How do sql variables work?

Question

I have a table with salary of multiple employees in a table. and i need to calculate zscore for these emplyees using a derived number (mean and stddevn), i know i can do this in the select statement. but if i wanted to use variables. i am new to sql and just trying out things here. please bare me if my question is stupid

declare @Weight int,
@weight2 int,
@Mean int,
@StdDevn int,
@Variance int
set @Weight = 30
set @weight2 = (POWER(@weight,2)/100)
select @Mean = AVG(salary) FROM [live].[dbo].[ProdKey]
select @StdDevn = STDEV(salary) FROM [live].[dbo].[ProdKey]
select @Variance = (POWER(@StdDevn,2)/100)
select * FROM [live].[dbo].[ProdKey], (([salary]-@Mean)/@StdDevn) as Zscore

Error desc - Must declare variable @Mean, Incorrect syntax near '-'

Answer 1

fixed it:) my final select statement was wrong. should have been select *, (...) as zscore from table name. it was earlier written as select * from table name, (...) as zscore

Answer 2

i prefer select statement ... that will be very easy ,
no idea about ur way of doing this 😕