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Oracle SQL Plus The Definitive Guide- P18

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Oracle SQL Plus The Definitive Guide- P18:Every day, computer professionals the world over wake up, travel to the office, sit down in front of a computer, and begin another day working with that database called Oracle. Programmers write queries and stored procedures. Database administrators monitor performance, make database changes, and perform other maintenance tasks. Operations people may need to back up or recover a database. | previous page page_146 next page Page 146 these types of constants in a user variable makes the script more maintainable because changes to the values can be made in one central place. The UNDEFINE Command The UNDEFINE command deletes a variable definition. If you have created a variable containing sensitive information such as a password you can use UNDEFINE to delete it when it is no longer needed. It s also not a bad idea to UNDEFINE all your variables at the end of a script so they don t linger and possibly affect the execution of other scripts that you run. Syntax for the UNDEFINE command The syntax for UNDEFINE looks like this UNDEF INE variable_name variable_name. where UNDEF INE Is the command and may be abbreviated to UNDEF. variablename Is the name of a user variable to delete. You can delete several variables with one command by listing them out separated by spaces. Deleting a variable The following example deletes the three variables created earlier when the DEFINE command was discussed SQL UNDEFINE fiscal_year SQL UNDEFINE my_publisher my_editor SQL DEFINE my_publisher symbol my_publisher is UNDEFINED SQL DEFINE my_editor symbol my_editor is UNDEFINED SQL DEFINE fiscal_year symbol fiscal_year is UNDEFINED As you can see after using UNDEFINE to delete the variables they can no longer be referenced. Controlling Variable Substitution As you use SQL Plus two problems will eventually arise concerning the use of substitution variables. The first problem you are likely to encounter is that you will need to use an ampersand somewhere in your script and you won t mean for it to be part of a substitution variable name. This is a very common problem and hap- previous page page_146 next page previous page page_147 next page Page 147 pens most often when you re using an ampersand in a quoted string or as part of a comment. The second problem which you may never encounter at all is that you may want to place a substitution variable smack in the middle of a word. .