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I defined a template that should link to a page when the the variable is non-empty:

[http://openlibrary.org/works/XXX/{{{ISBN}}} {{{ISBN|}}}]

But what's not working as perfectly is that when the variable ISBN is not given a value, a non-functional link is still displayed. How to improve this?

6

Assuming you have the ParserFunctions extension, you can use the #if function, as documented on MediaWiki:

This function evaluates a test string and determines whether or not it is empty. A test string containing only white space is considered to be empty.

{{#if: test string | value if test string is not empty | value if test string is empty (or only white space) }}

The template you described would look something like this, that is, the link is only displayed if the ISBN parameter is not empty:

{{#if: {{{ISBN|}}} | [http://openlibrary.org/works/XXX/{{{ISBN}}} {{{ISBN|}}}] }}
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