Question

I am trying to do some calculations in my query and i think postgresql is handling the math differently than mysql is but i’m not sure where the difference is. This is my query from my controller:

Invoice.find(params[:id], :joins => :invoice_line_items, :select => "invoices.id, SUM(invoice_line_items.hours * invoice_line_items.rate) as subtotal, SUM(invoice_line_items.hours * invoice_line_items.rate) - (SUM(invoice_line_items.hours * invoice_line_items.rate) * (invoices.discount_percentage / 100)) as total", :group => "invoices.id")

In my development environment i am using mysql and in my production environment i am using postgresql. These are the outputs that i am getting. For reference, the discount_percentage is 20.

MySQL:

 |  id  |  subtotal  |  total         |
----------------------------------------
 |  21  |  570.0000  |  456.00000000  |

Postgresql:

 |  id  |  subtotal  |  total         |
----------------------------------------
 |  9   |  570.0000  |  570.00000000  |

It looks like it’s something to do with the percentage and total. Anyone have any ideas? By the way, the MySQL result is what i am wanting.

Answer 1

Postgres is doing integer division.

Select                    MySQL   Postgres
-----------------------   -----   --------
select 999 / 100 as a     9.99    9
select 999 / 100.0 as a   9.99    9.99

So, change the x / 100 to x / 100.0 to get them both to behave the same.