Multiply kg and grams unit with price per kg - mysql

Question

I'm currently have a task when I have to calculate the values for the cost of supplies which I have to multiply priceperweight with weight. But in weight there's also kg and grams. When I multiply it, it gives the results like $3000 for 500 grams of Spinach. Here's my scheme. Please help. All help would be appreciated :D Here's my code! Thank you so much
select PricePerWeight * Weight from Supplies;

Answer 1

Normalize the weight to grams or Kg (I'll use grams) using CASE x WHEN y:
SELECT
s.*,
CASE s.metric
WHEN 'Kilos' THEN ( s.weight * 1000 )
WHEN 'grams' THEN ( s.weight * 1 ) ELSE NULL
END AS weightInGrams
FROM
supplies AS s
Then adding additional units is straightforward:
SELECT
s.*,
CASE s.metric
WHEN 'tonnes' THEN ( s.weight * 1000 * 1000 )
WHEN 'Kilos' THEN ( s.weight * 1000 )
WHEN 'grams' THEN ( s.weight * 1 )
WHEN 'jupiter' THEN ( s.weight * 1.9 * POW( 10, 27 ) * 1000 )
WHEN 'lbs' THEN ( s.weight * 453.592 ) ELSE NULL
END AS weightInGrams
FROM
supplies AS s
Assuming you're using ISO-compliant SQL, you'll need to use an outer-query or a CTE to then multiply by pricePerWeight, which I assume is in Kg:
Note that the queries below still use grams as the base unit of mass to avoid problems caused by integer division (as SQL does not make it easy to make sure numeric literals are processed as floats or decimals instead of integers, as integer division truncates rather than rounds).
The final value is the thePrice column in the queries below:
Option 1: Using an outer-query:
SELECT
s2.*,
( s2.pricePerWeight * s2.weightInGrams ) / 1000.00 AS thePrice
FROM
(
SELECT
s.*,
CASE s.metric
WHEN 'tonnes' THEN ( s.weight * 1000 * 1000 )
WHEN 'Kilos' THEN ( s.weight * 1000 )
WHEN 'grams' THEN ( s.weight * 1 )
WHEN 'jupiter' THEN ( s.weight * 1.9 * POW( 10, 27 ) * 1000 )
WHEN 'lbs' THEN ( s.weight * 453.592 ) ELSE NULL
END AS weightInGrams
FROM
supplies AS s
)
Option 2: Using a CTE:
WITH s2 AS (
SELECT
s.*,
CASE s.metric
WHEN 'tonnes' THEN ( s.weight * 1000 * 1000 )
WHEN 'Kilos' THEN ( s.weight * 1000 )
WHEN 'grams' THEN ( s.weight * 1 )
WHEN 'jupiter' THEN ( s.weight * 1.9 * POW( 10, 27 ) * 1000 )
WHEN 'lbs' THEN ( s.weight * 453.592 ) ELSE NULL
END AS weightInGrams
FROM
supplies AS s
)
SELECT
s2.*,
( s2.pricePerWeight * s2.weightInGrams ) / 1000.00 AS thePrice
FROM
s2