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.