You can define variables, called substitution variables, for repeated use in a single script by using the SQL*Plus DEFINE command. Note that you can also define substitution variables to use in titles and to save your keystrokes
(by defining a long string as the value for a variable with a short name).
To define a substitution variable L_NAME and give it the value "SMITH", enter the following command:
DEFINE L_NAME = SMITH
To confirm the variable definition, enter DEFINE followed by the variable name:
DEFINE L_NAME = "SMITH" (CHAR)
To list all substitution variable definitions, enter DEFINE by itself. Note that any substitution variable you define explicitly through DEFINE takes only CHAR values (that is, the value you assign to the variable is always treated as a CHAR datatype).
You can define a substitution variable of datatype NUMBER implicitly through the ACCEPT command. You will learn more about the ACCEPT command. To delete a substitution variable, use the SQL*Plus command UNDEFINE followed by the variable name.