Answer / esakkiraja

Both of these are substitution variable adn difference
between & and &&:

&--->the variable prefixed with an ampersand (&) to
prompt the user for a value.

eg:
SELECT employee_id, last_name, salary, department_id
FROM employees
WHERE employee_id = &employee_num ;

It should prompt the user to enter a value for employee_num


&&--->Mainly we used the double-ampersand (&&) if you want
to reuse the variable value without prompting the user each
time.

eg:SELECT employee_id, last_name, job_id, &&column_name
FROM employees
ORDER BY &column_name;

Here the user is asked to give the value for variable
column_name only once. The
value supplied by the user (department_id) is used both for
display and ordering of data.

Answer / fahad

&-it is used in run time.it ask d value as new and old value
&&-it repeats the duplicacy and it repeat d same output..
ex.single ampersent...
select employee_id,last_name,hire_date,salary from employees
where department_id=&department_id;
when u enter dis statement.
in output it vil ask d value..
enter value for department_id=



query for &&..
select employee_id,last_name,hire_date,salary from employees
where department_id=&&department_id;

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