.00
..
3 | type | credit
3 | total | 330.00
what is the best way to describe such select operation if you are to search the Internet for a solution to this problem. suppose I am MySQL.
You can do this with either join or group by. Here is the join method:
select ttype.order_id, cast(ttotal.meta_value as money)
from table ttype join
table ttotal
on ttype.order_id = ttotal.order_id and
ttype.meta_name = 'type' and
ttype.meta_value = 'credit' and
ttotal.meta_name = 'total';
If yo could have more than one total for an order, then you would still want to aggregate:
select ttype.order_id, sum(cast(ttotal.meta_value as money))
from table ttype join
table ttotal
on ttype.order_id = ttotal.order_id and
ttype.meta_name = 'type' and
ttype.meta_value = 'credit' and
ttotal.meta_name = 'total'
group by ttype.order_id
meta_value's data type is something like varchar so you would have to convert it first.
