If you want to use external value in SQL, you must use && or & expressions. You can use external value in column name, table name or variable.
Example
select * from employees where employee_id = &employeeid;
If you use to double expression (&&), this value makes a cache. You can use this cached value with single expression (&). But when your SQL Query is finished, you must do undefine this cached value. Because this variable is read in cache everytime. Your changes are not influence for variable.
Example
select &&column_name from employees where &column_name = &variable;
undefine column_name;
&&column_name wants one column name in employees table and make cache your value. After the system uses your value in &column_name. &variable is noncache. Because single expression (&).