How assigning a value to Variable is working in SSRS

Question

Recently started learning Reporting Services using Microsoft SQL Server 2016 Reporting Services, Fifth Edition, Brian Larson. I am creating this one report which uses UNION to create a dataset, FuelType, that will be used for assigning values to a dropdown list.

SELECT 'All' AS Fueltype, '_All' AS SortField
UNION
SELECT Description, Description FROM Propulsion ORDER BY SortField

Now there exists another dataset that will help generate the chart for me and there.

SELECT Propulsion.Description AS Fueltype, FuelPrice.PriceStartDate, FuelPrice.Price    
FROM  FuelPrice 
INNER JOIN Propulsion ON FuelPrice.PropulsionID = Propulsion.PropulsionID
WHERE        (YEAR(FuelPrice.PriceStartDate) = @year) AND  (Propulsion.Description = @PropulsionType) 
OR  (YEAR(FuelPrice.PriceStartDate) = @year) AND (@PropulsionType = 'All')
ORDER BY Fueltype, FuelPrice.PriceStartDate

I do not understand, how by using "@PropulsionType = 'All'" I am able to pass in all available values to the parameter.

For someone who has the book, this is under Chapter 6 :Fuel Price Chart, Version 2

Answer 1

SSRS isn't doing anything special. It's actually the query that's making the distinction to either pull all fuel types or just the one specified, though I suspect the book has a typo because it looks to be missing an extra pair of parenthesis. The real magic is supposed to be happening in the predicate, here:

WHERE  (YEAR(FuelPrice.PriceStartDate) = @year) AND (Propulsion.Description = @PropulsionType) 
   OR  (YEAR(FuelPrice.PriceStartDate) = @year) AND (@PropulsionType = 'All')

This really should be written in one of the two following ways:

WHERE  ((YEAR(FuelPrice.PriceStartDate) = @year) AND (Propulsion.Description = @PropulsionType))
   OR  ((YEAR(FuelPrice.PriceStartDate) = @year) AND (@PropulsionType = 'All'))

Or, another way:

WHERE (YEAR(FuelPrice.PriceStartDate) = @year) 
 AND  ((Propulsion.Description = @PropulsionType) 
        OR  (@PropulsionType = 'All'))

Basically what this is doing is in the event your report is passed the 'All' parameter, don't filter on Propulsion.Description. If a parameter is passed that is not 'All', you want to filter on that specific value. Taking SSRS out of it, you can see the working example via this dbfiddle.uk link.