Conditional summation in PostgreSQL

Question

I have a transactions table which contains a category (category_id), an amount (amount) and a flag (managed) which can be true or false.

I would like to display a list of all categories with the total amounts of managed and not managed transactions, e.g.

Category | managed_transactions | unmanaged_transactions
Cat 1    |     124000           |     54000
Cat 2    |     4000             |     0
Cat 3    |     854000           |     1000000

Is there a way to do something like

Select category_id,
       sum(amount) if (managed is true) as managed_transactions,
       sum(amount) if (managed is false) as unmanaged_transactions
from transactions

I'm obviously stuck on the if managed is true part...

Answer 1

Enjoy!

SELECT
  category_id,
  SUM( CASE WHEN managed THEN amount ELSE      0 END ) AS managed_transactions,
  SUM( CASE WHEN managed THEN      0 ELSE amount END ) AS unmanaged_transactions
FROM
  transactions
GROUP BY
  category_id
ORDER BY
  category_id