executing string selects in stored procedure

[D T]

I am used to writing stored procedures in microsoft sql server, where it is possible to write

declare sqlStr varchar(100);
set sqlStr = "select * from tablename";

execute (sqlStr)

is it possible to do this in mysql?

i need to write something along the lines of

if (a is not null) then
set sqlSTr = sqlSTr + parameter value
end if

thanks