Variables in mysql do not need to be declared in advance, and they can be used directly with "@variable name" when they are used.
The first usage: set @num=1; or set @num:=1; //Here to use variables to save data, use the @num variable directly
The second usage: select @num:=1; or select @num:=field name from table name where …
Pay attention to the above two assignment symbols, you can use "=" or ":=" when using set, but you must use ":= assignment" when using select
example：
SET @roleid=replace(uuid(), '-', '');
SET @userid=replace(uuid(), '-', '');
Variables in Oracle are declared in advance
example：
declare
roleid varchar2(32);
userid varchar2(32);
begin
roleid:=sys_guid();
userid:=sys_guid();