If you want to declare your variable as a loosely-typed user-defined variables are defined using '@', you can simply go:
SET @mean := .5
If not, then perhaps you want to define a variable in a stored procedure, then you can perhaps define as follows:
DECLARE mean INT;
SET mean = .5; // not sure you mean INT here though?
You may find this SO link useful for describing the differences in my sql variables. Also check out the mysql manual(user variables) / (local variables).
I think some of the confusion here may stem from the differing syntax that T-SQL uses from Mysql, but read the above link and use whatever method suits you best.