Question

Lets say I have such a table:

id|time|operation
1  2      read
2  5      write
3  3      read
4  7      read
5  2      save
6  1      open

and now I would like to do two things:

1. Divide all these records into two groups:
   1) all rows where operation equals to “read”
   2) all other rows.
2. Sum the time in each group.

So that my query would result only into two rows.

What I got so far is:

select 
 sum(time) as total_time,
 operation
group by
 operation
;

Although that gives me many groups, depending on the number of distinct operations.

How I could group them only into two categories?

Cheers!

Answer 1

group by can take arbitrary clauses, so

GROUP BY (operation = 'read')

will work. Essentially you’d be grouping on the boolean result of the comparison, not the value of the operation field, so any record which is “read” will be group 1, and any non-read will be group 0.