retrieving 8th record from emp table in scott instance

select * from emp where rownum<=8
minus
select * from emp where rownum<=7

update emp set comm = 
case 
when comm is null then 0
else
10
end

select e.*,decode(comm,null,0,-10) new_comm
from emp e

select nvl2(comm,0,-10) "New Comm" from employees;

select case when comm is null then 0 else -10 end from emp

select right(ename,5) as name from emp

select substr(ename,-5) from emp

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.

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