Variable Scope and Visibility

Among the most difficult things that I perceived in the very beginning of my career was the variable scope and visibility. So I decide to write about this topic last night. As I previously mentioned I am reading PL/SQL in my free time now a days. Talking about free time, by the way it’s really hard for me now days. Back to the topic, PL/SQL has a very compact and tight syntax. I mean for every Begin there must be an End. That is really impressive. And the optional parts of variable declaration and exception handling make your code very handsome looking. And if the code is written in Caps then it just kills. Try it out.

Variable scope means the boundaries in which a specific variable can be used. For example if you declare a variable then the variable will only be visible in the specific block you declared the variable in. This means is that the variable’s scope is limited to that block only. You cannot use the variable outside that block because when the block will end that variable will also end and the memory and other resources are returned back to the OS. So referring that variable outside the block will result in an error. If you have nested blocks (blocks inside other blocks) and you have declared a variable in the outer block then that variable can also be used inside the inner block, it is possible because inner block lies within the outer block and is a part of it.

Sometime it happens that a variable is in scope but is not visible. This can happen if you have declared a variable with the same name in both the outer and inner blocks of a nested blocks structure. Now if you try to use that variable in the inner block then the variable declared in the inner block will be used. The variable in the outer block will be hidden. In this type of situation we can say that the variable declared in the outer block is in scope but is not visible. Now if you want to use the variable of the outer block then you can use Labels. Labels are identifiers that identify the variables of specific blocks. They are written as:

<>,

Include this line in the declaration section of the outer block and you will be able to access the variables of outer block using this label anywhere no matter if the names are same in the inner blocks. To use this you will write:

Outer block.variable_name,

This will give the value of variable declared in the outer block.