Question

I’m looking for a solution to calculate a percentage rate of a boolean value:

Model:

Products 1 <-> n Components

Components.essential is a boolean attribute

What I want to calculate is something like:

SELECT products.*, count(comp_a.essential)/count(comp_b.essential) AS essential_percentage
From products
INNER JOIN components AS comp_a ON products.id = comp_a.product_id
INNER JOIN components AS comp_b ON products.id = comp_b.product_id
WHERE comp_a.essential = 1
GROUP BY products.id

Unfortunately this statement calculates way too many components as there are two INNER JOIN on the components model…

Any help on this?

Markus

Answer 1

Only need one join. Note the standard GROUP BY clause.

This gives “true” percent because I assume that’s what you mean (not false)

SELECT
  p.*, 
  count(CASE WHEN c.essential = 1 THEN 1 END) / count(*) AS essential_percentage
From 
  products p
  LEFT OUTER JOIN 
  components c ON p.id = c.product_id
GROUP BY 
  p.*;

or

  count(nullif(c.essential, 0)) / count(*) AS essential_percentage