Substitution variables are used to store temporary values.
For variables of type character or date is mandatory quoting.
Substitution variables can occur in a condition WHERE, in the ORDER BY clause, in the name of a table.
ORDER BY &order ;
The variable remains seted until SQL * Plus session is closed or UNDEFINE command is given for the variable in question.
SQL > DEFINE name1 = Brad
SQL > DEFINE name2 = Stuart
SQL > SELECT first_name, age
2 FROM students
3 WHERE first_name = ' &name1 '
4 OR first_name = ' &name2 ';